RU202134U1 - IR SPECTROMETRIC CELL FOR DETERMINING VOLATILE SUBSTANCES IN THE STEAM PHASE - Google Patents

Abstract

The utility model is aimed at creating an IR spectrometric cell that can be integrated into commercially available IR spectrometers and relates to the field of analytical chemistry, in particular to the analysis of materials using optical means, and can be used for the rapid identification and quantitative determination of volatiles in the vapor phase. by infrared spectrometry in the chemical, microbiological, food industries.

Description

The utility model relates to the field of analytical chemistry, in particular to the analysis of materials using optical means, can be used for identification and quantitative determination of volatiles in the vapor phase by infrared spectrometry (IR spectrometry) and can be used in the chemical, microbiological, food industries ...

The claimed technical solution relates to the field of analytical chemistry, in particular to the analysis of materials using optical means, i.e. with the use of infrared, visible or ultraviolet rays, it is a measuring cell for the headspace analysis of liquids, integrated into commercially available IR spectrometers.

Determination of the qualitative and quantitative characteristics of materials by the IR spectrometric method is based on the selective absorption of infrared energy. IR spectrometers are used to record the spectrum of absorbed IR radiation. Fourier transform IR spectrometers are currently the most widely used. The main components of these spectrometers are an IR source, an interferometer and a detector. Infrared radiation from the source enters the interferometer, where the radiation is modulated due to interference on the moving and stationary mirrors. Modulated IR radiation passes through the sample and is recorded by a detector. The resulting interferogram is processed by a computer to obtain a traditional spectrum. The Fourier transform is used for the calculation. FTIR spectrometers are single-beam devices, therefore, to compensate for the absorption of infrared radiation by atmospheric gases, spectrometer components, cuvette and solvents, a background spectrum (comparison spectrum) is recorded without a sample. Then the background spectrum is subtracted from the analytical one.

Until recently, the analysis of aqueous-organic mixtures by IR spectroscopy was limited by the capabilities of the "salt" optics of IR spectrometers. Known infrared spectrometric method, in which the determination of the content of solutes is carried out while removing the solvent. For this, an aliquot of the solution is evaporated until a dry residue is formed. The residue is transferred to an agate mortar, where it is ground. Then the test powder is mixed with a weighed portion of potassium bromide and thoroughly ground in a mortar. The resulting mixture is placed in a mold and pressed [Smith A. Applied IR spectroscopy: Per. from English. -M .: Mir, 1982, - p. 93-94]. The result is transparent or translucent tablets suitable for IR spectrometric analysis. The main advantage of the method is the absence of interfering absorption bands of IR radiation by the solvent. For the successful preparation of tablets with potassium bromide, prolonged (thorough) grinding of the mixture of the residue after evaporation of the solvent and potassium bromide powder is necessary. It is also necessary to press with a uniformly increasing load, preferably with evacuation. As a result, the preparation of tablets takes a considerable amount of time and requires the use of additional equipment: laboratory hydraulic press, vibrating mill and vacuum pump. Accordingly, the disadvantage of this method is the significant labor and cost of performing the analysis.

