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Method of infrared spectroscopy. The essence of the method of infrared spectroscopy. How FTIR Spectroscopy Works

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1 M. V. LOMONOSOV MOSCOW STATE UNIVERSITY FACULTY OF SCIENCES ON MATERIALS METHODOLOGICAL DEVELOPMENT INFRARED SPECTROSCOPY I.V. Kolesnik, N.A. Sapoletova Moscow 2011

2 CONTENTS 1. THEORY 4 Physical and chemical bases of the IR spectroscopy method 4 Optical spectroscopy. Infrared spectroscopy (IR) and Raman spectroscopy (RS). 4 Structure of atomic and molecular spectra. Rotational and vibrational spectra. 7 Vibrations of polyatomic molecules 8 Types of devices, schemes 11 Introduction 11 Principles of design and operation of IR spectrometers 11 Fundamentals of experimental technique: transmission, attenuated total reflection (ATR) and diffuse reflection spectra 17 Absorption spectra 17 Differential method 20 Technique of ATR methods SAMPLE PREPARATION TECHNIQUE 25 Technique for sample preparation and measurement of transmission spectra from samples pressed into thin tablets (using KBr as an example) 25 Preparation of tablets 25 Recording of spectra 26 Technique for sample preparation and measurement of transmission spectra from samples in suspensions (HCB, vaseline oil) 27 Suspensions 27 Grinding of KBr plates 29 Technique Sample preparation and ATR measurements 30 Introduction 30 Fundamentals 30 Materials used 31 ATR spectroscopy attachment SPECTRUM ONE USER MANUAL 33 Spectrometer construction 33 Tour of the spectrometer 33 Spectrum One attachments 35 Instrument internals 36 Instrument maintenance 37 Maintaining the Spectrum One 37 Moving the Spectrum One 37 Replacing the dryer 38 Measurement procedure 45 Procedure 45

3 Attachment for taking diffuse reflection spectra 52 Introduction 52 Package contents 53 Precautions 53 Installation 53 Attachment calibration 55 Sample analysis reduction reactions of quinones to hydrocarbons by IR spectroscopy 71 Task 4. Study of the process of hydrogen bond formation in ethanol solutions in carbon tetrachloride by IR spectroscopy 73 Task 5. Quantitative analysis REFERENCES APPENDIX. BRIEF TABLES OF CHARACTERISTIC FREQUENCIES 80 Frequencies of characteristic vibrations of bonds in organic compounds 80 Frequencies of characteristic vibrations of bonds in inorganic compounds 86

4 1. Theory Physical and chemical foundations of the IR spectroscopy method Optical spectroscopy. Infrared spectroscopy (IR) and Raman spectroscopy (RS). Spectroscopic methods of analysis are methods based on the interaction of matter with electromagnetic radiation. One of the most important concepts used in spectroscopy is the concept of spectrum. The spectrum is a sequence of energy quanta of electromagnetic vibrations absorbed, released or scattered during the transitions of atoms or molecules from one energy state to another. Rice. 1.1 Regions of the electromagnetic spectrum, , 152 p. The range of electromagnetic radiation extends from the longest-wavelength radiation of radio waves with wavelengths greater than 0.1 cm - to the highest-energy γ-radiation with wavelengths of the order of m (see Error! Reference source not found..1). Separate regions of the electromagnetic spectrum overlap. It should be noted that

5, the region of the electromagnetic spectrum that is perceived by the human eye is very small compared to its entire range. The nature of the processes occurring during the interaction of radiation with matter is different in different spectral regions. In this connection, spectroscopic methods of analysis are classified according to the wavelength (energy) of the radiation used. At the same time, optical spectroscopy is also subdivided according to the objects under study: into atomic and molecular. With the help of atomic spectroscopy, it is possible to carry out a qualitative and quantitative analysis of the elemental composition of a substance, since each element has its own unique set of energies and intensities of transitions between electronic levels in the atom. From molecular spectroscopy data, it is possible to extract data on the electronic structure of molecules and solids, as well as information on their molecular structure. Thus, methods of vibrational spectroscopy, including infrared (IR) spectroscopy and Raman spectroscopy (RS), make it possible to observe vibrations of bonds in a substance. The sets of bands in the IR and Raman spectra are as specific a characteristic of a substance as are human fingerprints. From these spectra, a substance can be identified if its vibrational spectrum is already known. In addition, IR and Raman spectra determine the symmetry and structure of unstudied molecules. The frequencies of the fundamental vibrations found from the spectra are necessary for calculating the thermodynamic properties of substances. Measuring the intensity of the bands in the spectra makes it possible to carry out quantitative analysis, study chemical equilibria and the kinetics of chemical reactions, and control the course of technological processes. Table 1.1 Relationship between spectroscopic methods and areas of the electromagnetic spectrum. Spectroscopic Spectral region Change their energy methods Nuclear physics 0.005 1.4 Å Nuclei X-ray 0.1 100 Å Internal electrons Vacuum UV spectroscopy nm Valence electrons UV spectroscopy nm Valence electrons Visible spectroscopy nm Valence electrons

6 areas NIR spectroscopy energy) Molecules (vibrational nm Molecules (vibrational, IR spectroscopy cm -1 rotational energy) Microwave 0.75 3.75 mm Molecules (rotational energy) spectroscopy Electronic Unpaired electrons (in 3 cm paramagnetic resonance magnetic field) Nuclear magnetic Nuclear spins (in a magnetic 0.6 10 m resonance field) As a result of the interaction of the radiation flux with matter, the intensity of the flux (I 0) decreases due to the processes of absorption (by the value of I A), reflection (IR) and scattering (IS). The magnitudes and intensity of the flow I that passed through the substance is expressed by the following relationship: I I I I I 0 A R S spectrum is not used due to difficulties in obtaining and recording emission spectra.In the IR region, to characterize the energy of photons, a quantity called the wave number is most often used: _ 1. (2) Its dimension is cm -1, i.e. this is the number of wavelengths that fit on a segment of 1 cm. The wave number is directly proportional to the energy: _ E h (3) In IR spectroscopy, the absorption (or transmission) spectrum is represented in the coordinates optical density (or transmission intensity) - wave number.

7 Structure of atomic and molecular spectra. Rotational and vibrational spectra. Atoms are characterized by discrete spectra consisting of individual spectral lines and line spectra. The number of spectral lines in them grows as the number of electrons in the outer electron shells increases. The spectra of molecules in the radio frequency range and the far IR region have a line character, and in the middle and near zones of the IR, UV and visible regions, striped spectra are observed. The appearance of bands in molecular spectra is associated with the existence of three types of motion in a molecule: electronic, vibrational, and rotational. The energy of a molecule E can be approximately represented as the sum of electronic E e, vibrational E v and rotational E r energies: E E E E (4) e v r those. it corresponds to a certain set of discrete energy levels. A qualitative diagram of the energy levels of a diatomic molecule is shown on Error! Reference source not found. For simplicity, it only shows two electronic levels of vibrational levels e v E e. Each electronic level has its own set of E v, and each vibrational level has its own set of rotational levels E r. When the energy of the electrons changes, the vibrational and rotational energies of the molecule simultaneously change, and instead of electronic, vibrational-rotational transitions are observed. The frequencies of the spectral lines corresponding to these transitions are determined by the expression r e, v, r e v r. Since the number of such lines is very large, the electronic-vibrational-rotational spectrum, usually called electronic, takes the form of wide overlapping bands. Electronic spectra of emission and absorption are observed in the range of nm (UV, visible and near infrared regions). For the same reason, vibrational spectra also have a striped structure (cm -1, middle and far zones of the IR region).

8 Fig. 1.2 Scheme of energy levels of a diatomic molecule, Vibrations of polyatomic molecules All possible positions of molecules in three-dimensional space are reduced to translational, rotational and vibrational motion. A molecule consisting of N atoms has only 3N degrees of freedom of motion. These degrees of freedom are distributed among the modes of motion differently depending on whether the molecule is linear or not. For molecules of both types, there are 3 translational degrees of freedom, and the number of rotational degrees of freedom for nonlinear molecules is 3, and for linear molecules 2. Thus, the vibrational degrees of freedom (Fig. 1.3.) account for: 3N-5 degrees of freedom for linear molecules, 3N-6 degrees of freedom for non-linear molecules. The main types of vibrations of a molecule are called normal vibrations. On Error! Reference source not found. normal vibrations of triatomic molecules are shown. More strictly, normal oscillations are those oscillations that occur independently of each other. This means that when a normal vibration is excited, there is no transfer of energy to excite other vibrations. In the case of normal vibrations, the atoms vibrate in the same phase and with the same frequency. Asymmetric motions of atoms lead to more complex vibrations. Each

9 the vibration of atoms in a molecule can be represented as a linear combination of several normal vibrations. From the point of view of the form of vibrations, there are: stretching vibrations (ν), which occur in the direction of chemical bonds and in which interatomic distances change; deformation vibrations (), in which the bond angles change, and the interatomic distances remain constant. When infrared radiation is absorbed, only those vibrations are excited that are associated with a change in the dipole moment of the molecule. All vibrations during which the dipole moment does not change do not appear in the IR spectra. Fig Various possibilities of movement of triatomic molecules. a) H 2 O molecule (nonlinear). b) CO 2 molecule (linear),

10 In experimentally obtained vibrational spectra, the number of bands often does not coincide with the theoretical one. As a rule, there are fewer bands in experimental spectra due to the fact that not all possible oscillations are excited, and some of them are degenerate. The experimental spectrum may also be richer in bands than the theoretical one due to the presence of overtones and complex oscillations. The frequencies of complex vibrations are equal to linear combinations of frequencies of various stretching and bending vibrations.

11 Types of devices, schemes Introduction The study of IR spectra of compounds allows obtaining significant information about the structure, composition, interaction of structural units (fragments) that make up the substance both in the solid state (crystalline or amorphous) and in solution. IR spectra also provide information about the state of molecules adsorbed on the surface of a substance or located inside its volume due to the presence of channels, pores, intervals between layers and intergranular spaces. The IR region of the spectrum covers wavelengths from the boundary of the visible region, i.e. from 0.7 to 1000 μm, which corresponds to 10 cm -1 lower limit of the vibrational frequencies of molecules. The entire IR region is conditionally divided into near, middle and far, or long-wavelength. Such a subdivision arose in connection with the properties of optical materials (transparency and linear dispersion). crystal KBr 25 μm (400 cm - 1). In connection with the creation, on the one hand, of cesium bromide and iodide prisms, and, on the other hand, of IR spectrometers with diffraction gratings and interferometers, the International Union for Pure and Applied Chemistry (IUPAC) recommended that the long-wavelength region below 200 cm -1 (low-frequency limit of the working range of the CsI prism corresponding to a wavelength of 50 µm). Of course, there are no fundamental differences between the intervals and cm -1, as well as the area above 400 cm -1, but the equipment and methods have their own specifics for each of the areas. The spectral interval below 10 cm -1 (λ > 1000 μm) is usually investigated by microwave and radio spectroscopy. Principles of design and operation of IR spectrometers Due to advances in the development of spectral instrumentation, there are now instruments of various designs that cover the entire range of infrared radiation. According to the principle of obtaining the spectrum, devices for the infrared region can be divided into two main groups: dispersive and non-dispersive.

12 Dispersive spectrometers As a dispersive device, prisms made of a material with a dispersion corresponding to the IR range and diffraction gratings are used. Usually, for the mid-IR region (cm -1), prisms of single crystals of KBr, NaCl and LiF are used. At present, prisms are of little use and have been practically superseded by diffraction gratings, which give a large gain in radiation energy and high resolution. But, despite the high quality of these devices, they are increasingly being replaced by Fourier spectrometers belonging to the group of non-dispersive devices. Single-beam and double-beam schemes Scanning dispersive IR spectrometers according to the illumination scheme are single-beam and double-beam. With a single-beam scheme, the absorption spectrum of the investigated is recorded on the intensity curve coinciding with the wavelength and together with the background absorption. Usually, a two-beam scheme is used, which makes it possible to equalize the background, i.e. full transmission line, and compensate for the absorption of atmospheric vapors of H 2 O and CO 2, as well as the attenuation of the beams by the windows of the cuvette and, if necessary, the absorption of solvents. Rice. 1.4 Block diagram of a two-beam scanning IR spectrometer: 1 source of IR radiation; 2 mirror system; 3 working beam and sample; 4 comparison beam and background compensator; 5 chopper-modulator; 6 entrance slit of the monochromator; 7 dispersive element (diffraction grating or prism with a Littrow mirror); 9 receiver; 10 amplifier; 11 mining motor; 12 - photometric wedge; 13 recorder; 14 sweep motor The block diagram of the dual-beam scanning IR spectrometer is shown in Fig.1.4. The registration of the spectrum is carried out as follows: IR radiation from source 1 is divided into two beams. The working beam passes through the sample, and the reference beam passes through some kind of compensator (a cuvette with a solvent, a window, etc.). By using

13 of the interrupter 5, the beams are alternately directed to the entrance slit 6 of the monochromator and through it to the dispersing element 7. When it is slowly rotated by the sweep motor 14, through the exit slit 8 of the monochromator to the receiver 9, narrow wavelengths cut by the slit, ideally monochromatic , rays. If the radiation of a given wavelength in the working beam and the comparison beam has different intensities, for example, it is attenuated in the working beam by the absorption of the sample, then an alternating electrical signal appears at the receiver. After amplification and conversion, this signal is fed to the mining motor 11, which drives the photometric wedge 12 (diaphragm) until the radiation fluxes are equalized (optical zero method). The movement of the photometric wedge is associated with the movement of the pen of the recorder 13 along the ordinate, and the rotation of the dispersing element is associated with the drawing of a paper tape or the movement of the pen holder along the abscissa. Thus, depending on the calibration during scanning, a spectral curve of the dependence of the transmission (absorption) as a percentage or the optical density of the sample on the wave number (or wavelength) can be recorded. Monochromators The fundamental part of scanning spectrometers is the monochromator. As a dispersing device in it, prisms made of materials transparent in the IR region with a suitable dispersion or echelette diffraction gratings can serve. Since the dispersion of materials is greatest at the long-wavelength limit of their transparency and rapidly decreases with decreasing wavelength, in the mid-IR region, replaceable prisms are usually used, made from single crystals of LiF, NaCl, KBr, and for the cm -1 region - from CsI. Non-dispersive devices The operation of Fourier spectrometers is based on the phenomenon of interference of electromagnetic radiation. For the manufacture of these devices, several types of interferometers are used. The Michelson interferometer is the most widely used. In this device, the infrared radiation flux from the source is converted into a parallel beam and then divided into two beams using a beam splitter. One beam hits the movable mirror, the second - on the fixed one. The rays reflected from the mirrors are returned by the same optical path to the beam splitter. These beams interfere due to the acquired path difference, and, consequently, the phase difference created by the movable mirror. As a result of interference, a complex interference pattern is obtained.

14 is a pattern that is an overlay of interferograms that correspond to a certain path difference and radiation wavelength. The combined light flux passes through the sample and enters the radiation receiver. The amplified signal is fed to the computer input, which performs the Fourier transform of the interferogram and obtains the absorption spectrum of the sample under study. The Fourier transform is a complex computational procedure, however, the intensive development of computer technology has led to the creation of small-sized, high-speed computers built into the spectrometer, which make it possible to obtain the spectrum and process it in a short time c.291] Fig Schematic optical diagram of the Michelson interferometer (without collimation bunches), . The radiation intensity curve of these sources, heated by current to high temperatures, has the form of a blackbody radiation curve. So, for example, in a globe at a temperature of ~ 1300 C, the maximum radiation intensity falls on the region of ~ 5000 cm -1 (~ 2 μm), and in the region of ~ 600 cm -1 (16.7 μm), the intensity drops by about 600 times. There are no good sources of radiation in the long-wavelength IR region at all. The main part of the thermal radiation of heated solids or the radiation of a gas discharge falls on the visible and near-IR spectral regions, and in the long-wavelength part, the radiation power of these sources is an insignificant fraction of the total power. For example, an arc lamp with a total radiation power of 1 kW gives here a power of only 10-1 W. Up to the low frequency limit of 200 cm -1, the thermal sources of IR radiation mentioned above are usually used, but they are very weak even in the range of cm -1, where the intensity curve I(λ) has a slope far from the maximum. Below 200 cm -1, a high-pressure mercury lamp is usually used as a source. In the upper part of its operating range, mainly thermal radiation of heated walls is used, and below the radiation flux of a mercury arc and plasma emission. IR detectors Sensitive thermocouples (“thermopillars”) or bolometers built on the principle of resistance thermometers are used as radiation receivers in spectrometers for the mid-IR region. Thermal receivers also include a pneumatic or optoacoustic receiver (Golay cell), in which thermal expansion of the gas occurs under the action of radiation. The gas is placed in a blackened chamber with a flexible wall that has a mirror-like outer coating. The movement of the light beam reflected by the mirror is recorded by a photocell. This receiver is usually made for the long-wave infrared region, where another group of receivers is also used: quantum or photon.

17 Fundamentals of Experimental Technique: Transmission, Attenuated Total Reflection (ATR), and Diffuse Reflectance Spectra Absorption Spectra General Principles If a substance is exposed to continuous infrared light energy and the transmitted light flux in a monochromator is decomposed into wavelengths (use a Fourier spectrometer) , then graphically display the dependence of the intensity of the transmitted light on the wavelength, you get the IR spectrum. Against the background of a continuous spectrum with intensity I o , absorption bands appear with wave numbers characteristic of a particular substance. Studies have shown that IR spectra are individual both for each chemical compound and for some atomic groups. Depending on the composition, structure, and nature of the bonds of a substance, its spectrum differs from the spectra of other substances in the number of bands, their position on the scale of wave numbers, and intensity. Therefore, IR spectra can be used to identify and qualitatively analyze chemical compounds for the presence of individual atomic groups. This is the first and simplest problem of vibrational spectroscopy. The second problem is related to the use of vibrational spectroscopy for the purposes of quantitative analysis. To solve it, one should only know the empirical dependences of the intensity of the bands in the spectrum on the concentration of the substance in the sample. The study of vibrational spectra for the identification of substances and quantitative analysis does not represent all the possibilities of this method, which is now widely used to solve problems of structural inorganic chemistry. Namely: a) to study the nature of chemical bonds, b) to study the symmetry of molecules and ions, c) to identify intermolecular interactions. When obtaining IR absorption spectra, substances can be in all three states of aggregation: gaseous, liquid and solid. The sample preparation technique and cell design depend on the state of aggregation of the substance. Cell windows are usually made from single crystals of salts, mainly of alkali and alkaline earth metal halides (most often from potassium bromide). The hygroscopicity of the latter and instability to temperature effects often cause significant difficulties in obtaining IR spectra.

18 Gases When recording the spectrum of gaseous substances, cuvettes with a distance between windows of 100 mm or more are used. For high-temperature recording of gas spectra, cuvettes about 1 m long have metal cups, the central part of which is heated by means of a spiral through which an electric current is passed. In order to prevent the diffusion and condensation of the vapors of the substance on the cooled windows, some inert gas is introduced into the cuvette. Since the amount of substance in the path of the light beam is determined by the temperature and pressure of the gas, these parameters must be carefully controlled to obtain greater accuracy of quantitative analysis. Liquids and solutions Two types of cuvettes are used for recording the spectra of liquids and solutions: collapsible and constant thickness. Collapsible cuvettes consist of two windows, an insert and a glass beaker. Their thickness can be varied by changing the height of the glass. Tightness is ensured by reliable optical contact of the end surfaces of the glass jar with the surface of the windows. Cuvettes of constant thickness consist of two glued windows, between which there is a gasket of a certain thickness made of Teflon or lead. The distance between the windows of liquid cuvettes is usually from 0.01 to 1 mm. When recording IR spectra of solutions, solvents are usually selected based on the fact that their transmission in the studied region of the spectrum is at least 25%. Very wide transmission regions are typical for such solvents as CCl 4, CS 2, CHCl 3, CH 3 CN, C 6 H 6 and some others. Water in a number of areas of the IR spectrum is not transparent. Its transmission range can be significantly expanded if, along with solutions in plain water, the spectra of solutions in heavy water (D 2 O) are recorded. Since solvents have their own absorption spectrum, it can be a problem to select a solvent in which a sufficient amount of the sample is dissolved and whose spectrum at the same time does not overlap with the absorption bands of the sample to be measured. A wide range of different solvents are used. Most of the organizations that labs use provide catalogs of the most common solvents, indicating areas of the spectrum where they are suitable for use. When recording the spectra of aqueous solutions, special non-hygroscopic materials should be used for the manufacture of cell windows: CaF 2, KRS, AgCl, Si, Ge.

