RU2806706C1 - Method for electrothermal atomic absorption analysis of composition of powder samples and device for its implementation - Google Patents

Abstract

FIELD: analytical chemistry.

SUBSTANCE: invention can be used for the analysis of geological samples and environmental objects containing lithogenic, biogenic and chemogenic-hydrogen components included in the composition of rocks, soils, bottom sediments, concentrates and suspensions of natural waters. The method of atomic absorption determination of elements with electrothermal atomization includes selecting analysis conditions, preparing a sample, assembling a graphite block of an electrothermal atomizer from units designed to form zones of evaporation, condensation, atomization, carrying out the first and second stages of analysis and obtaining results based on the characteristics of which the elemental sample composition is determined. At the stage of preparation and loading of the sample, its total mass from 1 to 25 mg is placed in the evaporation unit and divided into parts with a small volume in the working zones of the container channels, which comprises from 5 to 30 independent channels with equal internal volumes of the working zones, which makes it possible to create similar conditions of thermal decomposition, ashing and fractional evaporation of 5 to 30 parts of the sample during the analysis process. The porous structure of the container material and the large lateral surface of the working area of a separate channel in relation to the area of its entrance to the working area provide the opportunity to reduce emissions of sample fractions, where the container is installed in the working volume of the evaporation unit when assembling the electrothermal atomizer.

EFFECT: reduction in the level of interference, a decrease in detection limits and an increase in the reliability of determining the microelement composition of a substance.

8 cl, 2 dwg, 1 tbl

Description

Field of technology

The invention relates to atomic spectroscopy and is intended for use in analytical spectrometry, in particular, in electrothermal direct atomic absorption analysis of powdered organomineral samples.

State of the art

Direct determination of ultra-small amounts of elements in multicomponent compositions of powder samples using electrothermal atomic absorption analysis is complicated by non-selective interference and matrix influences in the analytical zone of the atomizer. To reduce the influence of interfering components on the analysis result, various methods are used. Currently, two-stage electrothermal atomizers operate effectively, in which, at the first stage, preliminary thermal decomposition and fractional evaporation of the powder sample under study are carried out and simultaneous deposition and condensation of component vapors on additional storage devices introduced into the design. At the second stage, in the process of heating the storage devices and atomizing the condensates deposited on them, vapor detection is carried out in the analytical zone. Two-stage electrothermal atomizers have different designs, including those associated with vapor condensation on remote elements [1,3], while increasing the number of condensations on the remote probe [3], with vapor condensation on the inner surface of the atomizers [2, 4, 5], and also using additional graphite porous diaphragms [4, 5].

However, in the atomic absorption analysis of powder samples of complex composition containing lithogenic, biogenic and chemogenic-hydrogen components (for example, rocks, soils, bottom sediments, concentrates and suspensions of natural waters), which is important for solving current environmental and geochemical problems, the well-known two-stage Electrothermal atomizers do not always provide reliable data due to the fact that the detection limits and reproducibility of results are greatly influenced by the action of a large number of organic and volatile mineral inclusions. Reducing interference by reducing the mass of the test substance during evaporation led to a low signal level and required additional loading of new samples into the evaporation zone and repeating additional condensation to increase the signal level. The general disadvantage of the considered technical solutions is that the known designs of atomizers do not provide sufficient protection of the condensation zone and analytical zone from emissions of large and small volatile aerosol particles from powdered organomineral samples placed in the evaporation zone. Thus, at the stage of fractional evaporation, due to particle emissions, the necessary transformation of the sample components and simplification of the condensate composition are not achieved, which reduces the efficiency of obtaining analysis results.