Famous [Smith A. Applied IR spectroscopy: Per. from English. -M .: Mir, 1982, - p. 154-155] earlier, the method of analyzing volatile organic liquids in mixtures with water by the method of multiple disturbed total internal reflection (MIPR) on a germanium crystal did not become widespread, because for the mid 60s of the twentieth century, this method was rare and costly. Modern methods of IR spectroscopy, in combination with waterproof spectral materials, make it possible to develop effective methods for the analysis of such "inconvenient" objects. An express and selective method for the determination of highly volatile organic liquids in mixtures with water with concentrations of more than 1 g / L turned out to be (IR spectroscopy combined with the registration of the IR spectrum by the method of disturbed total internal reflection (ATR) on a ZnSe crystal [Nekhoroshev SV Expert study Methyl alcohol and its aqueous solutions / SV Nekhoroshev, AV Nekhorosheva, MN Remizova, Kh.B. Tagizade // Forensic examination. 2011. No. 2 (26). P. 47-53]. At the same time, due to powerful intermolecular interactions in aqueous solutions of methyl and other alcohols, the position of some absorption bands in the IR spectra can significantly depend on their concentration, which negatively affects the measurement error of the analytical signal [Rai VK Spectroscopic studies of some aliphatic alcohols / VK Rai, SB Rai, ID Singh, Siyaram // Indian J. Phys. B. 2004. T. 78; No. 1. P. 81-86]. In addition, there are disadvantages and the very crystalline ZnSe, with which when registering IR spectra it is necessary to contact the analyzed solutions. This optical material is damaged by acid solutions with a pH of less than 5 and alkali solutions with a pH of more than 9, and also has insufficient hardness. However, in 2014, ATR attachments with diamonds grown and processed in Russia appeared on the domestic market of analytical equipment. With admissible optical properties, diamond is free from many of the disadvantages of crystalline ZnSe; possesses high hardness, thermal and chemical resistance. Today ATR attachments with diamond are most often used for recording IR spectra of hard elastic and brittle samples, because diamond is resistant to mechanical damage and minimizes labor-intensive sample preparation [Ezhevskaya T. Russian diamonds in FTIR spectrometry / T. Ezhevskaya, A. Bublikov, Yu. Palyanov, A. Khokhryakov // Analytics. 2015. No. 2 (21). S. 118-123].

An analogue of the proposed technical solution is an IR spectrometric cell [WO 2007060398 A1]. The device includes an inlet valve connected to the gas line through which an inert gas is supplied, which flows through the inlet valve into a cylindrical container characterized by a certain optical path and made of a material resistant to the action of the analyzed solution, two material windows are hermetically glued to the side parts of the container transparent in the infrared region. The container is equipped with a heating element for evaporation of the analyzed sample and a temperature sensor that allows you to control the temperature of the medium inside the container. In addition, the IR spectrometric cell is equipped with a capillary device for introducing the analyzed sample.

The disadvantage of the device is that this device is designed and reduces the analysis time for the registration of spectra in the stream, and not in a limited volume; in this device, the sample must have the ability to completely evaporate, otherwise non-evaporated residues will accumulate at the injection site (in the injector) and block the way for new portions of the sample; this device does not have a ventilation system (cleaning) of the optical cell from the vapors of the previous sample; in this case, the remains of the sample should condense and drain back into the liquid stream; this device only adjusts to a certain sample volume due to the diameter and filling of the inlet.

The closest to the proposed technical solution (prototype) is the device "IR spectrometric cell for the determination of volatile organic liquids in mixtures with water", RF patent for a useful model No. 160148 RU. It was found that for operational analytical control of the determination of volatile organic liquids in aqueous solutions by FTIR spectroscopy, the most effective of the known methods for recording spectra is the use of an ATR attachment with a diamond crystal equipped with a cover to prevent evaporation of sample components and a thermoelement to maintain a stable temperature. To reduce the effect of intermolecular interactions on the spectral characteristics of aqueous solutions of highly volatile organic liquids, it is recommended to record IR spectra at elevated temperatures (50-60 ° C). The most significant disadvantages of this solution are the high cost of the diamond optical element, as well as the influence on the shape and position of the analytical absorption band of volatile organic liquids in mixtures with water of powerful intermolecular interactions, which are inevitably present in such liquids as aqueous solutions.

The task to be solved by the claimed technical solution is to create an IR spectrometric cell for qualitative and quantitative analysis, which would ensure the elimination of the influence of intermolecular interactions in liquid samples, create the ability to analyze liquid samples in the vapor phase and analyze gas samples, reduce the cost and the time of analysis of highly volatile substances in water-organic mixtures and expanded the range of analyzed objects. The attachment is designed to work as part of commercially available FT-IR spectrometers and allows you to record the IR spectrum of vapors of a liquid sample in the range of wave numbers 4000-650 cm ^-1 .