19 Solids Solids can be removed as thin sections of single crystals (several hundredths of a mm thick) or films, but polycrystalline powders are much more likely to be encountered. To reduce light scattering by particles of such powders, their suspensions are prepared in some sufficiently viscous and transparent liquid for IR rays. Vaseline oil is usually used for this purpose. To prepare a suspension in vaseline oil, several tens of milligrams of the substance are carefully ground in an agate or jasper mortar with two to three drops of oil. The suspension is applied in a thin layer on a plate of potassium bromide and covered with a second plate. If the sample preparation operation is carried out in a dry chamber, even very hygroscopic substances can be analyzed in this way. To record the IR spectra of polycrystalline powders, they can also be pressed together with an excess of potassium bromide into tablets several millimeters thick. To obtain tablets, special vacuum molds and a pressure of several tons per 1 cm 2 are used. Tablets with potassium bromide can be used for approximate quantitative measurements of mixture compositions by band intensities. It should only be taken into account that during the preparation of tablets, labile complexes can decompose due to the heat released during pressing. In addition, ion exchange of some compounds with potassium bromide is possible, and strong oxidizing agents oxidize the bromide ion to bromine. Usually, the spectrum of a solid organic sample depends markedly on the crystal modification, so when working with solid samples, care should be taken that the polymorphic form of the sample is always the same. Quantitative Analysis Internal Standards Samples prepared in this way are difficult to quantify, since it is not possible to set the exact concentration in the paste, nor to quantitatively apply it to a certain area of the cuvette windows. To avoid this difficulty, you can use the internal standard method. An internal standard is selected that has IR absorption bands in the region where the sample does not have its own absorption bands. The defined mixture of internal standard and sample is mixed and dispersed as a paste as above. The ratio of the optical densities of the absorption bands of the sample and the internal standard is a measure of the concentration of the sample. Often used as internal standards

20, inorganic substances are used, since they usually have simple spectra with narrow bands, are easily crushed and form suspensions. For this purpose, PbCNS, CaCO 3, dodecanitrile, anthracene and metal stearates are used. Differential Method Once the sample is prepared for spectrum recording, the measurement technique must be chosen. It is usually sufficient to measure the height of the peak above the baseline and relate it to the concentration of the sample. In cases where high accuracy or high sensitivity is required, it is useful to use the differential method. In this case, a carefully prepared blank tablet or cuvette is placed in the comparison beam of a two-beam device, the spectrum of which is subtracted from the spectrum of the sample. The exclusion components present in the sample are usually introduced into the cuvette or comparison tablet in such concentrations that their spectra are completely compensated. The spectrum recorded under these conditions is the spectrum of only the components of interest to us without superimposing the spectra of excluded components present in the mixture. To improve the accuracy of the analysis, a known amount of the component to be determined can be added to the reference cuvette, and the intensity of the differential spectrum can be increased by increasing the layer thickness or increasing the gain of the instrument. If it is required to determine very small amounts of a substance, then the so-called double differential method can be used, which consists in the fact that the spectrum of the sample is recorded relative to some control substance, then the sample and the reference cuvette are interchanged and the spectrum is recorded on the same form. By measuring the positive and negative peaks together (doubling their height), an increase in sensitivity is obtained. When using this method, it is possible in favorable cases to determine ten-thousandths of a percent of a substance. Significance of pre-separation operations The more complex the composition of an unknown sample, the less the possibility of successful identification of its components by direct IR examination, so it is very important to use different separation methods before taking IR spectra. If you frequently analyze a particular type of sample, such as plastics, fragrances, or foodstuffs, you can develop a simple separation and analysis scheme that can almost completely identify

21 components of very complex mixtures. In these schemes, mixtures can be separated by solvent extraction, adsorption chromatography, ion exchange, preparative gas-liquid chromatography, followed by recording the IR spectra of the obtained fractions. Similar analytical schemes can be used to identify small impurities and pollutants, to characterize parallel products, etc. ATR Techniques The essence of the ATR method The ATR method is a form of spectroscopy, but it must be distinguished from other forms of reflection spectroscopy. Spectroscopy using conventional reflection is characterized by the fact that the radiation is incident on the surface of the sample and reflected into the monochromator, passing through a series of optical elements. Devices for these studies allow you to work with constant or variable angles of incidence. The usual specular reflection spectrum is not similar to the transmission spectrum. Another common reflection spectroscopy technique deals with thin films deposited on a highly reflective surface, such as aluminum, and the whole apparatus is placed in a conventional specular reflectance measurement setup. The spectrum obtained in this way is similar to the usual absorption spectrum. This type of reflectance spectroscopy is sometimes referred to as double transmission because the radiation passes through the sample, reflects off the mirror surface, passes through the sample again, and then enters the monochromator. The double pass technique is quite common, but its application is limited to those substances that can be prepared in very thin layers. It is unsuitable if the test specimens are very thick or very absorbent. The form of reflection spectroscopy that interests us is carried out in the case when light is incident on a sample from an optically denser medium (a medium with a higher refractive index) at an angle greater than the critical one, i.e., under conditions when ordinary total internal reflection should take place. However, part of the incident radiation penetrates the sample and is absorbed there in the wavelength ranges characteristic of the sample. As a result, the reflection turns out to be not total, but “broken total internal reflection”. The critical angle is the angle of incidence at which the angle of refraction is 90. The value of the critical angle of incidence can be found from the equation

22 : n p sin = n sin (5) where n p and n are the refractive indices of the crystal and sample, respectively; - angle of incidence; - angle of reflection. At the critical angle of incidence, the angle is 90, whence sin = 1. Hence, it is easy to obtain the value of the critical angle from the expression sin =n/n p, (6) It was found that four highly refractive crystals are most convenient for the ATR technique; thallium bromide iodide (KRS-5), silver chloride (AgCl), irtran-2 and germanium. They are listed according to their degree of applicability. To obtain an ATR spectrum, it is necessary that IR radiation pass into a crystal with a high refractive index, be reflected (one or several times) from the interface with a sample having a lower refractive index at an angle greater than the critical one, and leave the crystal into the monochromator. The resulting ATR spectrum is very similar to a conventional IR absorption spectrum. As the wavelength increases, the observed absorption bands in the ATR spectrum become more intense than the corresponding absorption bands in the ordinary spectrum. This is the most noticeable difference between the ATR spectra and the IR absorption spectra due to the wavelength dependence of the ATR. Another difference, less noticeable, is a slight shift in the maxima of the absorption bands. None of these differences creates serious difficulties when comparing ATR spectra with IR absorption spectra. As the angle of incidence approaches the critical angle, the observed ATR spectrum becomes very mediocre or poor in lines due to interfering refraction effects. But even as the deviation from the critical angle increases, the intensity of the absorption bands also decreases. If the refractive index of the crystal approaches the refractive index of the sample, then the ATR spectrum becomes very intense, i.e., the optical density of the bands increases. To obtain optimal ATR spectra, a compromise between these factors is necessary. The choice of a suitable crystal turns out to be a more important task than the choice of the range of incidence angles to obtain a good ATR spectrum. When choosing the optimal angle of incidence, spectroscopists try to work at angles much larger than the critical one. But not too low for the spectrum to be low-intensity and not so low that the ATR spectrum is distorted by refraction effects.

23 Apparatus for Obtaining ATR Spectra Numerous works on the experimental ATR technique were largely directed to the selection and use of crystals of various configurations. In this case, the conditions were chosen, and which can be used to obtain a single reflection, when the crystals were a prism or a half-cylinder, and multiple reflections (up to 20 or more times), when the crystals were given a special elongated shape. The largest number of ATR spectra was obtained on attachments placed in conventional IR spectrometers or spectrophotometers. The prefix consists of two systems of mirrors: one of them directs the radiation of the source into the crystal at a constant or variable angle of incidence; the second system of mirrors directs the radiation to the monochromator of the IR spectrometer. The ATR crystal and sample holder are designed to provide good contact between the crystal and the sample surface, for which some compression is provided. Similar mirror systems are also used in fixed-angle attachments, which have recently begun to become widespread. Such a removable attachment is placed in the cuvette section of the spectrometer. Now there are serial specialized spectrophotometers for obtaining ATR spectra. Selection of samples To obtain a satisfactory ATR spectrum, it is necessary to select the crystal material so that the optimal ratio of the refractive indices of the crystal and the sample is ensured, the angle of incidence must be selected, and good contact at the interface between the crystal and the sample must be ensured. The latter is the most important, since without good contact a satisfactory ATR spectrum cannot be obtained. The best ATR spectra are obtained from samples that have a sufficiently even flat surface. The smooth surface of samples such as films allows for good contact between the working surface of the crystal and the sample without damaging the crystal surface (which is important for its long service life). If the sample has an uneven surface, then it makes no sense to try to ensure its good contact with the crystal, for example, by applying great efforts. In this case, IR radiation will only scatter, the ATR spectrum will not work, and the crystal will either collapse or, at best, require repolishing. It is also not sufficient that the contact between the crystal and the sample is carried out at some points, and not over the entire surface. As in the previous case, the spectrum cannot be obtained. In cases where the surface

24 samples cannot be properly prepared without damaging them, and the ATR method should probably be abandoned altogether. To obtain the spectrum of a film deposited on a crystal, it is necessary to ensure its sufficient thickness, at which IR absorption would already be noticeable. This means that the layer thickness must be at least 0.001 mm. In some cases, ATR spectra can also be obtained from powdered samples, but this requires that they adhere to the crystal surface. There are very few such examples. A satisfactory ATR spectrum can be obtained for a finely divided powder. If the sample can be given the desired shape by pressing the powder, this also increases the chances of obtaining a spectrum of good quality. Working with solutions and liquids IR radiation from a crystal can penetrate into a liquid solution to a depth of 0.005-0.05 mm. If the analyzed component of the solution has sufficient absorption in such a layer thickness, then it is possible to obtain an ATR spectrum of satisfactory quality. For aqueous solutions, the recorded ATR spectrum will be only the spectrum of water to the extent that the radiation penetrates deeply into the liquid medium: at a penetration of 0.05 mm, the spectrum will practically be absent due to complete absorption by water. When preparing for measurements of the ATR spectrum, one should make sure that there will not be a chemical reaction between the sample under study and the crystal. In this case, the crystal may collapse, and the spectrum cannot be obtained.

25 2. Sample preparation technique Sample preparation and measurement of transmission spectra from samples pressed into thin tablets (using KBr as an example) Preparation of tablets 1. Powder grinding The size of crystallites in a sample strongly affects the quality of the obtained spectra due to radiation scattering processes. To avoid scattering effects, the particles in the sample powder to be used for tablet compression should be about 1 µm in size. To achieve such dimensions, the sample must be carefully ground in an agate or jasper mortar. Experienced operators judge the particle size of powders by tactile sensations. 2. Preparing the Mold After the powder has been carefully ground, it is placed into the mold after being weighed and thoroughly mixed with KBr. It should be noted that the state of the mold plays an important role; it must be absolutely clean and well polished. Before use, the mold is wiped with ethyl alcohol. The use of cotton wool and other fluffy materials is undesirable, it is recommended to use special lint-free wipes. 3. Pressing The powder placed in the mold is leveled with a spatula immediately before pressing to ensure uniform distribution of the substance in the mold volume during pressing. The mold with the powder and the inserted punch is placed in the press. The pressing process is carried out at a force of 6 atmospheres for two minutes. The preform should be depressurized gradually, as rapid depressurization of the sample can create stresses that can lead to undesirable cracking of the tablets. After the end of pressing, the tablet is removed from the mold and placed in a pre-prepared container for storing samples. Rolled paper envelopes can serve as a convenient container for storing tablets; they are convenient to use and store. 4. Mold service

26 For the preparation of high quality tablets, the molds must be thoroughly wiped regularly after completion of work to remove residues of the substance from the punch and the walls of the mold. To do this, it is recommended to use ethyl alcohol. The appearance of scratches on the working surfaces of the mold is highly undesirable, so molds must always be handled carefully and carefully. Recording spectra 1. Preparing the instrument for operation The spectrometer must be turned on in advance (min.) before the start of recording samples in order to warm up the radiation source. 2. Photographing the background Before taking samples, the air spectrum is taken in the spectrometer chamber. This spectrum will subsequently be automatically taken into account when obtaining the spectra of the samples. 3. Sample capture The finished tablet is fixed in the sample holder and placed in the spectrometer. To obtain the spectrum, the tablet must be sufficiently transparent, which is controlled by the amount of energy recorded by the spectrometer radiation receiver before the spectrum is recorded. The obtained spectra are stored in the form of a data table for their subsequent interpretation.

27 Technique of sample preparation and measurement of transmission spectra from samples in suspensions (HCB, vaseline oil) Suspensions One of the main methods for preparing solid samples for research is the method of preparing suspensions (suspensions, pastes) in vaseline oil or hexachlorobutadiene (HCB) that has proven itself over a long period of time. . Vaseline oil is a mixture of normal saturated hydrocarbons of average composition C 25. It contains practically no aromatic and unsaturated hydrocarbons, as well as other impurities, has sufficient viscosity and a suitable refractive index, which allow satisfactory spectra of solids to be obtained without much difficulty. The suspension is prepared by grinding and triturating the solid in liquid paraffin or HCB until sufficient dispersion is achieved. By squeezing the KBr windows with a layer of paste between them, the desired thickness is achieved. Then the windows fixed in the metal holder of the cuvettes are mounted on the spectrophotometer and the spectrum of the sample is recorded in the desired wavelength range. Simple in appearance, the process of preparing a suspension of satisfactory quality requires, in fact, great skill and skill. The suspension is usually prepared as follows. 5-10 mg of a solid substance is placed on a glass plate, then, using a dropper, a drop of oil is applied to the middle of the head of a glass pestle and the substance is vigorously crushed with it. Here, "pulverization" refers to the destruction of aggregates of small particles that make up crystalline, granular and powdery substances. After making about fifteen circular movements with the pestle on the glass plate, using a stainless steel spatula, collect all the crushed suspension from the glass and pestle to the middle of the plate and grind again. Usually the preparation of the suspension is considered complete after three such operations, sometimes you can limit yourself to two, although four or more operations may be necessary. The suspension may be too thick or too thin, in which case either oils or a solid must be added, respectively. However, the experimenter, having worked with various substances, soon

28 will learn to feel in what proportions oil and solid should be taken for any samples. A properly prepared suspension is usually translucent in visible light. When viewed from a slurry compressed between salt windows to the desired thickness, there should be no noticeable cracks, graininess, or other inhomogeneities in the film. If the inhomogeneities are visible to the eye, then the suspension will scatter short-wave radiation. In this case, the absorption and transmission maxima will be distorted, and the value of such a spectrum will be low, in the worst case, it will simply be wrong. The thickness of the suspension film required to obtain a satisfactory spectrum depends on the absorptivity of the sample. If the thinnest film that can be obtained gives too strong a spectrum, then the suspension should be diluted with oil and remixed. Conversely, if a very thick layer gives too weak a spectrum, then more sample should be added and everything remixed. Generally, properly prepared suspensions give excellent spectra for qualitative purposes. Thus, the simplest and generally satisfactory way to prepare a sample in order to obtain a spectrum of a solid for qualitative analysis is the suspension technique (of course, if applicable at all). However, this method also has some disadvantages. One of the disadvantages of the paraffin slurry spectrum is that in the oil's intrinsic absorption regions it is difficult or almost impossible to obtain absorption data for the sample itself. Vaseline oil itself is characterized by an absorption typical of long chain saturated hydrocarbons: a very strong band from 3000 to 2800 cm -1 (3.5 μm region), a strong band at about 1460 cm -1 (6.85 μm), a medium intensity band of about 1375 cm -1 (7.27 µm) and a faint band at about 722 cm -1 (13.85 µm). These bands are due to stretching and bending vibrations of bonds in the methyl, methylene, and methine groups of the molecules. However, this difficulty is easily overcome; it is only necessary to prepare and record the spectrum of the second suspension using a medium that does not contain hydrogen atoms. You can take hexachlorobutadiene, which does not absorb in areas where liquid paraffin has streaks. Having suspensions of the substance in hexachlorobutadiene and vaseline oil, it is possible to obtain the full spectrum of this substance free from the absorption bands of the dispersion medium.

29 Grinding KBr plates KBr crystals are the most commonly used material for cell windows, but their hygroscopicity leads to a number of difficulties. They may become cloudy during use. A small amount of water in the test substance and an organic solvent or high air humidity is enough to cause the windows to become cloudy sooner or later even with careful care. To remove haze, KBr plates have to be periodically polished. The polishing of KBr plates is such a simple matter that every serious spectroscopist should master this technique. This provides sufficient savings, and is also an easy exercise, useful for a specialist in a generally sedentary profession. Grinding and polishing of plates can be carried out using various abrasive materials, depending on the depth of damage. In the presence of deep damage, the plates are polished on fine sandpaper before polishing until large scratches are removed. In the absence of deep scratches, they are limited to polishing the plates on a paste (Cr 2 O 3), followed by polishing on a fabric (flannel). In this case, it is recommended to moisten the surface of the cloth with paste and clean cloth with ethyl alcohol. For grinding and polishing the plates, it is necessary to wear rubber gloves or fingertips, because. the plate becomes cloudy at the points of contact with the skin. Polishing is done in a circular motion. The polishing procedure described above seems to require some practice, skill and attention to detail, otherwise a smooth smooth plate may not work.

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Spectroscopy is a branch of physics and analytical chemistry devoted to the study of the interaction spectra of radiation (including electromagnetic radiation, acoustic waves, etc.) with matter. In physics, spectroscopic methods are used to study the various properties of these interactions. In analytical chemistry, for the detection and determination of substances by measuring their characteristic spectra, i.e. spectrometry methods.

The areas of application of spectroscopy are divided according to the objects of study: atomic spectroscopy, molecular spectroscopy, mass spectroscopy, nuclear spectroscopy, infrared spectroscopy, and others.

The method of infrared spectroscopy makes it possible to obtain information about the relative positions of molecules for very short periods of time, as well as to evaluate the nature of the relationship between them, which is of fundamental importance in the study of the structural and informational properties of various substances.

This method is based on such a physical phenomenon as infrared radiation. Infrared radiation is also called "thermal" radiation, since all bodies, solid and liquid, heated to a certain temperature, radiate energy in the infrared spectrum. In this case, the wavelengths emitted by the body depend on the heating temperature: the higher the temperature, the shorter the wavelength and the higher the radiation intensity. The radiation spectrum of an absolutely black body at relatively low (up to several thousand Kelvin) temperatures lies mainly in this range.

Infrared spectroscopy (IR spectroscopy), a section of molecular optical spectroscopy that studies the absorption and reflection spectra of electromagnetic radiation in the infrared region, i.e. in the wavelength range from 10-6 to 10-3 m. In the coordinates of the intensity of the absorbed radiation - the wavelength (wave number), the infrared spectrum is a complex curve with a large number of maxima and minima.

NIR spectrum of liquid ethanol

Absorption bands appear as a result of transitions between the vibrational levels of the ground electronic state of the system under study. The spectral characteristics (the positions of the band maxima, their half-width, intensity) of an individual molecule depend on the masses of its constituent atoms, the geometry of the structure, the features of interatomic forces, charge distribution, etc. Therefore, infrared spectra are highly individual, which determines their value in identifying and studying the structure connections.