To reduce the influence of emissions in the known device [4], chosen as a prototype, it was proposed at the first stage to carry out sequential repeated heating and evaporation of small samples in a crucible with the aim of sequential accumulation of atoms of the test substance on the inner surface of the second unit, which acts as a concentrator. However, this approach is not always effective, especially if the release occurs at the initial stage of heat treatment-ashing of an organomineral sample. In the prototype, to implement sequential repeated heating and evaporation of small sample samples in a crucible, partial dismantling of the block structure, consisting of three units, was carried out after the first heating and evaporation of the first sample sample. Next, a new assembly of the block was carried out, while another crucible with a new additional portion of the test sample was installed in the block. The final stage of the experiment, associated with heating the condensation zone and the analytical measurement zone, is carried out at the final stage after the completion of the process of accumulation and condensation of vapors of the test sample on the inner surface of the unit. The process of assembling and disassembling the unit takes a lot of time, is associated with the possibility of introducing contaminants, which impairs the reliability of the analysis results, and such a procedure requires energy consumption due to repeated heating of the evaporation unit. The design of the prototype to reduce particle emissions has a modification, complemented by a porous graphite diaphragm of small thickness, which closes the evaporation zone [4], while the sample is in a closed evaporation zone and there is no free escape of vapors. This position of the diaphragm is not successful when analyzing organomineral powders of complex composition. Due to the violent explosive processes of thermal decomposition of the sample components, the release of gaseous products together with large and small aerosol particles, and the deformation of the porous medium of the diaphragm, the atomization conditions and the efficiency of filtration of sample vapors change, which worsens and often eliminates the very possibility of obtaining results. Another disadvantage of the prototype design is the absence of any method of thermal insulation of the contacting walls of the crucible and the cylinder of the condensation zone, which greatly limits the possibility of optimizing the temperature regime of the stage of decomposition, ashing and fractional evaporation of powder samples.

The problem that is solved in the invention is to increase the reliability of determining the elemental composition of the analyzed powdered organomineral substance by reducing the level of interference and improving the lower limit of detection of sample components.

Other tasks are related to expanding the arsenal of technical means used to determine the elemental composition of organomineral substances and increasing the cost-effectiveness of analysis.

The objectives are solved due to the fact that in the device, a multi-channel container is additionally introduced into the internal volume of the evaporation unit of the test sample, in which the number of channels, the shape of the channels, as well as the placement of the channels relative to each other and the material from which the container is made are determined taking into account the possibility of creating close conditions of thermal decomposition, ashing and fractional evaporation of small quantities of sample in small channel volumes, as well as taking into account the presence of organic inclusions and the necessary reduction in particle emissions.

The solution to the problem is also achieved by the fact that in the method of atomic absorption analysis of powdered organomineral samples with two-stage electrothermal atomization, due to the use of a multichannel container, it is possible to distribute a sample dose from 1 to 25 mg in 5-30 channels with a small volume and carry out analysis without multiple loading of new portions of samples into the evaporation zone.

The essence of the invention

One of the aspects of the invention is a method for atomic absorption determination of elements with electrothermal atomization, including the selection of analysis conditions, sample preparation, assembly of an electrothermal atomizer block from units designed to form zones of evaporation, condensation, atomization, carrying out the first and second stages of analysis and obtaining results based on the characteristics of which determine the elemental composition of the sample. At the stage of sample preparation and loading, its total mass from 1 to 25 mg is placed and separated in the working areas of the container channels, which contains from 5 to 30 independent channels with equal internal volumes, which makes it possible to create similar conditions for thermal decomposition, ashing and fractional evaporation from 5 to 30 parts of the sample during analysis. The porous structure of the container material and the large surface of the working area of a separate channel in relation to the area of its entrance to the working area provide the opportunity to reduce emissions of sample fractions. In this case, at the first stage, the temperature of the evaporation unit, in which the multichannel container with the sample is located, is gradually increased to values from 1700 to 2200°C during a heating time of 20 to 60 s. At the second stage of obtaining results, the condensation zone and atomization zone are pulsedly heated to a temperature from 1600 to 2100°C for a time of 4 to 8 s.