FIG. 1 shows a diagram of the arrangement of the main parts of the proposed IR-spectrometric cell for the determination of volatiles in the vapor phase, side view.

This attachment is a heated gas cell (a horizontally placed cylindrical tube for pumping gaseous samples or samples of liquid substances in the vapor phase), the ends of which are closed by windows 4 and 5 made of optical material transparent in the range of spectrum registration. Tubes 12 and 13 are connected to the cuvette, equipped with pneumatic valves to isolate its internal volume during the spectrum recording. A through hole is made on the upper side wall of the attachment, in which a sample injector 8 is fixed, which allows, using a chromatographic microsyringe 10, to introduce a liquid sample into the inner volume of the cuvette 3 for evaporation and analysis, or a gaseous sample for analysis.

The cuvette body is made of durable corrosion-resistant heat-resistant material and has a cylindrical shape. The length of the cell body does not exceed 180 mm, its inner diameter is no more than 20 mm. The length of the optical path (the length of the inner space of the cuvette, limited by the inner surface of the end windows 4 and 5) is 150 mm. The inner volume of the cuvette is constant and does not exceed 50,000 ± 2,500 mm ³ . The operating temperature range of the cuvette is from room temperature (about 20 ° C) to 220 ° C.

The injector 8 is equipped with a silicone heat-resistant membrane 9 and provides a liquid or gaseous sample injection using a microsyringe 10. The inner diameter of the sample injection channel in the injector 8 should be 1.5 mm. As a microsyringe, standard chromatographic microsyringes with a capacity of 1, 10 and 50 μl are used.

Tubes 12 and 13 have an inner diameter of no more than 1 mm and are equipped with pneumatic valves 1 and 2 for hermetic isolation of the inner volume of the cuvette.

On the outer surface of the cuvette (with the exception of end windows 4 and 5) there are electric heating elements 6, a temperature sensor (not shown in the diagram) and a thermal insulation casing 11, which, together with an automatic temperature controller 7, allow the cuvette body to be heated from room temperature (about 20 ° C ) up to 220 ° С and for a long time (up to 8 hours) maintain the specified temperature within the specified range with an accuracy of ± 3 ° С.

To prepare the attachment for operation, it is placed in the cuvette compartment of the switched on IR spectrometer, after which the required temperature of the attachment is set with the help of the thermostat 7 and the attachment temperature is waited for it to heat up to the specified temperature. The attachment is ready for use.

To increase the efficiency of the attachment, the schematic diagram of the device shown in Fig. 1 has the following design features:

1) the material of the body of the attachment 3 is made of stainless steel, as of a more corrosion-resistant and heat-resistant metal, in comparison with aluminum and its alloys;

2) the direction of gas flows in the housing 3 of the attachment is made not "From window to window", but according to the scheme "From injector to windows", for which the injector 8 is connected by a tube 12 with a pneumatic valve 2, and two metal tubes 13 entering the cuvette are connected to pneumatic valve 1. These changes make it possible to halve the time for cleaning the inner space of the attachment from the previous sample, as well as to avoid the concentration of contaminants in the injector 8;

3) in the heating circuit of the set-top box body, ring heating elements 6 are used, which provide a more even distribution of heat compared to point heating elements;

4) pneumatic valves 1 and 2 for cleaning the inner space of the attachment from the previous sample are placed outside the thermal insulation casing 11 to exclude their overheating from the heated casing of the attachment 3 and to protect valves 1 and 2 from failure;

5) pipes 12 and 13 connecting the attachment and pneumatic valves are made of metal with an inner diameter of no more than 1 mm and a total length of no more than 300 mm, the internal volume of which will not exceed 0.025 cm ³ , which is less than 0.05% of the internal volume of the body attachments (about 50 cm ³ ) with an inner diameter of 20 mm, or less than 0.2% of the inner volume of the attachment body (about 12 cm ³ ) with an inner diameter of 10 mm. This allows you to reduce the measurement error by reducing the contribution of hard-to-reach places in the internal volume of the attachment;

6) from the ends of the device, two windows 5 and 6 of a material transparent in the IR region are hermetically fixed with threaded rings, which allows them to be replaced in case of damage, turbidity;

7) a chromatographic microsyringe 10 is designed to inject a liquid or gaseous sample into the inner volume of the cuvette.