The quantitative relationship between the intensity I of the radiation transmitted through the substance, the intensity of the incident radiation I0 and the quantities characterizing the absorbing substance is based on the Bouguer-Lambert-Beer law, i.e., on the dependence of the intensity of the absorption bands on the concentration of the substance in the sample. In this case, the amount of a substance is judged not by individual absorption bands, but by spectral curves as a whole in a wide range of wavelengths. If the number of components is small (4-5), then it is possible to mathematically extract their spectra even with a significant overlap of the latter. The error of quantitative analysis, as a rule, is a fraction of a percent.

In practice, the infrared absorption spectrum is usually represented graphically as a function of frequency  (or wavelength ) of a number of quantities characterizing the absorbing substance: transmittance T () = I ()/I0(); absorption coefficient А() = /I0() = 1 - Т(); optical density D() = ln = ()cl, where () is the absorption index, c is the concentration of the absorbing substance, l is the thickness of the absorbing layer of the substance. Because D() is proportional to () and c, it is commonly used for quantitative analysis from absorption spectra.

The method of infrared spectroscopy is a universal physical and chemical method, which is used in the study of the structural features of various organic and inorganic compounds. The method is based on the phenomenon of absorption by groups of atoms of the tested object of electromagnetic radiation in the infrared range. Absorption is associated with the excitation of molecular vibrations by infrared light quanta. When a molecule is irradiated with infrared radiation, only those quanta are absorbed, the frequencies of which correspond to the frequencies of the stretching, deformation and libration vibrations of the molecules.

To register the spectra of the surface of solids, the method of frustrated total internal reflection is used. It is based on the absorption by the surface layer of a substance of the energy of electromagnetic radiation emerging from a total internal reflection prism, which is in optical contact with the surface under study. Infrared spectroscopy is widely used for the analysis of mixtures and the identification of pure substances.

Identification of pure substances is usually carried out with the help of information retrieval systems by automatically comparing the analyzed spectrum with the spectra stored in the computer memory. Artificial intelligence systems are used to identify new substances (the molecules of which can contain up to 100 atoms). In these systems, based on spectral structural correlations, molecular structures are generated, then their theoretical spectra are constructed, which are compared with experimental data. The study of the structure of molecules and other objects by means of infrared spectroscopy implies obtaining information about the parameters of molecular models and mathematically reduces to solving inverse spectral problems. The solution of such problems is carried out by successive approximation of the desired parameters calculated using a special theory of spectral curves to the experimental ones.

The parameters of molecular models are the masses of the atoms that make up the system, bond lengths, bond and torsion angles, potential surface characteristics (force constants, etc.), dipole moments of bonds and their derivatives with respect to bond lengths, etc. Infrared spectroscopy makes it possible to identify spatial and conformational isomers, to study intra- and intermolecular interactions, the nature of chemical bonds, the distribution of charges in molecules, phase transformations, the kinetics of chemical reactions, register short-lived (lifetime up to 10-6 s) particles, refine individual geometric parameters, obtain data for calculating thermodynamic functions, etc.

A necessary stage of such studies is the interpretation of the spectra, i.e. determination of the form of normal vibrations, distribution of vibrational energy over degrees of freedom, selection of significant parameters that determine the position of the bands in the spectra and their intensity. Calculations of the spectra of molecules containing up to 100 atoms, including polymers, are performed using a computer. In this case, it is necessary to know the characteristics of molecular models (force constants, electro-optical parameters, etc.), which are found by solving the corresponding inverse spectral problems or by quantum chemical calculations.

In both cases, it is usually possible to obtain data for molecules containing atoms of only the first four periods of the periodic system. Therefore, infrared spectroscopy, as a method for studying the structure of molecules, is most widely used in organic and organoelement chemistry. In some cases, for gases in the infrared region, it is possible to observe the rotational structure of vibrational bands. This makes it possible to calculate the dipole moments and geometrical parameters of molecules, to refine the force constants, and so on.

As is known, in infrared spectroscopy, in the range of each chemical grouping of an organic molecule, there corresponds a certain set of absorption bands, which are well studied and listed in the corresponding reference books. It should be noted that in the process of removing the infrared spectrum, interference is created at certain wavelengths associated with the absorption of electromagnetic radiation by the bonds of O-H and C-H solvents.

The infrared spectrum of a biological sample is a total spectrum in which the absorption bands of various functional groups of organic substances and also water are superimposed. This phenomenon becomes more complicated in view of the interaction of individual types of vibrations of these groups, while the shape of the absorption bands is distorted and their maxima are shifted. Therefore, a large number of broad absorption bands with indistinct maxima are observed in the infrared spectra. Usually the interpretation of the infrared spectra of biological samples is very difficult, therefore, in order to facilitate the interpretation of the total spectrum, it is necessary to divide the biological sample into simpler components. This makes it possible to obtain a greater number of absorption bands for the substance under study and more accurately determine the composition of the components in the sample.

A positive feature of the method of infrared spectroscopy is that the absorption bands of the same type of vibration of the atomic group of various substances are located in a certain range of the infrared spectrum (for example, 3720-3550 cm-1 - the range of stretching vibrations of -OH groups; 3050-2850 cm-1 - groups -CH, -CH2, -CH3 of organic substances). The exact position of the absorption band maximum of the atomic group within this range indicates the nature of the substance (for example, a maximum of 3710 cm-1 indicates the presence of -OH groups, and a maximum of 3030 cm-1 indicates the presence of =C-H groups of aromatic structures).

Characteristic frequencies of functional

groups in organic compounds

However, if the object under study is not a mechanical mixture, but is a complex chemical compound, then the indicated features of the infrared spectra are not detected.

The number of characteristic absorption bands of atomic groups, their intensity and the position of the maxima observed in the infrared spectra give an idea of the structure of an individual compound or of the component composition of complex substances. The intensity of the absorption band is determined by a value numerically equal to the energy that the atomic or functional groups of the sample absorb when infrared rays pass through them. An important diagnostic indicator of absorption bands is the transmission value. This indicator and the concentration of the substance in the removed object are inversely proportional, which is used for quantitative determinations of the content of individual components.

The method of infrared spectroscopy allows you to explore the solid, liquid phases of the biological mass. This method makes it possible to study the sample as a whole, without its dissection and preliminary chemical treatments, and also to use small (up to 10 mg) samples.

Infrared spectroscopy is widely used for the analysis of biological fluids, in particular blood and its fragments, and recently oral fluid or mixed saliva has been increasingly used to diagnose and predict various diseases, however, the interpretation of the results obtained is complicated due to the multicomponent nature of the objects of study.

With infrared spectroscopy of blood and saliva, it is possible to quantitatively analyze only the functional groups included in the main components in analytical quantities. Therefore, the analysis of samples of these liquids is difficult, because. essentially analyzes their water base

In addition, recently the method has been increasingly used to characterize conformational and structural changes in proteins, lipids, phospholipids of cell biomembranes studied in biopsy specimens, as well as using fiber optic techniques.

Using this method, the pharmacokinetics of various drugs can be assessed. Reliably significant changes in the infrared spectrum of blood were revealed in diabetes mellitus. The possibility of using infrared spectrum indicators for early diagnosis of dental diseases and predicting dental caries in children has been proven. A study was made of rapid changes in the parameters of the infrared spectrum of blood for predicting, diagnosing and determining the severity of osteoporosis and the effectiveness of its treatment. The possibility of using infrared spectroscopy to study regeneration processes has been proved.

Infrared spectroscopy is also used in forensic analysis to study the mitochondrial genome for personality identification and paternity determination. a DIS80 genetic focus containing variable numbers of tandem duplications is identified.

Spectra are recorded using classical spectrophotometers and Fourier spectrometers.

Research IR spectrometer Varian Scimitar 1000 FT-IR

The main parts of a classical spectrophotometer are a source of continuous thermal radiation, a monochromator, and a non-selective radiation detector. A cuvette with a substance (in any state of aggregation) is placed in front of the entrance (sometimes behind the exit) slit. Prisms made of various materials (LiF, NaCl, KCl, CsF, etc.) and a diffraction grating are used as the dispersing device of the monochromator. Sequential extraction of radiation of different wavelengths to the exit slit and the radiation receiver is carried out by scanning by turning the prism or grating. Radiation sources - rods heated by electric current from various materials. Receivers: sensitive thermocouples, metal and semiconductor thermal resistances (bolometers) and gas thermal converters, the heating of the vessel wall of which leads to heating of the gas and a change in its pressure, which is fixed. The output signal has the form of a conventional spectral curve.

Advantages of devices of the classical scheme: simplicity of design, relative cheapness. Disadvantages: the impossibility of registering weak signals due to the low signal-to-noise ratio, which greatly complicates work in the far infrared region; relatively low resolution (up to 0.1 cm-1), long-term (for several minutes) registration of spectra.

Fourier spectrometers have no input and output slits, and the main element is an interferometer. The radiation flux from the source is divided into two beams that pass through the sample and interfere. The path difference of the beams is varied by a movable mirror reflecting one of the beams.

Block diagram of the Fourier spectrometer:

1 – radiation source; 2 - interrupter; 3 - beam splitter; 4 - movable mirror;

5 - fixed mirror; 6 - lens system; 7 - cell compartment;

8 - detector; 9 – analog-to-digital converter; 10 - controller;

11 - computer; 12 - digital printing; 13 - disk memory.

The initial signal depends on the energy of the radiation source and on the absorption of the sample and has the form of the sum of a large number of harmonic components. To obtain the spectrum in the usual form, the corresponding Fourier transform is performed using a built-in computer.

The most efficient use of FTIR equipment is possible only with proper preparation of the sample intended for analysis. When working on infrared Fourier spectrometers, one can use both traditional methods of sample preparation for infrared spectroscopy, as well as some new techniques, which are primarily due to a smaller amount of substance sufficient for analysis and the possibility of using additional devices (attachments).

Advantages of the Fourier spectrometer: high signal-to-noise ratio, the ability to operate in a wide range of wavelengths without changing the dispersive element, fast (in seconds and fractions of seconds) registration of the spectrum, high resolution (up to 0.001 cm-1). Disadvantages: manufacturing complexity and high cost.

All spectrophotometers are equipped with a computer that performs the primary processing of the spectra: accumulation of signals, their separation from noise, subtraction of the background and comparison spectrum (solvent spectrum), change of the recording scale, calculation of experimental spectral parameters, comparison of spectra with given ones, differentiation of spectra, etc. Cuvettes for Infrared spectrophotometers are made from materials that are transparent in the infrared region. CCl4, CHCl3, tetrachlorethylene, vaseline oil are usually used as solvents. Solid samples are often crushed, mixed with KBr powder, and compressed into tablets. To work with aggressive liquids and gases, special equipment is used. protective sputtering (Ge, Si) on cell windows. The interfering influence of air is eliminated by evacuating the device or purging it with nitrogen.

For the case of weakly absorbing substances (rarefied gases, etc.), multipass cells are used, in which the length of the optical path reaches hundreds of meters due to multiple reflections from a system of parallel mirrors. The matrix isolation method, in which the test gas is mixed with argon, and then the mixture is frozen, has become widely used. As a result, the half-width of the absorption bands sharply decreases, and the spectrum becomes more contrasting. The use of a special microscopic technique makes it possible to work with objects of very small sizes (fractions of a mm).

The preparation of solid samples for recording their infrared spectra is carried out in two ways:

1. The suspension method is grinding the sample to a finely dispersed state (particle size 2-7 μm) and preparing a suspension in an immersion liquid with a refractive index close to the sample. In this case, vaseline oil, fluorinated or chlorinated oils are usually used as a matrix. The resulting translucent paste is applied with a spatula to a window of optical material in the form of a thin uniform film.

Most often in expert practice vaseline oil is used as an immersion liquid. However, the spectrum of vaseline oil has absorption bands in the regions of 2900, 1460, 1380 and 725 cm-1. These bands are superimposed on the absorption bands of the sample and can be compensated either by using a reference cuvette or by subtracting the vaseline oil spectrum from the total spectrum. In practice, perfluorohydrocarbon oil is used in the study of substances in the region of 4000-1500 cm-1 (it does not absorb fluorinated oil), and vaseline oil is used in the study in the region of 1500-400 cm-1 (it absorbs little vaseline oil).

2. Pressing tablets with alkali metal halides is the main and most versatile method of sample preparation. It consists in thorough mixing of a finely ground sample with KBr powder in an agate mortar and subsequent pressing of the mixture in a mold, resulting in a transparent or translucent tablet. To obtain high-quality spectra, the degree of dispersion of the substance should reach a particle size of 2-7 μm (comparable to the wavelength of infrared radiation).

Sometimes a few drops of distilled solvent (carbon tetrachloride or hexane) are added to facilitate grinding, which evaporates during subsequent grinding. The best results are obtained when the mold is evacuated, which allows you to get rid of air inclusions in the tablets. For tablets, you can use potassium bromide for spectroscopy or qualification not lower than chemically pure, but previously dried from water. Potassium bromide should be dried at t ≈ 600°C for at least 6 hours and stored in a desiccator with a drying agent. Such careful preparation is necessary, because otherwise the resulting spectrum will have broad bands of adsorbed water in the regions of 3450 and 1630 cm-1.

From tablets with a diameter of 3, 5, 7 mm and more, it is possible to record the spectrum without additional devices. Tablets with a diameter of 1 and 2 mm must be examined using a microfocusing device. If the press-form does not allow obtaining tablets with a diameter of 1-3 mm, then a specially made, for example, from cardboard, round insert with a hole of the corresponding diameter cut out in the center can be used. Tablets with a diameter of 1-3 mm are used in the study of microquantities (up to 10-9 g) of the substance.

The KBr tablet compression method is useful for samples that are insoluble in common solvents, amorphous or have a stable crystalline structure and do not contain exchangeable ions.

It is known that the nuclei of molecules far from fixed positions relative to each other are in a continuous vibrational state. An important feature of these oscillations is that they can be described by a limited number of fundamental oscillations (normal modes). A normal mode is an oscillation in which the nuclei oscillate at the same frequency and in the same phase. Water molecules have three normal modes.

ν1 (OH) ν 2 (OH) ν3 (OH)

3656.65cm-1 1594.59cm-1 3755.79cm-1

Basic vibration frequencies of water molecules

The movements of the nuclei during vibrations ν1 (OH) and ν3 (OH) occur almost along the direction of the O-H bonds, these modes are usually called bond stretching vibrations (or δOH) or O-H bond stretching vibrations. During ν2 (OH) vibrations, the H nuclei move in the direction almost perpendicular to the O-H bonds, the ν2 mode is called the bending vibration of the H-O-H bond or the bending vibration of the hydrogen bond. The ν3 mode is called the asymmetric stretching vibration, in contrast to the symmetric stretching vibration ν1.

The transition of a water molecule from its ground vibrational state to an excited state described by the ν2 mode corresponds to the infrared band at 1594.59 cm-1.

Despite the fact that there are a large number of publications on the study of infrared spectra of water, information about the frequencies of oscillations and their assignment not only does not coincide, but is sometimes contradictory. In the spectrum of liquid water, the absorption bands are significantly broadened and shifted relative to the corresponding bands in the spectrum of water vapor. Their position depends on the temperature. The temperature dependence of individual bands in the spectrum of liquid water is very complex. In addition, the complication of the spectrum in the region of OH stretching vibrations can be explained by the existence of various types of associations, the manifestation of overtones and composite frequencies of OH groups in hydrogen bonds, as well as the proton tunneling effect (according to the relay-race mechanism). Such a complication of the spectrum complicates its interpretation and partly explains the contradiction in this regard available in the literature.

The hydroxyl group -OH is capable of strongly absorbing the spectrum in the infrared region of the spectrum. Due to their polarity, these groups usually interact with each other or with other polar groups, forming intramolecular and intermolecular hydrogen bonds. Hydroxyl groups that do not participate in the formation of hydrogen bonds usually give narrow bands in the spectrum, and the associated groups give intense broad absorption bands at lower frequencies. The magnitude of the frequency shift is determined by the strength of the hydrogen bond. The literature contains data on the assignment of absorption bands in the region of fundamental frequencies (2.5 - 6.0 μm (4000-1600cm-1)), as well as near (0.7-2.0 μm (14300-5000cm-1)) and far (20–16 µm (50–625 cm–1)).

The area of fundamental frequencies has been studied the most. For monomeric water, the bands at 3725 and 3627 cm-1 are assigned to the symmetric and antisymmetric vibrations of the OH group, and the bands at 1600 cm-1 are assigned to the bending vibrations of H-O-H. It should be noted that water dimers can have a cyclic structure with two hydrogen bonds (1) rather than an open one (2).

\ O – H H H

H - O / O. . . H–O

Structure of water dimers: 1 – cyclic; 2 - open

For liquid water, absorption bands are also observed in other regions of the spectrum. The most intense of them are 2100, 710-645 cm-1. When passing from water monomers to dimers and trimers, the maximum absorption of O–H bond stretching vibrations shifts towards lower frequencies. On the contrary, for H-O-H deformation vibrations, a shift towards higher frequencies is observed. The absorption bands at 3546 and 3691 cm-1 were attributed to the valence modes of dimers (Н2О)2. These frequencies are much lower than the valence modes ν1 and ν3 of isolated water molecules (3657 and 3756 cm-1, respectively). The 3250cm-1 band represents the overtones of deformation vibrations. Between frequencies of 3250 and 3420 cm-1, a Fermi resonance is possible (this resonance is a borrowing of the intensity of one vibration from another with their random overlap).

Assignment of frequencies in the spectrum of liquid water

The absorption band at 1620 cm-1 is attributed to the deformation mode of the dimer. This frequency is somewhat higher than the deformation mode of an isolated molecule (1596 cm-1). The shift of the band of deformation vibrations of water towards high frequencies during the transition from the liquid to the solid state is attributed to the appearance of an additional force that prevents the bending of the O-H bond. The deformation absorption band has a frequency of 1645 cm-1 and depends very weakly on temperature. It also changes little when passing to a free molecule at a frequency of 1595 cm-1. This frequency also changes little in salt solutions. It turns out to be quite stable, while temperature changes, salt dissolution, phase transitions significantly affect all other frequencies. Zundel (1971) suggests that the constancy of deformation vibrations is associated with the processes of intermolecular interaction, namely, due to a change in the bond angle of the water molecule as a result of the interaction of molecules with each other, as well as with cations and anions.

Infrared absorption spectra of water

in the region of fundamental frequencies.

Difficulties in using infrared spectroscopy in practice are not only technical, but are also associated with the lack of a technique that allows one to apply mathematical analysis in determining vibration frequencies and assigning them to one or another chemical bond.

Based on the results of infrared spectroscopy, it is possible to develop a chemically reliable, reproducible, standardizable method for the analysis of aqueous systems. In this regard, low-resolution infrared spectroscopy provides certain advantages, which allows one to determine the degree of influence of the substances present in the system under study on the structural organization of the aqueous base of solutions and biological fluids by fluctuations in transmission coefficients.

Improving the technical and economic performance of diesel engines is one of the main problems for shipowners. During operation, along with soot resulting from combustion, oil oxidation and degradation products, as well as engine wear particles, enter the lubricant. Improving the reliability of mechanisms and the rational use of lubricants depends on a number of reasons, among which the quality of the oils used is of great importance. At present, the use of modern instrumental methods of analysis is of great importance.

The content of mechanical impurities in most cases is determined by standard methods based on centrifugation or filtration of solutions of working oils in light hydrocarbon solvents, as well as paper chromatography and photometry in the visible region. However, each of the above methods has its drawbacks. For example, if the oil contains good dispersants, a large amount of sooty contamination remains after centrifugation in the centrifuge; when using the filtration method, the additive component is deposited on the filter, as a result, one can only get an idea of the degree of dispersion of insoluble contaminants; photometry in the visible region of the spectrum requires, as a rule, dilution of the test sample, which leads to an increase in time costs and is fraught with errors. The method of electron spectroscopy makes it possible to determine the number and size of particles, however, it is not suitable due to the technical complexity and duration of the analysis. In connection with the above, the photometric method in the infrared region has a number of significant advantages: rapidity, good reproducibility and comparability, no need for preliminary sample preparation for analysis and restrictions on the grades and quality of lubricating oils.