Another aspect of the invention is a device that makes it possible to implement the method, consisting of an atomic absorption spectrometer (9), power supplies (I), an electrothermal atomizer unit, including functionally connected units (1, 4, 6) intended for the formation of evaporation and condensation zones and atomization, characterized in that the unit for forming the evaporation zone (1) is configured to install a multi-channel container (2), either in a monolithic structure of a graphite crucible (Fig. 1), or on a replaceable graphite base (12) of a collapsible modification of the crucible (Fig. 2), consisting of a base and a hollow cylinder (13), while the multichannel container contains from 5 to 30 independent through channels (3), made vertically and parallel to each other, where each channel allows the formation of an independent zone of evaporation of a small mass of the sample ( 14). The total volume of the through channels (3) of the container (2) makes it possible to accommodate the total mass of the sample from 1 to 25 mg. In this case, the working area of each of the 5-30 channels has a large ratio of the surface area of the working area of the container channel in relation to the area of the channel inlet and ranges from 30:1 to 160:1. The container is made of porous gas-permeable graphite, which is part of the group consisting of MPG-6, MPG-7, MPG-8, which provides the possibility of uniform heating and evaporation of samples placed in the channels, and an additional reduction in the level of particle emissions from narrow channels.

List of figures

Fig. 1. Two-stage graphite crucible atomizer-evaporator in an orthogonal section.

Fig. 2. Cross-section of the evaporation zone assembly of a sample with a dismountable modification of the graphite crucible.

Description of the invention

In the process of studying the composition of samples using the method of atomic absorption analysis in devices with a two-stage operating cycle, we primarily considered the possibility of expanding the functions of the atomizer and reducing the intensity of the effects that arise in the evaporation zone when heating powder samples of complex composition containing lithogenic, biogenic and chemogenic-hydrogen components (for example , rocks, soils, bottom sediments, concentrates and suspensions of natural waters) placed in a graphite crucible. It is known that when such samples are heated and fractionally evaporated, violent, explosive processes of thermal decomposition of organic and mineral components are often observed, the interaction of decomposition products is possible, their sintering is possible, intense gas release and the release of large and small aerosol particles from the evaporation zone into the condensation zone and, creating interference at the stage of condensate atomization in the analytical zone, which worsens the metrological characteristics of the analysis when determining ultra-small amounts of elements. This drawback also applies to devices that use the stage of preliminary fractional condensation of sample vapor [1-5], including the design of the prototype.

Therefore, the search continues for technical solutions related to improving the atomization unit, in particular, the evaporation unit, and increasing the efficiency of the procedure for thermal decomposition, ashing and fractional evaporation of powdered organomineral samples.

In the process of working on improving the method of two-stage heating and atomization of the sample components and the device implementing the method, it was discovered that it is possible to increase the efficiency and reliability of the results obtained when studying samples of powdered organomineral samples of the test substance by dividing the total sample placed in the volume of the evaporation unit into mini doses in the container channels, in which the evaporation process takes place under closer conditions, and the intensity of the effects contributing to the release of particles is reduced. An additional contribution to reducing emissions is made by the selected shape of the channels in a multichannel container with a large ratio of the surface area of the working area of the container channel relative to the area of the channel inlet, as well as the use of container material containing a developed porous structure. All these technical solutions lead to a synergistic effect and contribute to a greater degree of atomization of the components in the process of diffusion-convective vapor transfer and a reduction in the entry of particles into the condensation zone. This reduces interference in the analytical zone at the condensate atomization stage, increases the signal-to-noise ratio and improves the lower detection limit of elements.

Below is a description of the device, which does not limit the use of other modifications; for example, the cross-sections of block nodes and the shape of the container may have other geometric features.