During operation, the set-top box can be in 2 alternating modes, shown in Fig. 2, which are started by the operator by switching toggle switches for controlling pneumatic valves:

1) pneumatic valves 1 and 2 are open and the internal volume of the attachment is purged (cleaned) with compressed gas from the vapors of the previous sample;

2) pneumatic valves 1 and 2 are closed, after which a sample is injected into the injector 8 of the attachment with a microsyringe 10, which evaporates in the inner space of the attachment and after that the spectrum of the sample vapors is recorded.

The use of the device makes it possible to transfer the analyte into the vapor phase in, in fact, the very volume of the container, bounded by the windows, at this time the IR spectrum of this vapor phase is recorded, which allows to reduce the analysis time. The sample in the vapor phase does not require heating and subsequent evaporation, and therefore it is possible to directly record the IR spectrum of the investigated vapor phase. Obtaining the corresponding spectrum of the vapor phase evaporated from a limited volume of liquid will make it possible to qualitatively and quantitatively characterize the composition of the evaporated liquid. The cell is placed in a standard cuvette holder for FT-IR spectrometers, commercially available.

The technical result provided by the given set of features is an improved quality of the analysis of materials using optical means, which can be used to identify and quantify volatile organic liquids in mixtures with water by infrared spectrometry, will allow the analysis to be performed quickly and economically, while maintaining high reliability of identification. and accuracy of determination. Compared with existing methods, the claimed method has the following advantages.

1. Reduces the time of analysis of samples from a limited volume, since there is no need for sample preparation.

2. The design features of the device allow the liquid sample to evaporate both completely and partially.

3. The design features of the device allow the device to operate in 2 alternating modes, which are triggered by the operator by switching the toggle switches for controlling the pneumatic valves.

4. The device has a ventilation system (cleaning) from the vapors of the previous sample and allows you to avoid contamination of the injector 8. The time for cleaning the inner space of the attachment from the previous sample is reduced by 2 times.

5. Design features of the device allow adjusting the sample volume by the position of the microsyringe plunger during sample injection.

6. Does not contain parts soluble or reacting with the analyzed solution and absorbing IR radiation, which allows the analysis of almost all volatile organic substances in mixtures with water.

7. The material of the device body is made of stainless steel to ensure the corrosion resistance of the inner volume of the cell and the ability to maintain temperature.

8. The design features of the device allow for uniform heating of the sample.

9. The design features of the device provide a decrease in the measurement error by reducing the contribution of hard-to-reach places in the internal volume of the attachment.

Claims (1)

An infrared spectrometric cell for the determination of volatiles in the vapor phase, enclosed in a thermal insulation casing, containing ring heating elements, equipped with a thermal sensor, characterized in that it contains inlet and outlet taps connected to a gas line through which an inert gas is supplied according to the "From injector to the windows ", entering through the inlet valve into a horizontally located cylindrical container, characterized by a certain optical path L and made of a material resistant to the action of the analyzed solution, in the end parts of which two windows are hermetically fixed by threaded rings from a material transparent in the infrared region , moreover, ring heating elements for evaporation of the analyzed sample and a thermal sensor are mounted in the container, which makes it possible to control the temperature of the medium inside the container, and in the center of the upper surface of the cylindrical container there is a hole insulated with an elastic membrane that excludes sample loss and contamination, through which a gas sample or a liquid sample in the vapor phase of the analyzed sample is injected with a microsyringe.

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