Differential infrared spectra are taken using potassium bromide cuvettes with a layer thickness of 0.1 mm. Since service oils are usually heavily contaminated with sooty deposits, in general the entire infrared spectrum of the service oil is higher than the infrared spectrum of fresh oil, and this "raise" of the spectrum is proportional to the contamination of the service oil. For analysis, wavelength regions are used in which the influence of oil absorption bands is minimal. It is most convenient to use a wavenumber of 2000 cm-1. Comparison of the change in absorption at a wave number of 1704 cm-1, determined by the baseline method and subtraction of the background at a wave number of 2000 cm-1, showed that such a subtraction of the "background" is quite legitimate and does not distort the dynamics of accumulation of oxidation products in oil.

Absorption change at wave number 1704 cm-1,

determined by the "baseline" method and "background" subtraction

Similar results were obtained when assessing the state of sulfonate additives in oil. This allows us to draw conclusions about the possibility of determining the content of mechanical impurities in oil by absorption at a wave number of 2000 cm-1. Since it is impossible to compare the absolute absorption values of the bands in the infrared spectra of working oils (oils contain different amounts of mechanical impurities), to compare the absorption values, the "background" was subtracted from them (the absorption value at a wavenumber of 2000 cm-1). In parallel, the content of mechanical impurities was determined using standard methods (GOST 6370 and the drop test method). The content of mechanical impurities C (wt.%) in the oil was determined by the formula:

 - wavelength, microns; l is the thickness of the working layer of the cuvette, µm (measured interferometrically); A - oil absorption at wavelength .

Comparison of the results obtained by this method and using the centrifugation method showed good reproducibility (correlation coefficient was 0.881). The time for one analysis is about 15 minutes.

Thus, the use of the method of infrared spectroscopy for determining the content of general contamination of engine oil is effective and allows you to quickly obtain a general assessment of the contamination of the working oil with soot, wear products of engine parts and oxidation of the hydrocarbon base of the oil.

Infrared spectroscopy finds application in the study of the structure of semiconductor materials, polymers, biological objects and directly living cells, as a method for studying the structure of molecules, it has become most widespread in organic and organoelement chemistry. In some cases, for gases in the infrared region, it is possible to observe the rotational structure of vibrational bands.

High-speed spectrometers make it possible to obtain absorption spectra in a fraction of a second and are used in the study of fast chemical reactions. With the help of special mirror microattachments it is possible to obtain absorption spectra of very small objects, which is of interest for biology and mineralogy.

Infrared spectroscopy plays an important role in the creation and study of molecular optical quantum generators whose radiation lies in the infrared region of the spectrum. The methods of infrared spectroscopy are the most widely studied in the near and middle regions of the infrared spectrum, for which a large number of various (mainly two-beam) spectrometers are manufactured.

The far infrared region has been mastered somewhat less, but the study of infrared spectra in this region is also of great interest, since in it, in addition to purely rotational spectra of molecules, there are spectra of vibration frequencies of crystal lattices of semiconductors, intermolecular vibrations, etc.

Yukhnevich G.V. Infrared spectroscopy of water. M. 1973.

Zatsepina G.N. Physical properties and structure of water. M. 1987.

Karyakin A.V. Kriventsova G.A. State of water in organic and inorganic compounds. M. 1973.

Antonchenko V.Ya., Davydov A.S., Ilyin V.V. Fundamentals of water physics. Kyiv. 1991.

Privalov P.L. Water and its role in biological systems.// Biophysics 1968. v.13. No. 1.

Gribov L.A. Introduction to molecular spectroscopy. M. 1976.

Mitchell J., Smith D. Aquametry: Per. from English. M. 1980.

Eisenberg D., Kautsman V. Structure and properties of water. : Per. from English. L. 1975.

Rakhmanin Yu.A., Kondratov V.K. Water is a cosmic phenomenon. Cooperative properties, biological activity. M. 2002.

Verbalovich V.P. Infrared spectroscopy of biological membranes. The science. Kazakh SSR. Alma-Ata. 1977.

Bellamy L., Infrared spectra of molecules, trans. from English, M., 1957;

Cross A., Introduction to practical infrared spectroscopy, trans. from English, M., 1961;

Kazitsyna L.A., Kupletskaya N.B. Application of UV, IR, NMR and mass spectroscopy in organic chemistry. M.: Publishing House of Moscow. un-ta, 1979.

Yaroslavsky N. G., Methods and equipment of long-wavelength infrared spectroscopy, "Advances in physical sciences", 1957, vol. 2.

INFRARED SPECTROSCOPY

The number of absorption bands in the spectrum of IR radiation, their position, width and shape, the magnitude of absorption are determined by the structure and chemical. the composition of the absorbing in-va and depend on its aggregate, temperature, pressure, etc. Therefore, the study of vibrational rotation. and cleanly rotate. spectra by I. s methods. allows you to determine the structure of molecules, their chemical. composition, inertia of molecules, magnitude of forces acting between atoms in a molecule, etc. Due to the uniqueness of the relationship between the structure and its mol. spectrum I. s. widely used for qualities. and quantities. spectral analysis. Changes in the parameters of the IR spectra (shift of absorption bands, changes in their width, shape, absorption value) occurring during the transition from one state of aggregation to another, during dissolution, changes in temperature, pressure, make it possible to judge the magnitude and nature of intermolecular interactions. I. s. also finds application in the study of the structure of PP materials, polymers, biol. objects and directly living cells. High-speed spectrometers make it possible to obtain fractions of s and are used in the study of fast-flowing chem. reactions. The use of special mirror micro attachments makes it possible to obtain absorption spectra of very small objects, which is of interest for biology and mineralogy. I. s. plays an important role in the creation of IR lasers and the study of their emission spectra. The use of tunable frequency IR lasers as radiation sources makes it possible to obtain IR spectra with very high resolution (see LASER SPECTROSCOPY).

Physical Encyclopedic Dictionary. - M.: Soviet Encyclopedia. . 1983 .

INFRARED SPECTROSCOPY

(IR spectroscopy) - section of optical. spectroscopy, including the study, acquisition and application of emission, absorption and reflection spectra in the IR region of the spectrum (see. Infrared radiation). IR spectra are obtained and examined in principle by the same methods as the corresponding spectra in the visible and UV regions, but with the help of special. spectral instruments intended for use in the IR region, usually equipped with mirror focusing optics (see Fig. Spectral instruments) and IR-sensitive receivers (see Receivers of optical radiation). I. s. deals with the arr. the study of molecular spectra, since the majority of vibrations are located in the IR region. and rotate. spectra of molecules. Besides, in And. the radiation spectra of atoms and ions, which arise during transitions between close energy levels (for example, Zeeman sublevels; see Zeeman effect) reflection and absorption spectra of crystals and other solids, a number of molecules, semiconductor and molecular lasers, etc. dissociation of molecules or changes in their structure. Only for sufficiently chemically and thermally stable molecules (usually consisting of a small number of atoms) and stable chemical. radicals (eg, CO, CO 2 , H 2 O, HCl, HF, CN, NO, etc.) possibly emission spectra (such radicals are used as active media in molecular IR lasers). IR spectra of selective reflections apply ch. arr. in the study of the spectra of single crystals, inorganic. solids, minerals, etc., the substance can be examined in decomp. states of aggregation, at various temperatures and pressures, solids in decomp. states. Absorption I. s. makes it possible to obtain absorption spectra of substances that are colored and opaque in the visible region, brightly luminescent substances, etc. Using frequency-tunable IR lasers, absorption spectra are recorded with a significantly higher than in the traditional way. classical methods, resolution. a molecule made up of N atoms, has 3N- 6 vibrations. frequencies of normal vibrations (in the presence of symmetry, some degenerate) and 3 frequencies of rotation. In the IR absorption spectra, only those molecular frequencies are observed at which the dipole moment changes during vibrations, i.e., the derivative of the dipole moment is nonzero R along the corresponding normal coordinate q:R pl R q No. 0 (see Selection rules). Purely rotational IR absorption bands are observed only for polar molecules. Every substance has a certain set of properties. oscillating and rotate. frequencies, so the IR absorption spectrum is an individual characteristic of the Islands. v or wavelength TO. In the classic absorption I. s. from a source with a continuous IR spectrum (Fig. 1)

Rice. 1. Schematic diagram of a single-beam IR spectrometer: Q - source of continuous IR spectrum; M 1 - illuminator mirror; M 2 - condenser mirror; WITH - cuvette with the test substance; M - ; Si and S 2 - input and output slits of the monochromator; D- radiation receiver; A - amplifier; I - measuring or recording device.

through a cuvette with the test substance; The radiation passing through the substance is directed to the entrance slit of the monochromator, and from its exit slit - to the radiation receiver. It is then amplified and measured or recorded by the plotter during the scanning process. In laser I. with. the dependence of the intensity of the radiation of a narrow-band IR laser (more often a semiconductor with a tunable frequency) transmitted through the substance on the frequency of the laser radiation in the process of its tuning is measured. v) passing through a cuvette with a substance of radiation with a wavelength l (or a wave number v(cm~ 1) = 1/l) and the quantities characterizing the absorbing substance is given by the generalized Bouguer-Lambert-Beer law: I( v)=I" 0 ( v)exp3[- k(v)cd], Where k(v) - absorption index characterizing the absorbing substance, With - the concentration of the absorbing substance in solution (c=1 for a pure substance), d- thickness of the absorbing layer of the substance (cuvettes), I 0 ( v)=b( v)I 0 ( v), I 0 ( v) - incident on the cell (perpendicular to its windows), b( v) -coefficient. the transmission of the cell itself, which takes into account reflection losses from the windows of the cell. Usually, the IR absorption spectrum is presented graphically as a function of v(or l) quantities characterizing only the absorbing substance:
coefficient transmission T( v)=I( v)/I" 0 ( v),
coefficient absorption A(v)=/I" 0 ( v)=1-T( v),
optical density D( v)=lnI" 0 ( v)/I 0 ( v)=ln=k( v)cd,
and absorption rate k( v)=D( v)/cd.
Value D(v) is linearly related to k(v) And With, therefore it is usually used in quantities, analysis by absorption spectra. In practice, the Bouguer - Lambert - Vera law is also expressed as: , where e( v)=0,434k(v) - attenuation indicator. In this case D( v)=lgI" 0 ( v)/I(v)=e( v)cd. The Bouguer - Lambert - Vera law is valid at a low intensity of the incident radiation flux, i.e. e. in when the population of energy level changes slightly and T(v)does not depend on I 0 ( v). In addition, the beam is monochromatic. radiation passing through the cuvette must be parallel, and the molecules absorb the radiation independently of each other [i.e. e. k(v) should not depend on With]. The last assumption allows us to generalize this law to the case of a mixture of several absorbing substances: I( v)=I" 0 ( v)l0 -D( v) , Where

- sum optical densities mixture components. This ratio underlies the quantities, absorptions. molecular spectral analysis(however, it does not hold in some real mixtures). Definition T(v) and respectively A(v)And D(v) is reduced to an independent sequential measurement

Rice. 3. Absorption spectrum of liquid indene in the region of 2.5-16 μm. The thicknesses of the cuvettes at which this region of the spectrum was obtained are indicated above.

quantities I( v) and I 0 ( v) and the subsequent definition of I( v)/I 0 ( v)=bT( v). A number of methods have been developed to obtain the b value. Double-beam spectrophotometers directly record the ratio I( v)/I 0 ( v).Basic parameters of the IR absorption spectrum - the number of absorption bands, their position (determined v or l at the absorption maximum), width and shape of the bands, absorption at the maximum. They are determined by chem. the composition and structure of the molecules of the absorbing substance, and also depend on the state of aggregation of the substance, temperature, pressure, nature of the solvent (in the case of solutions), etc. The IR spectra of gaseous substances at low pressures, obtained using high-resolution spectrometers, have a characteristic oscillatory-rotate. structure (Fig. 2) with a large number of narrow rotations. lines (see Molecular Spectra). Width rotating components.

Rice. 2. Absorption spectrum of gaseous methane (CH 4) (rotational-vibrational band in the region l=3.3 μm).

structure is tenths and even hundredths of cm -1 and increases with gas pressure. Oscillatory-rotate. the bands in the spectra of liquids expand and merge into broad, almost structureless bands, the width of which is 5-20 cm -1 (Fig. 3). The width of the bands in the IR spectra of crystals is somewhat smaller than that of liquids, which is associated with an ordered arrangement of particles in the crystal. lattice. vibrations of molecules is associated with great difficulties. However, it fluctuates. absorption bands determined. chem. bonds and groups of atoms, as experience has shown, have similar frequencies, regardless of which molecules they are part of. Characteristic limits frequencies of some chem. bonds and groups of atoms are given in table. Analysis of IR absorption spectra with the help of allows you to decompose complex overlapping absorption bands into separate components, which are then easier to attribute to a definition. types of normal vibrations of molecules.
* in - valence, in a - valence asymmetric, in c - valence symmetric vibrations; e - various forms of deformation oscillations.
(in the region of 50-1000 μm) and especially the absorption spectra of rarefied gases obtained using high-resolution instruments, including tunable lasers, are used to determine the structure of molecules, their moments of inertia and the values of dipole moments, the energy of interatomic interactions , mechanical coefficients. anharmonicity, rotational constants, etc. The characteristic nature of vibration frequencies allows for the analysis of complex organic. compounds and especially unknown compounds. I. s. used for the analysis of isomers (Fig. 4, see isomerism of molecules) for research of a structure of semiconductor materials, polymers, biol. objects and directly living cells. I. s. plays an important role in the development and study of IR molecular lasers. High-speed IR spectrometers allow you to obtain absorption spectra in a fraction of a second and are used in the study of fast-flowing chem. reactions. With the help of special With spectral instruments, it is possible to obtain absorption spectra of very small objects, which is of interest for biology and mineralogy. In the case of strongly absorbing substances, from which it is not possible to create a thin layer, methods are used to obtain IR absorption spectra. frustrated total internal reflection(ATR). To obtain IR absorption spectra, a large number of dec. spectrometers. Spectrometers with prism monochromators make it possible to obtain spectra with a resolution of d v~l-3 cm -1 and are used only to study the spectra of condensed matter. Serial spectrometers with diffraction. monochromators make it possible to obtain spectra with a resolution up to d v~0.2 cm -1 , unique diff. spectrometers - d v~0.02-0.05 cm -1 and are used to study the spectra of rarefied molecular gases. Permission Fourier spectrometers can reach d v~0.005 cm -1 . When using frequency-tunable lasers, the spectral resolution of the IR spectra

Rice. 4. Absorption spectra o-, m - and p-isomers of liquid cresol; the arrows mark the characteristic absorption bands of individual isomers.

absorption is determined by the width of the laser generation line; when using semiconductor lasers, it reaches d v~10 -3 -10 -4 cm -1 , and gas lasers - somewhat higher, although the frequency tuning range is usually small. Some IR spectrometers have a built-in mini-computer, which is used for registration and automatic. processing of IR spectra: determining the frequencies of absorption bands, their intensities, etc. Since the 70s. in I. s. method has become widespread photoacoustic spectroscopy to obtain IR absorption spectra of gases, solids and especially dispersed media. submillimeter spectroscopy). Lit.: Bellamy L., Infrared spectra of molecules, trans. from English, M., 1957; Application of spectroscopy in chemistry, trans. from English, M., 1970; Cross A., Introduction to practical infrared spectroscopy, trans. from English, M., 1961; Applied infrared spectroscopy. [Sat. Art.], ed. D. Kendall, trans. from English, M., 1970; High resolution infrared spectroscopy. Sat. st., trans. from French, English, M., 1972; Malyshev V. I., Introduction to experimental spectroscopy, M., 1979. V. I. Malyshev.

Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-Chief A. M. Prokhorov. 1988 .

Goal of the work: learn to decipher the spectrum of minerals, master the skills of qualitative analysis of minerals.

Instruments and accessories: spectrophotometer, card file of spectra of minerals.

infrared spectroscopy. General concepts

Spectroscopy is the science of the interaction of electromagnetic radiation with matter, which provides information about the substance itself, the atoms and molecules that make up the substance, about its structure and properties. Spectroscopy uses the entire range of electromagnetic radiation, including gamma rays, x-rays, infrared rays, visible and ultraviolet rays, microwave radiation, and radio frequencies. The method of absorption spectroscopy is based on the interaction of electromagnetic radiation with matter.

Depending on the object of study, spectroscopy is divided into atomic and molecular. Atomic spectroscopy studies the structure and properties of atoms, molecular - the structure and properties of molecules. The method of spectroscopy is spectral analysis. Spectral methods of analysis use the ability of atoms and molecules to absorb and emit electromagnetic radiation.

Infrared spectroscopy is a branch of spectroscopy that includes the acquisition and study of infrared spectra. Infrared spectroscopy is mainly concerned with the study of molecular emission, absorption, and reflection spectra, since most of the vibrational and rotational spectra of molecules are located in the infrared region. Infrared spectroscopy is as specific as human fingerprints. A substance can be identified from the spectra if its spectrum is known. The method of infrared spectroscopy makes it possible to determine the state of water in a mineral, the nature of isomorphic impurities, the degree of structural order, the assignment of minerals to a certain structural type, etc.

Main characteristics of electromagnetic radiation

Electromagnetic radiation has the following main parameters: wavelength λ, frequency ν or wave number and the corresponding radiation energy E.

Wavelength is the distance traveled by a wave in one period. The main units for measuring wavelengths in the UV and visible region are nanometers (1 nm = 10 -9 m), in the IR region - micrometers (1 μm = 10 -6 m). The wavelength depends on the refractive index of the medium in which the radiation propagates. The speed of propagation of radiation in different media is different, therefore, to characterize a certain section of the spectrum, frequencies or wave numbers are used that do not depend on the medium.

Radiation frequency n is the number of oscillations in one second; it is equal to the ratio of the propagation speed of radiation (the speed of light c) to the wavelength.

The frequency is measured in reciprocal seconds s -1 or hertz (1Hz = s -1).

wave number shows how many wavelengths fall on 1 cm of the radiation path in vacuum and is determined by the ratio . The dimension of wave numbers is cm -1 . The wave number is related to the radiation frequency: , where с is the speed of light in vacuum (с ≈ 3×10 8 m/s).

Table 3.1

Wavelengths of electromagnetic radiation

Type of radiation	Wavelength range
Gamma radiation
x-ray
ultraviolet

infrared	760 nm - 300 µm
microwave	300 µm - 300 mm
radio waves	From 300 mm to several kilometers

Radiation energy , where h is Planck's constant (h = 6.62 × 10 -31 J × With.). The set of wavelengths (or frequencies) is the emission spectrum. The division of the electromagnetic spectrum into a number of regions (Table 1.1) is not sharp and is mainly associated with the method of obtaining and recording radiation of various wavelengths (or frequencies) and using various optical materials.

Molecular spectra. IR spectroscopy method

The optical spectra of molecules are obtained by changing the three types of internal energy of molecules: the energy of electrons; vibration energy of atoms in a molecule relative to some equilibrium position; energy of rotation of the entire molecule, like a top, around its own axis, that is

E \u003d E el + E to + E time.

Each of these types of internal energy for the molecules of a given substance has its own set of energy levels. The distance between the levels, their number and relative location is completely determined by the structure of the molecules of the substance.

Exciting one or another type of internal energy of molecules, molecular spectra are obtained: rotational; oscillatory; electronic.

To excite the rotational spectrum, a small energy is needed - 0.005 - 0.025 eV, for the vibration of atoms in a molecule - 0.05 - 0.5 eV, for the excitation of electronic spectra - 5 - 10 eV. However, it is not possible to obtain electronic and vibrational spectra in pure form. Simultaneously with the excitation of vibrations of atoms, the speed of rotation of the entire molecule also changes. Therefore, the spectrum turns out to be vibrational-rotational.

Analysis by molecular absorption spectra is based on the use of the Bouguer-Lambert-Beer law.

To obtain absorption spectra, it is necessary to direct the radiation necessary for the excitation of one or another type of internal energy to the substance. Excitation of electronic spectra is carried out by ultraviolet and visible radiation, vibrational spectra require quanta of infrared radiation, rotational - quanta of microwave radiation or far infrared radiation.