The device (Fig. 1) consists of three cylindrical units (1,4, 6), placed vertically one above the other and forming an evaporation zone, a condensation zone and an atomization zone with an analytical zone, power sources (11), radiation source (8), spectrometer (9). The units are made with the possibility of heating them with electric current and the possibility of blowing them with a protective inert gas (for example, argon). The design is based on the design of a two-stage crucible atomizer [4]. Different types of spectrometers can be used as detection devices. Atoms of the elements being determined can be detected by emission and absorption spectrometry, as well as other methods.

The cylindrical evaporator unit (1) is made on the basis of a graphite crucible. The second cylindrical unit (4), located above the crucible with a small heat-insulating gap, forms a condensation zone (5) on the inner surface of the cylinder. The third cylindrical unit (6), fixed above the unit (4) without a gap, provides the possibility of forming an atomization zone with a conical inner surface that passes into a translucent analytical zone (7) for recording the components of the condensate under study in a through cylindrical hole and with the possibility of localizing vapors to increase the level signal. The axis of the hole coincides with the optical axis of the light flux from the radiation source (8) passing through the analytical zone to the spectrometer (9). Each of the three cylindrical units is equipped with a pair of independent electrical contacts (10) with the possibility of cooling them with running water, which are in contact with their outer surface. Independent contacts provide the possibility of separate heating of the evaporator from the power source (11) and subsequent heating of the second and, with some advance, the third unit from independent power sources. The nodes that form the evaporation zone, condensation zone, atomization zone, i.e. crucible and two cylinders, made of graphite, selected from the group MPG-6, MPG-7, MPG-8, with a density from 1.65 to 1.8 g/cm ³ with a predominant pore size from 1400 to 8000 nm [6] . To reduce diffusion losses of elements, these units can preferably have a pyrolytic coating or be made entirely of pyrographite.

The main attention in solving the technical problem was directed to the design of a new element of the device, which is a multi-channel container (2), installed in the internal working volume of a graphite crucible (1). The powder organomineral sample (14) under study is placed in the channels (3) of the container (2).

The graphite crucible (1) can be made monolithic, as shown in Fig. 1 or represent a collapsible structure from a replaceable graphite base (12), on which a hollow cylinder (13) is installed, forming the working volume of the crucible (1) as shown in Fig. 2. The collapsible design further simplifies the ability to replace the used container (2) without significant disassembly of the device. The container has a cylindrical shape and is made of porous gas-permeable graphite. To place the test sample in the volume of the container (2), from 5 to 30 through holes are made - channels (3) whose axes are parallel to each other. Before starting an experiment using non-metallic tweezers (for example, medical ones), the container (2) is installed in the internal volume of either a monolithic (Fig. 1) or a collapsible structure of a graphite crucible (Fig. 2). At the next stage, a prepared crushed sample sample with particle sizes less than 0.1 mm is placed in the crucible. Container (2) contains from 5 to 30 independent through channels, which provide the possibility of forming independent zones of evaporation of a small sample mass with a total dose of the test sample from 1 to 25 mg. This makes it possible to create similar conditions for thermal decomposition, ashing and evaporation of the sample by increasing the surface of contact of particles with the inner surface of each channel. At the same time, an additional reduction in the level of emissions of large and small aerosol particles during gas formation during the heating period is ensured due to physical contact and interaction of sample particles with the porous and structured inner surface of each of the container channels and by increasing the degree of atomization of the sample components during diffusion-convective vapor transfer .