In the method of IR spectroscopy, the most widely used is the study of IR absorption spectra arising from the passage of IR radiation through a substance. Each substance has its own vibrational spectrum. The number of absorption bands in the spectrum, width, shape, intensity are determined by the structure and chemical composition of the substance. This makes it possible to conduct qualitative and quantitative analyzes of the substance in all aggregate states using IR spectra.

Qualitative Analysis

To conduct a qualitative analysis of samples by infrared spectra, it is necessary to interpret the infrared spectrum. This requires a combination of experimental data with theoretical calculations. The study of the infrared spectra of substances is currently carried out by two methods: the identification of characteristic frequencies and the comparison of the spectra of complex substances with the spectra of individual compounds.

Method of characteristic frequencies. Molecules that have the same chemical groups often have the same frequencies in the spectrum. These frequencies are called characteristic.

The infrared spectrum is deciphered as follows: the identification of absorption bands begins with the strongest and highest frequency bands in the region of stretching vibrations of the OH bond. According to the tables of characteristic frequencies, the absorption band is assigned to the vibration of a particular bond. The presence of one or another connection is confirmed by the deformation absorption band related to this connection.

comparison method. Identification of an unknown compound by the infrared spectrum is carried out by comparing its spectrum with reference spectra. This requires an extensive file of reference spectra; at the same time, the most important factor is the standardity of the conditions for their registration. At present, there are numerous atlases of organic and inorganic compounds.

Identification of substances by the infrared spectrum is completely reliable only if the spectrum under study exactly matches the spectrum of the standard in position (frequency), shape and relative intensity of all bands, that is, the entire spectral curve.

Work order

I part:

Prepare mineral samples for IR spectra.
Take IR spectra of your chosen minerals or study the spectra of taken minerals.
Decipher the spectra of minerals.
Make the assignment of the bands, starting from the region of stretching vibrations of the OH bond, then bending vibrations. Find the bands responsible for SiO bonds in the spectrum.
Compare the absorption bands you found with the mineral card file and determine the mineral you removed.

II part:

Obtain captured spectra of known minerals.
Decipher the spectra of minerals, find the characteristic frequencies for this mineral.
Make the assignment of the bands.
Make tables of the frequencies you found, for example:

Table of frequencies in the spectrum of muscovite and their assignment

Muscovite

		Interpretation

Table of absorption bands in the IR spectra of minerals

Control questions

How are molecular spectra obtained?
Qualitative analysis using IR spectroscopy.
The Bouguer-Lambert-Beer law.
How is IR quantitative analysis performed?
How are the spectra of minerals and rocks interpreted by the comparison method?
What are characteristic frequencies?
How is the absorption bands identified using characteristic frequencies?
What are the characteristic features of the IR spectra of layered minerals?
How to distinguish muscovite from phlogopite by the stretching vibrations of the OH bond in micas?
How to distinguish hydrated micas from less hydrated micas using the IR spectrum?

FEDERAL STATE BUDGET EDUCATIONAL INSTITUTION

HIGHER PROFESSIONAL EDUCATION

"VORONEZH STATE UNIVERSITY"

(FGBOU VPO "VSU")

Faculty of Pharmacy

Department of Pharmaceutical Chemistry and Pharmaceutical Technology

COURSE WORK

Infrared spectroscopy and its practical application in pharmaceutical analysis

VORONEZH 2014

Introduction

The essence of the method of infrared spectroscopy

IR absorption spectra of organic compounds

2.1 Hydrocarbons

1.1 Saturated hydrocarbons

1.2 Alkenes

1.3 Conjugated hydrocarbons

2 Organic compounds with functional groups

2.1 Organic compounds containing oxygen

Group characteristic frequencies

Spectroscopy in the near infrared region (NIR)

Fourier transform spectroscopy

Methods and techniques for sample preparation in IR spectroscopy

Conclusion

infrared spectroscopy sample pharmaceutical

Introduction

In connection with the expansion of the pharmaceutical market for drugs, more and more accurate and informative methods of analysis are required. The need to use physicochemical methods of analysis, in particular spectral methods, is due to the requirements of the 12th Pharmacopoeia of the Russian Federation. One of the promising methods of analysis is IR spectroscopy.

Spectroscopy is a branch of physics and analytical chemistry devoted to the study of the interaction spectra of radiation (including electromagnetic radiation, acoustic waves, etc.) with matter. In physics, spectroscopic methods are used to study the various properties of these interactions. In analytical chemistry - for the detection and determination of substances by measuring their characteristic spectra, i.e. spectrometry methods.

Infrared (IR) spectroscopy is characterized by a wide information content, which creates the possibility of an objective assessment of the authenticity and quantitative determination of medicinal substances. The IR spectrum unambiguously characterizes the entire structure of the molecule. Differences in chemical structure change the nature of the IR spectrum. Important advantages of IR spectroscopy are specificity, speed of analysis, high sensitivity, objectivity of the results obtained, and the possibility of analyzing a substance in a crystalline state. IR spectroscopy can be used not only to quantify medicinal substances, but also to study such chemical transformations as dissociation, solvolysis, metabolism, polymorphism, etc. .

.The essence of the method of infrared spectroscopy

Infrared spectroscopy is a section of molecular optical spectroscopy that studies the absorption and reflection spectra of electromagnetic radiation in the infrared region, i.e. in the wavelength range from 10 -6before infrared spectrum is a complex curve with a large number of maxima and minima. Absorption bands appear as a result of transitions between the vibrational levels of the ground electronic state of the system under study. The spectral characteristics (the positions of the band maxima, their half-width, intensity) of an individual molecule depend on the masses of its constituent atoms, the geometry of the structure, the features of interatomic forces, charge distribution, etc., therefore, infrared spectra are highly individual, which determines their value in identification and study connection structures.

Quantitative relationship between the intensity I of the radiation transmitted through the substance, the intensity of the incident radiation I 0and quantities characterizing the absorbing substance is based on the Bouguer-Lambert-Beer law: I = I 0e -ccl , i.e., on the dependence of the intensity of the absorption bands on the concentration of the substance in the sample. In this case, the amount of a substance is judged not by individual absorption bands, but by spectral curves as a whole in a wide range of wavelengths. If the number of components is small (4-5), then it is possible to mathematically extract their spectra even with a significant overlap of the latter. The error of quantitative analysis, as a rule, is a fraction of a percent.

The parameters of molecular models are the masses of the atoms that make up the system, bond lengths, bond and torsion angles, potential surface characteristics (force constants, etc.), dipole moments of bonds and their derivatives with respect to bond lengths, etc. Infrared spectroscopy makes it possible to identify spatial and conformational isomers, to study intra- and intermolecular interactions, the nature of chemical bonds, the distribution of charges in molecules, phase transformations, the kinetics of chemical reactions, register short-lived (lifetime up to 10 -6c) particles, clarify individual geometric parameters, obtain data for calculating thermodynamic functions, etc.

Infrared spectroscopy has a number of advantages over spectroscopy in the visible and ultraviolet regions, since it makes it possible to trace the change in all the main types of bonds in the molecules of the studied substances. When using infrared spectroscopy to determine the qualitative and quantitative composition of natural mixtures, there is no destruction of substances, which allows them to be used for subsequent studies. As is known, in infrared spectroscopy, in the range of each chemical grouping of an organic molecule, there corresponds a certain set of absorption bands, which are well studied and listed in the corresponding reference books. At the same time, it should be noted that in the process of removing the infrared spectrum, interference is created at certain wavelengths associated with the absorption of electromagnetic radiation by the bonds of O-H and C-H solvents.

A positive feature of the method of infrared spectroscopy is that the absorption bands of the same type of vibration of the atomic group of various substances are located in a certain range of the infrared spectrum (for example, 3720-3550 cm -1- range of stretching vibrations of -OH groups; 3050-2850 cm -1- groups -CH, -CH 2, -CH 3organic matter). The exact position of the maximum of the absorption band of an atomic group within this range indicates the nature of the substance (for example, a maximum of 3710 cm -1indicates the presence of -OH groups, and a maximum of 3030 cm -1- about the presence of groups =С-Н aromatic structures).

However, if the object under study is not a mechanical mixture, but is a complex chemical compound, then the indicated features of the infrared spectra are not detected.

The absorption of organic substances in different parts of the infrared range is determined by the chemical groups that make up the molecule, or rather the bonds that form them, so the method allows you to determine related substances in total, according to the characteristic absorption zones. Infrared spectroscopy is widely used for the analysis of biological fluids, in particular blood and its fragments, and recently oral fluid or mixed saliva has been increasingly used to diagnose and predict various diseases, however, the interpretation of the results obtained is complicated due to the multicomponent nature of the objects of study.

In medicine, infrared spectroscopy has been used in recent years to determine certain substances in biological fluids: blood, urine, saliva, lacrimal fluid, bile, milk, to identify certain vitamins, hormones, and other biologically active substances. In addition, recently the method has been increasingly used to characterize conformational and structural changes in proteins, lipids, phospholipids of cell biomembranes studied in biopsy specimens, as well as using fiber optic techniques.

This method can be used to evaluate the pharmacokinetics of various drugs. In diabetes mellitus, significantly significant changes in the infrared spectrum of blood were revealed, and the possibility of using infrared spectrum indicators for early diagnosis of dental diseases and predicting dental caries in children was proved. A study was made of rapid changes in the parameters of the infrared spectrum of blood for predicting, diagnosing and determining the severity of osteoporosis and the effectiveness of its treatment. The possibility of using infrared spectroscopy to study regeneration processes has been proved. Infrared spectroscopy is also used in forensic analysis to study the mitochondrial genome for identification and paternity purposes.

.IR absorption spectra of organic compounds

In the study of organic compounds, absorption of infrared radiation in the region of 2-50 μm (5000-200 cm-1 ).

To obtain IR spectra, optical instruments are used, in which heat sources and radiation receivers are used, and halide salts (LiF, CaF2, NaCl, KBr, Csl) serve as the material of prisms. In modern instruments, salt optics have been replaced by diffraction gratings.

When recording IR absorption spectra, the wavelength parameter is microns (µm) or frequencies in reciprocal centimeters (cm -1). Intensity is expressed as a percentage of transmission or absorption, and only in some cases in optical density.

Infrared spectra can be measured for gaseous, liquid and solid compounds. To measure the spectra of gaseous compounds, special gas cells are used. Liquid compounds are applied in the form of a film on plates of a material that is transparent in the area under study (for example, KBr, NaCl, Csl, KCl). From the solids, a suspension is prepared in vaseline oil, which is placed between salt plates. It must be borne in mind that vaseline oil itself is highly absorbent at 3000-2800 cm -1, 1460 and 1380 cm -1. Therefore, to study the absorption of substances in this area, instead of vaseline oil, perhaloid hydrocarbons are used. It is possible to obtain spectra of solids by pressing them with potassium bromide and taking the spectrum of the resulting plate. However, sometimes the substance interacts with potassium bromide, which leads to a distortion of the spectrum. Infrared spectra can also be measured for solutions. Since there are no solvents that are transparent over the entire region of the spectrum, the measurements of the IR spectra of solutions are usually made only for narrow regions. To study aqueous, acidic and alkaline solutions, cuvettes made of water-insoluble materials (fluorite, silicon, germanium, and other materials that are transparent in the IR region) are used. The dependence of the oscillation frequency on the force constant and the atomic mass makes it possible to estimate the position of the absorption bands of individual groups.

The force constants of bending vibrations are significantly less than the force constants of stretching vibrations, so the bands of bending vibrations are located in the region of lower frequencies.

Figure 2 shows the absorption regions of the structural elements of organic compounds.

Rice. 1. Absorption of some solvents at a layer thickness of 0.1 mm

Rice. 2. Absorption regions of some structural elements

In the area of 1400-900 cm -1there are absorption bands corresponding to vibrations of the C-C, C-O, C-N groups, as well as many deformation vibrations. As a result of the strong interaction of these vibrations, it is impossible to assign absorption bands to individual bonds; however, the entire set of absorption bands in this region is an individual characteristic of compounds. This area is therefore called the ?fingerprint area? .

Rice. 3. IR absorption spectra of isomeric hexanes: a - k-hexane; b - 2-methylpentane.

According to the IR spectra in the area? Fingerprints? even isomeric hydrocarbons can be identified, the determination of which in other ways causes great difficulties. The spectra of two hexane isomers shown in Figs. 3 .

1 Hydrocarbons

1.1 Saturated hydrocarbons

The IR spectra of hydrocarbons are characterized by the appearance of absorption bands due to vibrations of the C-C and C-H bonds.

Hydrocarbon bands associated with the characteristic frequencies of C- ? (methyl, methylene and methane groups) are in three regions: 3000-2800, 1400-1300 and about 700 cm-1 .

Absorption in the region of 3000-2800 cm -1due to C-H stretching vibrations. It manifests itself as a complex absorption band, in which the peaks at 2962 and 2872 cm -1belong to vibrations of the methyl group (asymmetric ?AS and symmetrical - ?S), and peaks at 2926 and 2853 cm -1belong to the stretching vibrations of the methylene group (?AS And ?S).

The position of these absorption bands is well preserved in all types of aliphatic hydrocarbons. The intensities of the bands depend on the number of methylene and methyl groups in the hydrocarbon molecule. Methyl group C- ? has a relatively weak absorption band. ?CH about 2890 cm -1, which is overlapped by intense absorption bands of CH2 and CH3 groups.

Absorption in the region of 1400-1300 cm -1and about 700 cm -1due to deformation vibrations of C-H bonds. The methyl group has both symmetric and asymmetric bending vibrations. Band at 1460 cm -1corresponds to the asymmetric bending vibration of methyl groups, band 1380 cm -1- symmetrical vibration. The splitting of this band into a doublet can serve as a sign of the gemdimethyl group. Methylene groups are characterized by four types of deformation vibrations: scissor, fan, torsional and pendulum. The absorption band corresponding to the scissor vibrations of the methylene groups is located at 1467 cm -1. Stripes 1467 cm -1methylene and 1460 cm -1methyl groups are superimposed, and in the spectra of branched hydrocarbons are difficult to distinguish. In normal hydrocarbons at n>5, the methyl group band at 1460 cm -1appears, in the form of a shoulder on a strip of 1467 cm -1. Pendulum vibrations of methylene groups are in the region of 790-720 cm-1. Their position is determined by the length of the carbon chain. So, for C2H5, the frequency of pendulum oscillations is 790-770 cm -1, for C 3H 7she has a value of 743-734 cm -1. For n=4, the corresponding band is observed at 725-720 cm -1(for solid samples, sometimes in the form of a doublet). Pendulum oscillations CH 2-groups can be used to detect polymethylene chains. The bands corresponding to torsional and fan vibrations of methylene groups and bending vibrations of methane groups are located in the region of 1300 cm -1; are rarely used for identification due to their insufficient characteristic and low intensity. The intensities of the absorption bands of bending vibrations of the methylene group increase linearly with an increase in the number of CH groups 2. The absorption bands corresponding to skeletal vibrations are relatively weak and appear in two regions: 1100 - 700 cm -1and below 500 cm -1. The first region is associated with stretching vibrations of the carbon skeleton, the second - with deformation. Experimental studies of a large number of hydrocarbons have shown that in the presence of an isopropyl group in the molecule, bands appear in the spectrum at 1170 and 1145 cm -1, tert-butyl - at 1255 and 1210 cm -1, in the presence of a quaternary carbon atom, absorption is observed at 1215 and 1195 cm -1. These data can only be given as evidence in favor of the presence of these groups, but not as unambiguous evidence.

Deformation vibrations of the carbon skeleton below 500 cm -1, in the IR spectra are practically not studied.

2.1.2 Alkenes

The introduction of a multiple bond into the molecule of an organic compound leads to the appearance of absorption bands that characterize this bond and changes the position of the absorption bands of the groups directly associated with it. In monoolefins with C = C stretching vibrations, an absorption band is associated in the region of 1680-1640 cm -1. This vibration is not strictly valence, since along with ? stretching the C = C bond causes a change in bond angles ?-C \u003d C. In centrosymmetric ethylene compounds, the vibration ?C=C is inactive due to the symmetry prohibition (since symmetric vibrations occur without a change in the dipole moment), in asymmetric molecules the intensity of the absorption band increases as the double bond of the chain finally shifts. The position of the C = C stretching vibration band within the interval 1680-1640 cm -1depends on the degree of substitution at the C = C bond and the geometry of the molecule.

The position of the C=C bond absorption band in cycloalkenes depends on whether it is endo- or exocyclic. For endocyclic frequency ?С=С slightly increases as the angle in the cycle increases, for exocyclic bond С=С frequency ?C=C increases with increasing intensity of the cycles.

The frequency of stretching vibrations \u003d C- ? observed at 3010-3095 cm -1, and the value ?=C-H is determined by the degree of substitution: =CHR is characterized by oscillation with ?=3040-3010 cm -1, for group =GH 2an oscillation appears with a frequency of 3095-3075 cm -1. Bands of plane deformation vibrations of bonds =C- ? are of low intensity, are located in the region of bending vibrations of methylene and methyl groups, and are usually not used for the purposes of structural analysis. On the contrary, the bands of nonplanar deformation vibrations ?-C=C- ? in the spectra of alkenes are very characteristic, located in the region of 1000-800 cm -1and quite intense. Absorption at 970-965 cm -1characteristic of the transisomer.

By absorption in the region of 1000-800 cm -1it is possible to determine with a good degree of certainty the terminal vinyl R-CH =CH 2and the methylene group R 2C=CH 2. The vibration of the rest of the alkene molecule is not significantly affected by the multiple bond.

2.1.3 Conjugated hydrocarbons

The conjugation of two C=C bonds leads to the appearance of two absorption bands in the region of 1650-1600 cm -1. The splitting is explained by mechanical interaction and a change in the forms of normal vibrations. The intensity of the bands is increased compared to the absorption intensity of the corresponding non-conjugated compound. In polyenes, several bands appear in this region, sometimes merging into one broad band, and with an increase in the number of conjugated bonds, the absorption bands shift towards lower frequencies.

Fig.4. Absorption spectrum of clotrimazole

When double bonds are conjugated with an aromatic ring, the shift of the C = C band is usually less than when aliphatic bonds are conjugated. The intensity of the band strongly increases along with an increase in the intensity of the absorption bands of skeletal vibrations of the aromatic ring in the region of 1600-1500 cm-1 .

A particularly strong interaction of vibrations occurs in the case of cumulated bonds; all three carbon atoms of the allene group participate in the vibration, giving two absorption bands: an intense one at about 1950 cm -1(?AS) and weak near 1050 cm-1 (?S) .

2.2 Organic compounds with functional groups

The introduction of functional groups leads to a significant change in the IR spectrum of the corresponding hydrocarbon. As well as for hydrocarbons, a set of bands in the region of 1400-700 cm -1is an individual characteristic of each substance. In addition, bands specific to each functional group appear in the spectrum.

2.2.1 Organic compounds containing oxygen

The most characteristic absorption bands appear in the regions of 3600-3000 cm -1(stretching vibrations of the О-Н group) and 1400-1000 cm -1(vibrations associated with the C-O-H grouping). Stretching vibrations O- ? are characteristic, since they involve a light hydrogen atom. They are observed in a wide frequency range (3600-2500 cm -1), which is associated with the ability of the hydroxyl group to form hydrogen bonds. The formation of a hydrogen bond affects the position and shape of the OH stretching vibration bands.

The free, non-associated hydroxyl group of alcohols and phenols has a narrow absorption band in the region of 3670-3580 cm -1. This band is usually observed in dilute solutions of hydroxyl-containing compounds in inert solvents. The identification of the bands of the free OH group is not difficult, since the other fundamental vibrations do not give bands in this region, and the intensity of the overtones is much lower.

Fig.5. Ethanol absorption spectrum

The participation of the hydroxyl group in the formation of intermolecular hydrogen bonds is manifested in the shift of the absorption band towards lower frequencies and a significant increase in its intensity. The formation of hydrogen bonds between alcohol molecules leads to the appearance of dimers and polyassociates. Dimers are characterized by the appearance of a sharp absorption band in the region of 3550-3450 cm -1in the spectrum of polyassociates, a wide band is observed in the region of 3400-3200 cm -1. A characteristic feature of intermolecular hydrogen bonds is a change in the nature of the spectrum in the region of 3600-3200 cm -1with a change in the concentration of a hydroxyl-containing compound in an inert solvent (Fig. 6): at low concentrations of a substance, the spectrum has a narrow absorption band corresponding to a free hydroxyl group (Fig. 6a). An increase in concentration leads to the appearance of dimers and polyassociates, and in the spectrum, along with the band of the free OH group, absorption appears in the longer wavelength region (Fig. 6b). A further increase in concentration is accompanied by an increase in the absorption intensity of the band of the bound OH group and a decrease in the intensity of the band of the free hydroxyl group (Fig. 6c, d).

Rice. Fig. 6. IR absorption spectra of a hydroxyl-containing compound at an alcohol concentration in CCl4: a - 0.01 M; b - 0.1 M; c - 0.2 M; g-1.0 ?

The formation of intermolecular hydrogen bonds with polar compounds, such as ethers, ketones, amines, etc., is accompanied by a band shift ?OH in the area of 3550-3450 cm -1. At the same time, a slight shift of the absorption bands of the electron donor group (10–20 cm -1) to the low frequency region. The participation of the OH group in the intramolecular hydrogen bond leads to the appearance of a narrow absorption band in the region of 3590-3420 cm -1. The hydrogen bond of the chelate type appears as a very wide blurred absorption band in the region of 3200-2500 cm -1. In contrast to intermolecular hydrogen bonds, the nature of the absorption of compounds with intramolecular hydrogen bonds in inert solvents does not depend on concentration.

Thus, the IR spectra in the region of 3600-3000 cm -1make it possible to study hydrogen bonds in organic compounds. The study of the dependence of the position and intensity of the absorption bands in this region on the concentration of the hydroxyl-containing compound makes it possible to determine the nature of the hydrogen bond. The presence of a polar C-O bond causes the appearance of an intense absorption band in the range of 1200-1000 cm -1due to the participation of this group in skeletal vibrations. In addition, in the region of 1400-1250 cm -1intense absorption bands appear, which are associated with planar bending vibrations of the OH group. There are works in the literature, according to which it is possible to distinguish between primary, secondary, tertiary alcohols and phenols by the position of the absorption bands in the region of 1400-1000 cm 1. However, for the purposes of structural analysis, these data should be used with caution. Ethers. Vibrations of the C-O-C group of ethers are not characteristic. However, in the region of 1200-1000 cm -1ethers have an intense absorption band associated with the participation of the polar C-O-C bond in the vibration. The position of this band is not constant, it depends on the structure of the ether: for example, in alicyclic ethers this band is located in the region of 1150-1060 cm -1, and for aromatic and unsaturated esters it is observed in the region of 1270-1200 cm -1. An ether bond cannot be unambiguously proven from IR absorption spectra. The introduction of oxygen affects the position of the absorption bands of the methyl and methylene groups directly attached to the oxygen atoms. Thus, the band of symmetric stretching vibrations of the methyl group of aliphatic esters (ROCH 3) shifts to 2830-2815 cm -1. In aromatic ethers (ArOCH 3) absorption bands of the CH3 group are observed at 2850 cm -1. Stretching vibrations C- ?-bonds at the epoxy ring appear at 3050-2990 cm -1, CH fluctuations in ?,?-unsaturated ethers - at 3150-3050 cm-1 .

2.2 Organophosphorus compounds

In the spectra of phosphines, sharp absorption bands of medium intensity are observed in the region of 2440-2350 cm -1, due to stretching vibrations of the bond ?-?. Phosphine itself absorbs at 2327 and 2421 cm -1. Group ?-? does not participate noticeably in the formation of hydrogen bonds. Group ?-Ag is characterized by absorption bands in the intervals 1450-1435 and 1005-995 cm -1.The oscillation associated with the P = O group appears in the region of 1350-1175 cm -1. This frequency is reduced by 50-80 cm -1if the P = O group participates in the formation of hydrogen bonds. In phosphorus compounds containing the ROOH group, the absorption band ?OH manifests itself in the region of 2700-2560 cm -1(wide and unsharp). with grouping ?-O-Ar bind absorption bands in the region 1240-1190 cm -1and a less intense band around 1030 cm -1. grouping ?-O-Alk causes absorption in the range of 1050-995 cm -1. In the area of 970-930 cm -1a band may appear due to the oscillation of the group ?-O-R. P = S bonds cause absorption in the 800-600 cm region -1. Phosphorus compounds are characterized by the appearance of intense absorption at 980 cm -1, the nature of which has not been elucidated.

Fig.7. Absorption spectrum of thiamine hydrochloride

2.3 Infrared spectra of polymers

A polymer is made up of macromolecules that are built from an integer number of relatively small repeating units. The molecular weight of the polymer is typically between 10 3to 10 6a.u. The content of many atoms in a polymer molecule leads to a significant number of normal vibrations, so one would expect a complex form of the IR spectrum of the polymer. However, most polymers are characterized by relatively simple spectra, which is due to the following reasons. In real polymers, each repeating structural unit (unit cell) is characterized by certain normal vibrations, which can change as a result of the interaction between neighboring structural units. If there is little or no interaction at all, their normal vibrations have similar frequencies and appear in the spectrum as a single absorption band (degenerate vibrations). Strong interaction leads to a change in the frequencies of normal vibrations of structural units, which is expressed in the splitting of the absorption band in the spectrum. In the spectrum of a polymer chain consisting of N linked repeating units, each band will split into N components. The amount of band splitting depends on the strength of the interaction of structural units. The interaction of these units in the case of characteristic vibrations of individual functional groups is usually small, but it turns out to be significant for skeletal vibrations. As a result of averaging close vibration frequencies, individual peaks in the absorption bands disappear, and the spectrum becomes simpler.

Another reason for simplifying the spectra is the existence of strict selection rules that determine the activity of molecular vibrations. According to the selection rules in the infrared region, only those normal vibrations are active that are associated with a change in the electric moment of the system, and the selection rules do not depend on force constants, but only on the type of symmetry. In molecules that do not have symmetry, all normal vibrations are active.

The task of qualitative analysis of polymers by IR spectra is to determine at least the main components when their quantity and concentration in the mixture are unknown.

A necessary condition for the applicability of the method is the presence of mutually non-overlapping sections in the spectra of the substances that form the mixture. For a polymer sample that does not contain impurities, the IR method can establish with a high degree of reliability not only the main structure of the polymer chain, but even its minor details, due to the method of production or processing. For example, individual spectra of plasticizers, antioxidants, and other additives are used to exclude non-polymer absorption bands from the spectrum.

Along with the qualitative determination of the structure of molecules, IR spectroscopy makes it possible to obtain quantitative data on the content of certain groups of atoms, bonds, and their combination in a molecule. After determining the quantitative content of the molecular structures included in the polymer, its composition is found. However, compared with other spectroscopic methods (electron and radio spectroscopy), in some cases, IR has lower sensitivity and accuracy, which excludes the possibility of determining a low content of functional groups.

Quantitative analysis of the composition of polymers is based on the fact that each of the analyzed monomeric groups has its own specific chemical structure and, consequently, its own characteristic absorption bands. All substances, except for optical isomers, have different IR spectra. Which are often called "fingerprints" of the molecule. The ICS method is almost universal in its capabilities; samples may be liquid, solid, gaseous, colorless or colored. The most reliable information can be obtained for high molecular weight samples of a regular structure with a linear chain configuration, when the contribution of end groups, anomalous links, and irregularities of other types is minimal. In cases where the task of establishing the mutual arrangement of atoms in a molecule is not posed, but it is required to determine only the quantitative content of the corresponding groups, the question is reduced to choosing an analytical absorption band, measuring its intensity, and comparing the latter with the intensity of the same band in a reference compound. As far as possible, an isolated absorption band is chosen as an analytical one; the presence of a crust in the spectrum is due to the presence of the desired combination of atoms in the sample under study. When obtaining polymers, processing, predicting their performance in products, it is necessary to control the structure of polymer molecules, which in most cases can be solved by spectroscopy methods. For example, IR spectra can be used to judge the relative content of one or another type of unsaturated groups in polyolefins obtained by different polymerization methods, the branching of their macromolecules, and the structure of the polymer chain. Thus, the difference in the structure of the molecules of 1,4-trans- and 1,4-cis-polybutadienes is reflected in the IR spectrum (Fig. 5), and allows obtaining information about the structure of rubber.

Rice. Fig. 8. IR absorption spectra of polybutadiene with a predominance of 1,4 - cis - (1) and 1,4 - trans isomer (2)

3. Infrared radiation and vibrations of molecules

The IR absorption spectrum is a unique physical property of its kind. There are no two compounds, except for optical isomers, with different structures but identical IR spectra. In some cases, such as polymers with similar molecular weights, the differences may not be very noticeable, but they are always there. In most cases, the IR spectrum is a "fingerprint" of a molecule, which is easily distinguished from the spectra of other molecules. In addition to the fact that absorption is characteristic of individual groups of atoms, its intensity is directly proportional to their concentration. Thus, measuring the absorption intensity gives, after simple calculations, the amount This method is almost universal in its capabilities.Samples can be liquid, solid or gaseous.They can be organic or inorganic, although inorganic substances sometimes do not give well-defined spectra.Under normal conditions, only monatomic gases are transparent to IR radiation and non-polar molecules (Ne, He, O 2, N 2, H 2). Another limitation is that such a common solvent as water has a very strong absorption in the IR region and, in addition, dissolves cell windows, which are used as plates of salt crystals. The IR spectroscopy method is usually not very sensitive to impurities, if they do not exceed 1%. This, of course, can be both a boon and a disaster, it all depends on the point of view and the problem being solved. Similarly, the fact that the positions of the characteristic absorption bands for many groups are different from one molecule to another may be distressing, but this confirms the individuality of the absorption spectrum and gives more insight into the structure of the molecule than if the bands were unchanged.

Infrared radiation is called radiation with wavelengths from 0.5 to 1000 microns. In the IR range, transitions between the vibrational and rotational energy levels of molecules appear. Chemical bonds in molecules experience oscillatory motions. The vibrational energy of molecules is quantized, that is, the absorbed energy does not change continuously, but abruptly. As a result, the vibrational (infrared) spectrum of a molecule is a series of peaks (absorption bands) corresponding to different vibrational energy transitions. Most vibrational transitions in molecules of organic compounds occur in the wavelength range ? from 2.5 to 25 µm. In units of wave numbers ? = 1/? (c m -1), values reciprocal to wavelengths, this interval is 4000-400cm -1. It is in this range of wave numbers that the IR spectra of organic and natural compounds are recorded.

4. Characteristic frequencies of groups

The concept of "characteristic group frequency" summarizes in short the empirical observation that certain groups of atoms - functional groups in organic compounds - lead to the appearance of absorption bands with characteristic frequencies. These groups behave, as it were, in isolation and independently from the rest of the molecule, since their absorption frequencies change little when passing from one compound to another. In a simple diatomic X-Y molecule, the frequency of the stretching vibration depends on the strength of the internuclear bond and on the masses of both atoms. Therefore, in polyatomic molecules, the appearance of a characteristic frequency (X-Y) is possible provided that this band belongs to the vibrational motion belonging only to this X-Y grouping. But this condition is not fulfilled exactly, since when the fundamental vibration is excited, all atoms must be in motion, even if it is insignificant. In fact, the remainder of the molecule affects to a small extent (usually about 5%) the position of the characteristic frequency of the group, and the direction and magnitude of the frequency shift are of great importance for structural analysis, since they often allow attributing absorption to any particular variant of the functional group. So, the carbonyl group absorbs, as it was found, in the region of 1820-1620 cm -1, but saturated and unsaturated acyclic ketones specifically absorb around 1720 and 1680 cm -1respectively. This kind of "second order" effect, due to intramolecular factors, bears a close resemblance to the "chemical shift" of nuclear magnetic resonance exhibited by methyl and methylene groups. The infrared spectrum of a compound also depends on the physical state. These influences are usually small, but they can become large if significant intramolecular forces appear, such as hydrogen bonding; in this case, the correct assignment of group frequencies requires increased attention.

Characteristic absorption frequencies of some groups of atoms.

4.1 Application of group characteristic frequencies

Ideally, each functional group should have a characteristic region of frequencies and intensities, and to some extent this actually happens. For example, the main vibrations of the unbound hydroxyl group and the secondary amine (OH) and (NH) absorb quite distinctly at ~3610 and 3400 cm -1respectively; groups with a triple bond can be detected by the appearance of their valence absorption in the region of 2100 cm -1, and although these bands are sometimes very low intensity, they are still considered characteristic since most organic matter is relatively transparent between 2700 and 1800 cm -1. In practice, bands above 1500 cm -1are very easily assigned to functional groups such as OH, NH, C=O and C=C, but below this frequency the spectrum is more complex and defines the individual molecule rather than its functional groups. Therefore, when interpreting the infrared spectrum, it is convenient to first consider the region of "functional groups" (above 1500 cm -1) and then the so-called "fingerprint" area (below 1500 cm -1). This division is not absolute, since a significant number of important characteristic frequencies actually appear below 1500 cm. -1, it just serves as a first approximation. The complexity of the "fingerprint" region results from the intense vibrational interaction that takes place between adjacent C-C, C-N and C-O bonds in the molecule, and as a result, these numerous "skeletal" vibrations represent the movement of the entire molecular fragment and cannot be assigned to specific structural units. On the other hand, the characteristic nature of the frequencies of functional groups is due to the absence of interaction between vibrations localized in a given particular grouping and vibrations of the rest of the molecule. This means that the vibrational motion responsible for the absorption of light with a characteristic frequency refers mainly to the atoms of the group under consideration. This situation can arise in two cases: first, whenever a light atom, such as hydrogen or deuterium, is attached to a much heavier atom, such as carbon; secondly, always when one bond in a molecule is much stronger than adjacent bonds.

5. Registration of infrared spectra

Modern spectrometers make it possible to record the IR spectra of gaseous, liquid, and solid samples. To obtain the IR spectrum of an organic or natural compound, only 1 to 10 mg of the substance is needed. Registration of IR spectra is carried out in cuvettes made of potassium bromide KBr or sodium chloride NaCl - materials that do not absorb IR radiation in the studied range. It is customary to write IR spectra as a dependence of the IR radiation transmission (%) on the wave number ? = 1/? (cm -1). Therefore, the peak maxima corresponding to the highest absorption of IR radiation are directed downwards. In most cases, the IR spectra of organic and natural compounds are recorded either in the form of solutions of substances in chloroform CHCl 3, carbon tetrachloride CCl 4, carbon disulfide CS 2, or in the form of solid transparent tablets obtained by pressing under pressure a finely ground mixture of a substance with potassium bromide. Sometimes they use the method of shooting the IR spectrum of a substance in the form of a finely ground suspension in vaseline or mineral oil. In the case of recording IR spectra of compounds in solutions or suspensions, it is necessary to subtract the absorption bands of solvents or a suspending medium. When interpreting the IR spectra of substances obtained in solutions in CHCl 3and CCl 4, it should be taken into account that in the intrinsic absorption zones of these solvents, the assignment of spectral lines can be ambiguous. When registering the IR spectra of organic and natural compounds, absorption lines of impurities in the samples are often observed. This is usually a water signal around 3450 cm -1, fluctuations in carbon dioxide (as impurities from the atmosphere) at 2360-2325 cm -1. Sometimes samples are contaminated with silicone lubricants that have streaks at 1625 cm. -1and 1100-1000 cm -1, or phthalates, appearing as a peak at 1725 cm -1. Be aware that IR cuvettes made from KBr and NaCl are sensitive to traces of water and become cloudy and fail over time. Therefore, samples and solvents must be thoroughly dried before taking IR spectra.

6. Spectroscopy in the near infrared region (NIR)

Spectrometry in the near infrared region (NIR spectrometry, English NIR) is a method based on the ability of substances to absorb electromagnetic radiation in the wavelength range from 780 to 2500 nm (from 12500 to 4000 cm-1 ).

Absorption in the NIR range is associated, as a rule, with the overtones of the fundamental vibrational frequencies of the C-H, N-H, O-H and S-H bonds and their combinations. The most informative range is the region from 1700 to 2500 nm (from 6000 to 4000 cm-1 ) .

The analysis of information extracted from the NIR spectra is carried out using chemometric algorithms that require the creation of a primary data array. As part of the applicability of the method, NIR spectrometry allows you to directly or indirectly conduct a qualitative and quantitative assessment of the chemical, physical and physico-chemical characteristics of the analyzed object, including the evaluation of the following characteristics:

hydroxyl and iodine number, degree of hydroxylation;

crystal form and degree of crystallinity;

polymorphic form or pseudopolymorphic form;

degree of dispersion of particles and others.

NIR spectrometry has the following capabilities:

ease of sample preparation or no preparation;

measurement speed;

non-destructive nature of the analysis;

possibility of simultaneous evaluation of several parameters (indicators);

the possibility of remote control, including in process flows in real time.

Devices. Both specialized NIR spectrophotometers and other spectrophotometers capable of operating in the near IR region of the spectrum are used.

NIR spectrophotometers consist of:

radiation source, for example, a quartz lamp (incandescent lamp) or its equivalent;

monochromator (diffraction grating, prism, optical-acoustic filter) or interferometer (spectrophotometers with Fourier transform);

a recording device - a detector (based on silicon, lead sulfide, indium arsenide, indium-gallium arsenide, mercury telluride, cadmium, deuterated triglycine sulfate, etc.);

sample placement device and/or remote fiber optic sensor.

Samples are placed in glass or quartz cuvettes, vials, glass beakers, capsule or tablet holders, and other devices. Spectrophotometers can be equipped with a cuvette compartment, an integrating sphere (an integrating sphere is an optical component consisting of a spherical cavity coated with a highly reflective material, the sphere is designed to obtain spectra of inhomogeneous samples), external modules for measuring the transmission of highly scattering samples, automatic sample feeders , fiber optic probes. The choice of one or another device for analysis depends on the type of sample and the chosen method of measurement. Therefore, devices that implement several approaches to measurement are recommended for use. Data processing and analysis of the obtained results are carried out using special software. Each measurement mode (transmission, diffuse reflection, and their combination) must have its own verification procedure, including verification of the correct wavelength setting and photometric noise verification.

Checking the correct setting of the wavelengths. To check the correctness of the wavelength setting, the spectrum of a standard sample is recorded, which has characteristic absorption maxima and minima, and the obtained wavelength values are compared with the declared characteristics. For transmission and reflection modes, to determine the correct setting of wavelengths, it is most common to use oxides of rare earth elements, water vapor in the atmosphere, methylene chloride, and others as standard samples. In devices with a Fourier transform, the scale of wave numbers is linear over the entire operating range, and to check the accuracy of the installation, it is sufficient to use one standard sample with the control of the declared characteristics by one absorption band. Instruments of other types may have a non-linear nature of the wave number scale and require verification of the declared metrological characteristics for at least three peaks (one or more standard samples) covering the entire operating range. The error in setting the wavelengths should be no more than ±1 nm (or its equivalent value of the wave number) in the wavelength range up to 1900 nm and no more than ±1.5 nm for the wavelength range?1900 nm.

The reproducibility of the wavelength setting must comply with the requirements of the manufacturer or the requirements of regulatory documents in force on the territory of the Russian Federation.

Checking photometric linearity. To check photometric linearity, the NIR spectra of standard samples with known transmission/reflection values are recorded and a graphical dependence of the obtained transmission/reflection values on known values is plotted. The result of constructing such a dependence should be a straight line with an intersection at the center of coordinates (0.00 ± 0.05) and a tangent of the slope of the straight line (1.00 ± 0.05). To check the photometric linearity in the reflection mode, polymers doped with carbon or analogues in the amount of at least 4 samples in the range of reflection values of 10-90% are used as standard samples. To check the photometric linearity in the transmission mode, filters in the amount of 3 samples with transmission values of 10-90% and a 100% transmission line are used as standard samples (the transmission spectrum of an empty channel is recorded). Checking photometric noise. To estimate photometric noise when measuring transmission, a 100% air line is recorded; when measuring reflectance, record a line of 100% using suitable standard samples with a reflectivity of at least 99%. In this case, the 100% line means a measurement in which the standard sample is the measured sample and the background at the same time. At high absorption values, the photometric noise is evaluated using standard samples with transmission or reflectance values of about 10%. The photometric noise must be in accordance with the manufacturer's specification.

Measurement methods. The NIR spectrum is the dependence of the corresponding photometric quantity (optical density (A), transmission (T), reflectance (R) and derived quantities) on the wavelength or frequency of the radiation. When measuring in the NIR region, the following methods are implemented:

measurement of absorption (or transmission) during the passage of radiation through the sample;

measurement of radiation reflected or scattered from the sample;

a combination of the above methods.

Measurements are always carried out relative to the background.

Transmission measurement. Transmission is a measure of the reduction in intensity of radiation as it passes through a sample. This principle is implemented in most used spectrophotometers, and the result can be presented directly in units of transmission (T) and/or optical density (A). The spectrum of air or reference medium is used as a background. The method is applicable to solid and liquid samples, including dispersed systems. As a rule, special preparation of samples for transmission measurements is not required. To measure the spectrum of liquid samples, use vials or cuvettes with a suitable optical path length (typically 0.5-22 mm), as well as fiber optic transmission sensors. diffuse reflection. The diffuse reflectance method measures the reflectance (R), which is the ratio of the intensity of the light reflected from the sample (I) to the intensity of the light reflected from the background, or the reciprocal logarithmic value of this ratio (AR).

A high R-value surface is used as a background: plates of gold, perfluorinated saturated polymers, ceramic plates and other suitable materials. The method is used to analyze solid samples using an integrating sphere or fiber optic sensors operating in reflection mode. In the latter case, for the reproducibility of the results obtained, it is necessary to ensure the stability of the measurement conditions, in particular, the relative immobility of the sensor, the degree of pressure, and other conditions. Transmission-reflection method. This method is a combination of transmission and reflection due to the special design of the cuvettes and sensors, in which the radiation passes through the sample twice, which allows the analysis of samples with low absorbing and scattering power.

The spectrum of air or reference medium is used as a background.

The method is applicable to liquid, including inhomogeneous samples.

To record the spectrum, the test sample is placed in a cuvette with a mirror or other diffuse reflector. It is possible to use a fiber optic sensor that is immersed in the sample.

Factors influencing measurement results.

Sample temperature. The temperature of a sample can affect both its transmission and its reflection. Temperature control is important in the analysis of thermally labile objects, in which a difference of several degrees can lead to significant spectral changes: solid samples containing water, dispersed systems, amorphous objects, and so on.

Moisture and residual solvents. The presence of water and residual solvents can affect the nature of the spectrum and the results of the analysis. The need and conditions for drying should be specified in monographs.

The thickness of the sample determines the degree of transmission. With an increase in the layer thickness, an increase in absorption is observed. Therefore, in comparative measurements of transmission, the thickness of the sample must be the same or taken into account. When measuring reflection, the layer thickness is not of fundamental importance, but it must be taken into account that the layer thickness must be comparable to the depth of penetration of the beam into the sample. In case of insufficient thickness, an additional reflective material is placed behind the sample, for example, a stamp with a gold coating.

Optical properties of the sample. When analyzing solid samples, it is necessary to ensure that the sample is as homogeneous as possible, since differences in particle density or particle size affect the nature of the spectrum. The spectra of physically, chemically or optically heterogeneous samples should be recorded either with an increased beam size or using devices that rotate the samples during measurements. In this case, it is desirable to measure each sample several times with subsequent averaging of the spectra.

Polymorphism. Differences in the crystal structure (polymorphism) affect the spectrum, which makes it possible to distinguish crystalline or amorphous forms from each other based on their NIR spectrum. When conducting an analysis, it is necessary to take into account the reference spectrum, which crystal structure (modification) is used in the analysis method.

Sample age. Sample properties can change over time, and these changes can cause spectral differences for the same samples. These changes should be taken into account when building calibration models, both for identification purposes and for quantitative analysis purposes.

Identification.

Identification in NIR spectrometry is based on the principle of identical spectra of the same substance. To carry out identification, a library of reference spectra is initially created (hereinafter referred to as the "library"), an optimal mathematical model is selected for processing the spectra and implementing algorithms for their comparison, that is, an identification method is created. Next, the library is validated in conjunction with the selected mathematical model (see the section "Validation of the identification method"). Identification is carried out by comparing the spectrum of the test sample with the spectra in the library (see section "Data Analysis"). Creation of spectrum library. The library contains collections of spectra that carry characteristic information about each object of analysis. For each set of spectra, the optimal identification parameters are determined using appropriate methods and algorithms. These settings are valid for the entire library. For nearby objects that are indistinguishable at given settings, sublibraries are created in which other methods of spectrum preprocessing and analysis algorithms can be used. The number of spectra in the library is not limited, but with a large number of them, the identification of chemically similar substances is difficult. The library includes spectra of substances that meet the requirements, the authenticity of which is confirmed by other pharmacopeial methods. To take into account possible variations in the properties of each type of analyzed objects, the spectra of several batches (series) are recorded. The registration of the spectra is carried out under the similarity of the conditions of measurements and primary processing, which are pre-optimized for all analyzed objects and remain constant during subsequent measurements. The library does not include spectra that are random outliers. Spectra preprocessing methods. Preliminary processing of the spectra is recommended in order to increase the information content of the obtained results and reduce the effect of spectral variations. Primary data processing may include calculation of the first or second derivative, vector normalization, multiplicative scatter correction, and other methods, including combined ones. It should be taken into account that mathematical processing can lead to the loss of information or the appearance of errors-artifacts. The choice of a mathematical model and algorithms must be justified.

Data analysis. Comparison of the spectra of the tested samples during identification is carried out with individual or averaged spectra in the library, for example, by means of correlation analysis. The identification method must be validated. The validation of an identification method is intended to demonstrate its suitability for the intended analysis. Method validation involves verification of indicators of specificity and stability. Specificity indicates that each object whose spectrum is contained in the library will be positively identified and will be different from other objects, while objects not included in the library are negatively identified. Stability indicates that slight changes in conditions (eg temperature, air humidity, vibrations, sample temperature, degree of compaction of the material, immersion depth of the probe, layer thickness, etc.) do not affect the results and reliability of identification.

Quantitative analysis.

Method development (calibration). When developing a technique for quantitative analysis (calibration), changes in the intensity of absorption or reflection in the spectrum are correlated with changes in the properties and/or composition of substances. At the same time, spectra of samples with known values of their chemical composition and/or their properties, confirmed by other pharmacopoeial methods, are recorded. Since chemometric algorithms do not allow extrapolations, it is necessary that the calibration concentration range be larger than the expected range of analyzed concentrations or properties. Calibration samples, if possible, should be evenly distributed within the range of working concentrations. The registration of the spectra is carried out while observing the experimental parameters, factors affecting the results of measurements and primary processing, which are pre-optimized for all analyzed objects and remain constant during subsequent measurements. The library does not include spectra that are random outliers. The calibration model is optimized using an appropriate spectrum preprocessing technique, spectral domain selection, and a mathematical algorithm.

Spectra preprocessing methods. Carry out in the same way as described in the "Identification" section. The choice of a mathematical model and algorithms must be justified.

Data analysis. Any reasonable mathematical algorithm can be used for calibration. Since a strong overlap of absorption bands is observed in the near-IR range, quantitative analysis is carried out with the implementation of predominantly chemometric algorithms, for example, such as the partial least squares method (PLS), the principal component regression method (PCR). ) and others.

Validation of the calibration model. Validation of a calibration model involves demonstrating its suitability for a given task. In this case, such indicators as specificity (selectivity), linearity, working range of concentrations (analytical area), correctness, precision and stability should be evaluated.

To demonstrate specificity, there are the following approaches:

the selected spectral range or absorption band is related to the analyzed property of the object (for example, concentration, moisture content, etc.) and correlates with the photometric value;

demonstrates that changes in the composition of the placebo within the working range of concentrations do not significantly affect the results of the quantitative measurement of the analyzed sample;

other reasonable approaches are allowed.

During the linearity validation, it should be shown that the results obtained by the NIR method with the implementation of the selected processing algorithm are comparable with the results obtained by another standard method. The coefficient of determination (r2), the correlation coefficient (r) or another criterion that determines the suitability of the calibration method can be chosen as an acceptance criterion. The working concentration range defines the interval in which the declared validation parameters are observed. Test results outside this range are not acceptable. The correctness of the methodology should prove the absence of a significant systematic error or the validity of the correction factors introduced into the calculations, if any. Correctness is assessed by comparing the results obtained using the calibration model and the results obtained by the standard method. Precision describes the degree of scatter in the results of an analysis. In this case, both intralaboratory and interlaboratory precision should be evaluated. The robustness of the quantitation procedure indicates that slight changes in conditions do not affect the results of the quantitation.

Emissions. When analyzing the NIR method, one should take into account, correct or reasonably exclude outlier results both within the operating range and outside this range. Outliers that are within the operating range are subject to further investigation and, if informative, may be included in the model. Emissions outside the calibration range are also subject to analysis. If the obtained results are confirmed by the standard method, the spectra of such samples can be entered into the calibration model with subsequent revalidation of the method.

Revalidation. An NIR identification or quantification method that has been validated and found suitable for use needs subsequent periodic validation (revalidation). When deviations are established, the method must be corrected. The need for revalidation depends on the nature of the changes.

The NIR method needs to be revalidated if:

a new object is added to the library (for qualitative analysis);

there are prerequisites for changing the characteristics of objects whose spectra are already included in the library (changing the production technology

(synthesis), composition, quality of packaging feedstock, etc.);

other changes and/or inconsistencies were found in the properties of the analyzed objects or the methodology.

Method transfer. When transferring identification and quantitative analysis techniques from one instrument to another, the spectral characteristics of the spectrophotometers used (resolution, wavenumber range, etc.) should be taken into account. Spectrophotometers with high photometric and wavenumber accuracy (for example, Fourier spectrophotometers) allow direct transfer of both qualitative and quantitative analysis methods without additional manipulations. If direct transfer is not possible, various mathematical methods are applicable to transfer models. After transferring methods, they need to be revalidated.

Data storage. Data storage is carried out in electronic form in accordance with the requirements of the software. At the same time, it is necessary to save the original spectra that have not been subjected to mathematical processing for the purpose of their possible further use in optimizing libraries or methods.

A number of studies demonstrate the wide possibilities of NIR spectroscopy in relation to the identification of pharmaceutical substances and drugs. Moreover, in some cases, with the appropriate calibration of the methods, it is also possible to establish the origin (manufacturer) of drugs. One of the main advantages of this method is that there is practically no sample preparation. Moreover, in some cases, substances and preparations can be scanned through the packaging. Diffuse reflection spectroscopy in the near-IR region of the spectrum is a unique physical method that makes it possible to determine a significant number of parameters in products with a complex chemical structure. Instruments based on this method (IR analyzers) and representing a new generation of spectrometers are the most promising environmentally friendly instruments for express determination of a wide range of quality indicators of food and agricultural products and materials. Weak absorption in the near infrared region and the use of diffuse reflection from the analyzed sample makes it possible to directly analyze the product, which virtually eliminates complex sample preparation and significantly increases the measured concentrations.

7. Fourier transform spectroscopy

Fourier spectroscopy is an optical spectroscopy method that makes it possible to obtain a spectrum as a result of the inverse Fourier transform of the interferogram of the studied radiation, which depends on the optical path difference of the two beams and is the Fourier image of the spectrum (radiation energy distribution function over frequency).

The complex of equipment that performs these operations is called a Fourier spectrometer (FS). As a rule, it includes, in addition to a two-beam interferometer, an illuminator, a radiation detector, an amplifier, an analog-to-digital converter, and a computer.

The interferometer contains two mutually perpendicular mirrors - fixed and movable, and a semitransparent beam-splitting plate located at the intersection of the incident radiation beams and the beams reflected from both mirrors. The radiation beam from the source, falling on the plate, is divided into two. One of them is directed to a fixed mirror, the second - to a movable mirror; then both beams, reflected from the mirrors, exit the interferometer through the beam splitter in the same direction. Next, the radiation is focused on the sample and enters the radiation detector. Two beams differ from each other by the optical path difference, the value of which varies depending on the position of the movable mirror. As a result of beam interference, the intensity of the resulting light flux periodically changes (modulates). The modulation frequency depends on the frequency of the incident radiation and the displacement of the movable mirror.

Being much more complicated than conventional spectrometers, Fourier spectrometers have a number of advantages over other spectral instruments.

With the help of FS, it is possible to register the entire spectrum simultaneously. Due to the fact that an input aperture larger than the slit of spectral instruments with a dispersive element of the same resolution is acceptable in an interferometer, Fourier spectrometers have a gain in luminosity compared to them, which makes it possible to: reduce the time of recording spectra; increase the signal-to-noise ratio (the so-called "Felgett advantage"); increase resolution; reduce the size of the device.

All IR - spectrophotometers, regardless of design, have common elements: radiation source, optical system, receiver, signal amplification system.

Sources of radiation. An ideal source for IR spectroscopy would be a high-intensity monochromatic emitter, continuously tunable over a wide frequency range. Despite the fact that there are frequency-tunable lasers, currently the most common are sources heated to a temperature of 1200 - 1400 K with a wide radiation region: globar (silicon carbide), Nernst pin (zirconium, thorium, yttrium oxides), nichrome spiral, platinum wire with ceramic coating. In the far IR region, the radiation from the walls of a low-pressure mercury lamp is used. The emissivity of heat sources obeys Planck's law for black body radiation. Research is underway on the use of terahertz radiation (submillimeter range) in spectroscopy.

Optical systems. The purpose of the optical system is to direct the radiation of the source along the desired path with minimal losses. The use of reflective mirrors with an external coating (sprayed aluminum, antireflection coatings) avoids chromatic aberration. Reflective optics can have flat, spherical, parabolic, elliptical, or toroidal surfaces. A large number of types of optical systems of spectrophotometers have been developed. Classical schemes of spectral devices are considered in the special literature

Radiation receivers. Receivers of IR radiation are divided into two groups: thermal and photoelectronic. The first group includes thermoelements (thermocouples), bolometers (resistances with a large temperature coefficient), pneumatic receivers, pyroelectric receivers. Pyroelectric detectors (based on triglycine sulfate (NH2CH2COOH)3 H2SO4) are used in interferometers due to their high sensitivity in the wide IR region. The operation of photoelectronic semiconductor receivers, which include photoresistors and photodiodes, is based on the phenomenon of the internal photoelectric effect. In the near-IR range, photodiodes based on germanium and InGaAs solid solution are most common. In the mid-IR range, liquid nitrogen-cooled photodiodes based on a solid solution of HgCdTe (MCT Mercury-Cadmium-Tellurium) are used. Semiconductor detectors for operation in the low-frequency region require cooling to low (nitrogen or helium) temperatures. The band gap determines the long-wave sensitivity limit of photoelectronic receivers.

Optical materials. Since ordinary optical glasses absorb medium and long-wavelength IR radiation, single crystals of various salts are used as materials for the manufacture of cell windows and beam splitters. In internal reflection spectroscopy, materials with high refractive indices are used. Some features and advantages of Fourier transform spectroscopy consist in the fact that in classical spectrophotometers the spectrum is recorded in time as the spectrum leaving the monochromator sequentially moves along the exit slit. This process is called wavenumber scanning. Fourier transform spectrometers belong to the type of multichannel instruments, which leads to a significant reduction in energy losses. Felgett and Jacquinot independently showed that the reconstruction of the spectrum using the Fourier transform of the interferogram has a great advantage over sequential, element-by-element registration of the same spectrum. Using one receiver, it is possible to study all spectral elements simultaneously, just as it is done in the case of photographic registration of spectra. In this regard, Felgett called the method "multiplex spectrometry" .

8. Methods and techniques for sample preparation in IR spectroscopy

The variety of techniques for preparing samples for IR imaging is almost limitless, and the researcher must choose the one that best suits the specific problem, taking into account the properties of the object under study. Here are some basic sample preparation techniques.

Due to the fact that the physical state of the sample can strongly affect its IR spectrum, it is advisable to determine in advance the hierarchy of the methods used. The sequence of their application is determined by the tasks assigned to the researcher. For example, in a laboratory conducting chemical work of a general nature, it is advisable to record the spectra of liquid non-volatile samples in the form of liquid films or drops crushed between salt plates. The IR spectra of volatile liquids are recorded in thin cuvettes or in the form of solutions, if the substance is soluble, while taking into account the intrinsic absorption of the solvent. For organic powdered substances, the following sequence is logical: 1) suspension in vaseline oil (or other diluent), 2) tablet with KBr or diffuse reflectance spectrum, 3) solution, 4) pyrolyzate. Techniques such as frustrated total internal reflection (ATR) are usually reserved for special cases (e.g. polymeric materials).

liquid samples. One of the simplest sample preparation methods is the liquid film method. It is used to obtain high-quality panoramic spectra of non-volatile, non-reactive, insoluble liquids. A drop of substance is squeezed between two salt plates or placed on a flat glass surface and then "wiped off" by the salt plate. It is desirable that within the cross section of the light beam of the spectrometer, the thickness of the sample be more or less the same, without air bubbles. Obviously, the spectra obtained in this way are not very reproducible and are not suitable for quantitative processing (the thickness of the absorbing layer is unknown). To obtain spectra of resins or varnishes soluble in volatile solvents, a thin layer of the corresponding solution deposited on a salt window is carefully dried under a heat lamp, hair dryer, or in a vacuum desiccator, achieving complete removal of the solvent. In some cases, researchers prefer the preparation of samples in the form of solutions, although this method is more laborious than others, its advantage lies in high reproducibility and the ability to perform quantitative measurements.

Fig.9. Collapsible liquid cell

solvent requirements. The choice of solvent is always the result of a compromise. Since all standard organic solvents have IR spectra, it is necessary to choose those in which the substance is sufficiently soluble and which have transparency windows in the analytical regions of the spectrum. The solvent must be chemically inert, cleanable and dryable. In those regions of the spectrum where the solvent transmission drops below 30%, the measurement sensitivity will be reduced, and the noise and measurement errors will increase.

Choice of concentration. Most organic substances give acceptable spectra in the region of 625 - 4000 cm -1in a cuvette with a thickness of 0.1 mm at concentrations of about 1 g / 10 ml. When working below 600 cm -1higher concentrations may be needed. In the case of highly absorbent fluorine or organosilicon compounds, the concentration can be reduced to 0.2 g/10 ml. For compounds containing polar groups, one must keep in mind the possibility of manifestation in the spectra of intermolecular interactions (for example, hydrogen bonds).

The thickness of the absorbing layer. The choice of cuvette thickness can be influenced by the amount of sample available or its solubility. Very thin cuvettes (<0,05 мм) трудно изготавливать, заполнять и опорожнять, а в кюветах толщиной более 0,2 мм поглощение растворителя может оказаться слишком сильным. Удобными для работы являются кюветы толщиной 0,1 мм. В специальных случаях для анализа следовых количеств в узких областях высокой прозрачности растворителя могут использоваться кюветы с толщиной поглощающего слоя до 1см. Перед приготовлением образца с большой толщиной поглощающего слоя, необходимо проверить, пропускание растворителя и убедиться в его чистоте.

The determination of the cell thickness is based on the fact that regular maxima and minima due to interference are observed in the spectrum of an empty clean cell with plane-parallel windows.

aqueous solutions. Due to the very strong absorption of liquid water in the IR region, the use of aqueous solutions is limited to such special areas as biological research. The use of liquid cuvettes with a thickness of less than 10 μm made of waterproof materials - Ge, Si, Zn, Se makes it possible to conduct studies in a significant part of the "fingerprint" area. Since during deuteration the vibration frequencies shift to the region of low wave numbers, sometimes heavy water is used as a solvent.

solids.

Difficulties in preparing samples of solids, which are insoluble in common solvents for IR spectroscopy, most often arise when they are ground to fine powders that form suspensions (suspensions - mulls) in liquid paraffin or KBr.

In both cases, the goal is to create a uniform distribution of particles in the beam, reduce scattering and improve the transmission of light by suspended particles in a medium that has a refractive index close to that of the sample (immersion).

Suspensions in vaseline oil. Vaseline oil (nujol) is widely used for the preparation of suspensions, but its disadvantage is a strong absorption in the valence region (2800-3000 cm -1) and deformation (1350-1500 cm -1) vibrations of CH bonds. This difficulty can be overcome by using chlorinated or fluorinated hydrocarbons.

Rice. 10. Passage of light through a scattering medium: a) without an immersion liquid, b) in the presence of an immersion liquid (for example, vaseline oil).

The size of the crushed particles should be less than the wavelength of IR radiation. To do this, a small amount of the substance (usually not more than 0.5-2 mg) is ground into a fine powder, mixed with vaseline oil, the resulting homogeneous paste is carefully applied to the salt window and crushed into a thin layer by the second window. With poor grinding, the spectra are unresolved and sometimes distorted due to the Christiansen effect (the dispersion of the refractive index in the region of the absorption band affects).

Tablets with KBr. The suspension method in KBr, also called the tablet pressing method, was first proposed in 1952. It consists in thoroughly mixing a finely divided sample with KBr powder (or other alkali metal halide), followed by pressing the mixture in a mold, resulting in a transparent or translucent tablet. The best results are achieved when the mold is evacuated, which allows you to get rid of air inclusions in the tablets. The advantages of the tablet compression method are as follows: 1) the absence of most interfering absorption bands, 2) the ability to control the sample concentration, 3) the convenience of sample storage. The disadvantages of the method include: 1) the possibility of changing the crystal structure of polymorphic substances during grinding and pressing, 2) the manifestation in the spectra of adsorbed water, which is always present in a certain amount in hygroscopic KBr (1640 and 3450 cm-1), 3) in some cases, chemical interaction of KBr with the sample substance (for example, with organometallic compounds).

Diffuse reflection is used to obtain IR spectra of powdered substances, especially in the near spectral region. When using special attachments for Fourier spectrometers, the technique turned out to be useful for quantitative analysis, in particular, of drugs.

· Pyrolysis. When all attempts to obtain an IR spectrum fail, difficult samples are subjected to pyrolysis or dry distillation, followed by analysis of the IR spectra of volatile products. In many cases, the spectra of pyrolyzates are similar to those of the starting compounds. In this way it is possible to identify, for example, polyurethanes.

· There are attachments to Fourier spectrometers that allow you to record the IR spectra of chromatographic fractions as they exit the gas chromatograph.

· Cuvettes with diamond windows. To study the properties of solids and phase transitions in them, it is necessary to record IR spectra at high pressures, up to 10,000 atm. Currently, cuvettes with IR-transparent windows made of type IIa natural diamonds (diamond anvils) are not exotic. The optical aperture of such a cuvette is small and a special micro-illuminator, a light condenser, may be required to use it.

· Spectroscopy of internal reflection. Among the methods of sample preparation, frustrated total internal reflection spectroscopy occupies a special place. This method is widely used to obtain surface spectra of "uncomfortable" objects such as filled resins, composite materials, crude rubber or foodstuffs. It is based on the absorption by the surface layer of the test sample of electromagnetic radiation emerging from the total internal reflection prism, which is in optical contact with the surface under study. To register ATR spectra, special attachments are required, which are placed in the cuvette section of a standard spectrometer. The ATR spectra are almost identical to conventional absorption spectra (Fig. 11).

Fig.11. IR spectra of a polypropylene film - at the top is the absorption spectrum, at the bottom is the ATR spectrum

In the simplest cases, there are no special problems in the preparation of gaseous samples. In the case of aggressive gases and vapors, special materials must be used to make the cuvette. Sheet polyethylene can be used as windows. Gaskets are best used with inert materials such as Viton or Teflon, as other materials can contaminate samples through adsorption and desorption.

To reduce the effect of band broadening due to collisions, the pressure in the cuvettes is usually adjusted to atmospheric pressure with dry nitrogen. This procedure increases the sensitivity to trace constituents and also allows quantitative measurements.

In cases where high sensitivity is required, such as in atmospheric pollution studies, multi-pass gas cells with long optical paths are very useful. The industry produces gas cells with an optical path length of up to 120 m, and in the literature there are reports of special cells with a total path length of up to 1 km, which made it possible to achieve a sensitivity of 0.1 - 1 ppb. Traces of noxious and noxious atmospheric vapors can be adsorbed on charcoal in adsorption tubes and then eluted with a solvent for IR identification. Coal cooling to liquid nitrogen temperature increases the determination efficiency up to 80 - 100%.

9. Equipment for IR spectroscopy

The entire IR region is conditionally divided into the near one in the range of wave numbers 4000-12500 cm -1, in which electronic and vibrational transitions are observed; main and middle from 625 to 4000 cm -1, associated mainly with vibrations of molecules; and far from 50 to 625 cm -1, in which rotational transitions, vibrations in heavy molecules, in ionic and molecular crystals, some electronic transitions in solids, torsional and skeletal-deformation vibrations in complex molecules, for example, in biopolymers, are observed. At present, spectroscopy has received the greatest development in the mid-IR region, in which the majority of serial instruments operate.

The general design of an IR spectrometer includes: a radiation source, a dispersive system (monochromator), and a recording element (detector). The specificity of IR radiation leads to features in the design of each element.

Spectrum region ?, cm -1?, μm Application Near 12500-40000.8-2.5 Qualitative and quantitative analysis for H-containing functional groups, many sorbents of the main frequencies from the mid-IR region. Middle 4000-6252.5-16.7 Vibrational or main IR region. Typical absorption lines of functional groups, mainly in the range of 1400-4000 cm -1, and the range 600-1400 is called the "fingerprint" region. Study of skeletal and torsional vibrations, as well as lattice vibrations of solids.

The radiation source in the IR spectrometer must cover a large range of wavelengths. Of these, the most common rods are made of silicon carbide (globar) or oxides of rare earth elements (Nernst pin), heated by current up to 1500 and 800 C. The radiation intensity curve of these sources has the form of a blackbody radiation curve. They give powerful IR radiation, but mainly in the near IR region and rapidly decreasing with increasing wavelength (this change in power is compensated by the programmed opening of the entrance slit of the device). In the long-wave part of the IR spectrum, high-pressure mercury-quartz lamps are used.

Rice. 12. Silicon carbide rod (a) (globar) and Nernst pin

Rice. 13. High pressure mercury-quartz lamps

In a monochromator, dispersive elements can be prisms in the IR region of materials with a suitable dispersion or diffraction gratings. Prisms made of glass or quartz are not applicable because they do not transmit infrared radiation, and usually prisms made from some salts are used. Prisms require a sufficiently large dispersive power, but it decreases with decreasing wavelength. So the NaCl prism provides an accuracy of about 2 cm -1in the area of 650 cm -1and about 30 cm -1in the area of 3000 cm -1. Therefore, 3-4 interchangeable prisms of LiF, NaCl, KBr single crystals are usually used. The dispersion of the prism material usually varies greatly with temperature, so temperature control of the device is necessary. Salt optics should be protected from high humidity.

Currently, diffraction gratings are increasingly used as dispersive elements. They have more dispersion, which depends little on wavelength and almost does not depend on temperature, but gratings can give superimposition of higher-order spectra, which requires the use of good spectral filters in the device.

Rice. 14. Diffraction grating

The detection of infrared radiation is based primarily on its thermal effect. For the mid-IR region, sensitive thermocouples (thermopillars) and resistance thermometers (bolometers) covered with black are used as radiation receivers. Pneumatic receivers (Golay cell) are also used, in which the gas in a blackened chamber with a flexible wall changes pressure under the action of radiation. In the long-wavelength region, another group of receivers is also used: photonic receivers with photoconductivity.

Among IR spectrometers, the most common are dispersive scanning devices, in which the spectra are sequentially scanned and recorded using a single-channel receiver. According to the lighting scheme, such devices are single-beam and two-beam. In single-beam spectrometers, a single spherical mirror is used to illuminate the slit.

Now, a two-beam system is more often used, which makes it possible to equalize the background, that is, the full transmission line, and compensate both for absorption by atmospheric H 2O and CO 2and attenuation of the beams by the cuvette windows and the solvent. A simplified block diagram of a two-beam scanning IR spectrometer with a diffraction grating is shown in fig. 15.

Rice. 15. Scheme of an infrared double-beam spectrophotometer

IR radiation from source 1 is divided into two beams by a system of mirrors 2. The working beam passes through the cell with the sample 3, and the comparison beam passes through the background compensator 4. With the help of the disk modulator 5, the beams are alternately directed to the entrance slit of the monochromator 6 and through it to the diffraction grating 7, which decomposes the radiation into a spectrum and directs it to the exit slit 8. The monochromatic image of the slit falls on the receiver - bismuth bolometer 9. In the absence of the test sample, the intensities of the working beam and the reference beam are the same, in the receiver the signals from these beams are subtracted; there is no signal at the output. When the working beam is absorbed by the test substance, rays of different intensity fall on the receiver, as a result of which an alternating signal appears in the receiver. After amplification and conversion of the signal, the pen of the recorder 10 is set in motion. When the grating is slowly rotated, the slit 8 successively cuts out narrow sections of the spectrum, and a curve of dependence of transmission on the wavelength is drawn on the recorder tape.

Serial one- and two-beam spectrophotometers used to study low molecular weight compounds have sufficient resolution to study most polymers. However, for work in the far region of the spectrum, which plays a very important role in the study of polymers, special vacuum spectrophotometers with diffraction gratings are needed.

Modern spectrometers make it possible to record the IR spectra of gaseous, liquid, and solid samples. To obtain the IR spectrum of a polymer or organic compound, only 1 to 10 mg of a substance is needed.

In most cases, the spectra of compounds are recorded either in the form of solutions of substances in chloroform, carbon tetrachloride, carbon disulfide, or in the form of solid transparent tablets obtained by pressing under pressure a finely ground mixture of a substance with potassium bromide. Sometimes I use the method of shooting the IR spectrum of a substance in the form of a finely ground suspension in vaseline or mineral oil.

In the case of recording the IR spectrum of compounds in solutions or suspensions, it is necessary to subtract the absorption bands of solvents or a suspension medium.

Rice. 16. IR spectrum of chloroform

10. Specific features of pharmaceutical analysis

Pharmaceutical analysis is the science of chemical characterization and measurement of biologically active substances at all stages of production: from the control of raw materials to the assessment of the quality of the obtained drug, the study of its stability, the establishment of expiration dates and the standardization of drug dosage forms. Pharmaceutical analysis has its own specific features that distinguish it from other types of analysis. These features lie in the fact that substances of various chemical nature are subjected to analysis: inorganic, organoelement, radioactive, organic compounds from simple aliphatic to complex natural biologically active substances. The range of concentrations of analytes is extremely wide. The objects of pharmaceutical analysis are not only individual drugs (substances), but also mixtures containing a different number of components.

Pharmaceutical analysis methods need to be systematically improved in connection with the creation of new drugs and the continuous increase in the requirements for their quality. Moreover, the requirements are growing both for the degree of purity of the drug, and for the quantitative content. Therefore, it is necessary to widely use not only chemical, but also more sensitive physical and chemical methods to assess the quality of drugs.

The requirements for pharmaceutical analysis are high. It should be sufficiently specific and sensitive, accurate in relation to the standards stipulated by the GF, FS and other ND, carried out in short periods of time using the minimum amount of tested drugs and reagents. Pharmaceutical analysis, depending on the tasks, includes various forms of drug quality control: pharmacopoeial analysis, stage-by-stage control of drug production, analysis of individually manufactured medicinal products, express analysis in a pharmacy, and biopharmaceutical analysis.

Pharmacopoeial analysis is an integral part of pharmaceutical analysis. It is a set of methods for the study of drugs and drugs, set out in the State Pharmacopoeia or other regulatory documentation. Based on the results obtained during the pharmacopoeial analysis, a conclusion is made about the compliance of the drug with the requirements of the Global Fund (FS, FSP). In case of deviation from these requirements, drugs are not allowed for use.

The conclusion about the quality of drugs can be made only on the basis of the analysis of the sample (sample). The procedure for its selection is indicated either in a private FS or in the general article of the GF XI (issue 2).

Performing a pharmacopoeial analysis allows you to establish the authenticity of the medicinal product, its purity, to determine the quantitative content of the pharmacologically active substance or ingredients that make up the dosage form. Although each of these steps has a specific purpose, they cannot be considered in isolation. They are interrelated, mutually complement each other and reflect the complex nature of drug quality assessment. For example, melting point, solubility, pH of an aqueous solution, etc. are criteria for both authenticity and purity of a medicinal product. These features of pharmacopoeial analysis significantly distinguish it from the norms and requirements for analysis methods used in the State Standards (GOST) and technical specifications (TU).

The FS describes the methods of appropriate tests in relation to one or another pharmacopoeial drug. Many of these methods are identical. In order to unify the methods of analysis, the Global Pharmacopoeial Articles (GPM) are included in the Global Pharmacopoeia, which systematize information on the performance of tests for a number of ions and functional groups, as well as unified methods of quantitative determination. To summarize a large amount of private information on pharmacopoeial analysis, the main criteria for pharmaceutical analysis and the general principles of testing for authenticity, purity and quantitative determination of drugs will be considered.

To check the quality of drugs, testing centers or drug quality control centers (QCCLS) must have the necessary arsenal of analytical equipment. The article gives recommendations on the choice of equipment, evaluates the strengths and weaknesses of the instruments and laboratory materials on the market, and presents the main range of equipment for the CCCCL.

Issues of organization of a microbiological laboratory, equipment for testing for bacterial endotoxins, organization of the activities of the CCCCL, quality management system will be presented in the following publications.

Mid-range infrared (IR) spectroscopy (4000 to 400 cm-1) is currently the number one method for authenticating pharmaceutical substances. It can also be applied to drugs (i.e., dosed drugs ready for use), but modern pharmacopoeial analysis suggests in this case a preliminary extraction of the effect the active substance from the dosage form. (There are studies that demonstrate the possibility of direct acquisition of IR spectra of preparations at a relatively high content nii of the main substance in the preparation.)

The method of IR spectroscopy is pharmacopoeial. In the State Pharmacopoeia (SP) XII (part 1, p. 62) there is a corresponding general pharmacopoeial article (GPM) "Spectrometry in the infrared region".

A modern IR spectrometer usually works according to the Fourier transform principle, i.e., it uses an interferometer, which distinguishes it favorably from dispersive instruments.

It should also be noted that a modern IR spectrometer is a device with great capabilities, not all of which are required for routine drug quality control. By this when acquiring such boron for CKKLS, it is necessary to choose a set of devices, attachments and software for the main device, which will be really in demand.

In the vast majority of cases, two methods are used to obtain IR spectra:

pressing tablets with potassium bromide (basic option);

obtaining a suspension in vaseline oil.

To get the tablets you need:

special press with mold mi and other devices;

spectroscopically pure bro potassium mide (KBr for IR spectroscopy);

appropriate holders in the sample compartment of the instrument.

To obtain a suspension in vaseline oil, you need:

mortar that does not contain pores (for example Mer, agate) with the same pestle (the pores accumulate moisture, which must be avoided in the sample);

spectroscopically pure mineral (vaseline) oil (oil for IR spectroscopy);

glasses made of potassium bromide or other material that is transparent in the working range of the IR spectrum (the suspension is placed between the glasses);

appropriate holders in the sample compartment of the instrument. When obtaining IR spectra of liquid substances, potassium bromide glasses, which are also used to scan the spectra of suspensions, may be suitable.

Naturally, different manufacturers can constructively implement the acquisition of spectra in different ways, which should be consulted when purchasing the device.

It should also be taken into account that other options for sample preparation provided for by the GPM may also be in demand. They should also be discussed with the supplier when purchasing the instrument.

Conclusion

Infrared spectroscopy is becoming more and more widespread every year as a very valuable physical method for studying the structure of molecules and as a powerful analytical method.

The introduction of spectroscopy into the field of pharmaceutical analysis of drugs served as a powerful impetus for its development and improvement. Due to the variety of applications, high accuracy of results and sensitivity of detection, a significant reduction in the time of analysis, IR spectroscopy has reached the highest degree of economic efficiency.

List of used literature

1.Averko-Antonovich I.Yu. , Bikmullin R.T.: Methods for studying the structure and properties of polymers. Textbook / KSTU. Kazan. - 2002. - 604 p.

Arzamastsev A.P., Dorofeev V.L. Modern requirements for Standardization and quality control of medicines. // New pharmacy. Effective management. 2007 - No. 4 - S. 54-57.

Arzamastsev A.P., Dorofeev V.L. Standard samples for pharmacopoeial analysis. // Questions of biological medical and pharmaceutical chemistry. 2010 - No. 5С. 6-10.

Bellamy JI. Infrared spectra of complex molecules M.: 1983, 432 p.

Becker Yu. World of Chemistry. Spectroscopy. Translation from German by L.N. Kazantseva. Moscow -2009. - 522s.

Brand J. Application of spectroscopy in organic chemistry. M.: Mir. - 1967. - 280 p.

State Pharmacopoeia of the Russian Federation XII Edition Part 2, Moscow 2010

Kesler N, Methods of infrared spectroscopy in chemical analysis. M.: Mir, 1984, -287 p.

Koptyug V.A. Atlas of spectra for forensic divisions of the Ministry of Internal Affairs of the USSR. Issue 8. Novosibirsk - 1991 -477s.

Krishchenko V.P. Near infrared spectroscopy. M.: 1997. -638s.

Cross A.D. Introduction to Practical Infrared Spectroscopy Translated from English. M.: Foreign Literature, 1981. - 110 p.

Krylov AS, Vtyurin AN, Gerasimova Yu. V. Data processing of infrared Fourier spectroscopy. Toolkit. Krasnoyarsk, Institute of Physics SB RAS, 2005. - 48 p.

Kuptsov A.Kh., Zhizhin G.N. Fourier Raman and Fourier IR spectra of polymers. M.: Fizmatlit, - 2001. -316 p.

Nakanishi K. "Infrared Spectra and Structure of Organic Compounds" M.: Mir, - 1965. -216p.

Norris K.H. Devices for near infrared spectroscopy // Application of spectroscopy in the near infrared region for product quality control (4th collection of scientific papers on IR). M.: Interagrotech, 1989.-S. 5-10.

Pentin Yu.A. , L.V. Vilkov: Physical methods of research in chemistry. - M.: Mir. - 2003. - 367 p.

Sadchikova N.P., Arzamastsev A.P., Titova A.V. The method of near-IR spectroscopy in the quality control system of medicines (review) // Problems of biological medical and pharmaceutical chemistry. -2010.-№1. pp. 16-20.

Sadchikova N.P., Arzamastsev A.P., Titova A.V. The current state of the problem of using IR spectroscopy in the pharmaceutical analysis of drugs // Khim.-farm.zh. 2008. - No. 8. - S. 26-30.

Tarasov K.I., Spectral instruments, "Engineering", L., 1968.

Tarasevich B.N. Fundamentals of IR spectroscopy with Fourier transform. Sample preparation in IR spectroscopy. Moscow - 2012. -22p.

Tarasevich B.N. IR spectra of the main classes of organic compounds. Reference materials. Moscow -2012. -52s.

Tarutina L.I., Pozdnyakova F.O. Spectral analysis of polymers. - L.: Chemistry. - 1986. - 248 p.

Slivkin A.I. Atlas of IR spectra of medicinal substances. A.I. Slivkin, O.V. Trineeva, E.E. Logvinova, A.S. Chistyakova, L.Yu. Yakovlev, I.I. Mekhantiev. Publishing and Printing Center of Voronezh State University - 2013.- 172p.