When carrying out atomic absorption determination of elements using electrothermal atomization of powders, the following steps are performed: selection of analysis conditions, preparation of a powder sample and its loading into a crucible and placement in the container channels, assembly of an electrothermal atomizer block from units intended to form an evaporation zone, a condensation zone and atomization zones, with their preliminary heat treatment at a temperature of at least 2000°C to remove contaminants; carrying out an analysis based on the characteristics of which determines the elemental composition of the sample. At the sample preparation stage, particles of the test sample with a size of less than 0.1 mm (usually from 0.05 to 0.08 mm) are obtained, prepared in accordance with known recommendations for sample preparation [7]. At the loading stage, using non-metallic tweezers (for example, medical ones), a container (2) is installed in the internal volume of a monolithic (Fig. 1) or collapsible modification of the crucible (Fig. 2), and then the total mass of the sample is placed in the crucible and distributed into doses in the channels container. For better packaging of the powder sample, a technique known in spectrometry is used to shake the evaporator assembly. The same procedure is carried out with powder reference samples to construct calibration graphs. In this case, the number of container channels, the shape of the channels, as well as the placement of the channels relative to each other and the material from which the container is made are selected taking into account the physicochemical properties of the powder sample to ensure the possibility of creating similar conditions for thermal decomposition, ashing and fractional evaporation of a small mass of samples in container channels, as well as taking into account the presence of organic inclusions and the necessary reduction in the emission of particles from the evaporation zone.

At the next stage, the complete assembly of the units of the electrothermal atomizer block is carried out from the units intended to form the evaporation zone, the condensation zone and the atomization zone.

At the final stage, two-stage heating and recording of the analytical signal are carried out. Wherein:

a) at the first stage, electrical heating is carried out with a gradual increase in temperature from 1700 to 2200 ° C for 20 to 60 s of the first unit for thermal decomposition, ashing and fractional evaporation of the sample placed in the container, with simultaneous condensation of the evaporated components on the inner surface of the second node; b) at the second stage, while heating the first node continues, the second node of the condensation zone is pulsedly heated and, with some advance, the third node of the atomization zone to a temperature from 1600 to 2100°C for a time of 4 to 8 s, while the interaction of the light flux occurs in the analytical zone with sample pairs and registration of the analytical signal.

The use of a new method of sample placement, in which the total dose is divided into small volumes in independent working zones of the container channels, makes it possible to reduce the level of emissions of large and small aerosol particles compared to the prototype during gas formation during the heating and evaporation of the sample. The interaction of sample particles with the large porous and structured inner surface of each of the container channels makes it possible to increase the degree of atomization of the components due to the uniform heating of small doses during diffusion-convective vapor transfer.

Example 1 Comparison of non-selective absorption absorption signals in different types of crucible atomizers.

It is known that the metrological characteristics of determining elements in samples of complex composition (detection limits, reproducibility) deteriorate with increasing level of non-selective absorption (often up to an absorption unit or more, which greatly complicates the possibility of obtaining results).

A comparison of non-selective absorption absorption signals was carried out when determining elements (Ag, Cd, Bi, Tl) in powder samples placed in crucible atomizers using different methods.

Table 1 shows experimental data on the magnitude of nonselective absorption absorption signals when determining elements in organomineral samples characterized by intense evaporation of the base and emission of particles. In the experiments, one stage of fractional condensation was used during the evaporation of sample samples weighing 3-20 mg. The experimental data showed that the use of a crucible atomizer containing a multichannel container, in which the total mass of the sample was divided and distributed into independent channels of the container, compared with the prototype, in which the total mass of the sample was placed directly at the bottom of the crucible, reduces the level of non-selective absorption. As a result, an increase in the analytical signal/noise ratio and the necessary reduction in the detection limits of elements (at least 2-5 times) and an increase in the reliability of determination results are achieved.

Industrial applicability

The proposed device is intended for electrothermal atomic absorption determination of the microelement composition of powder samples containing lithogenic, biogenic and chemogenic-hydrogen components (for example, rocks, soils, bottom sediments, concentrates and suspensions of natural waters), which is important for solving current environmental and geochemical problems . The device and method are designed to select optimal conditions for analysis by reducing factors affecting the limits of detection and reliability of determination of ultra-small amounts of elements due to intensive processes of thermal decomposition of powdered organomineral samples, interaction of decomposition products, release of gases and the release of large and small aerosol particles from atomizer evaporation zones.

The device has significant advantages over two-stage atomizer devices for analytical spectrometry, for example, using external probes. The use of a multichannel container to accommodate a large mass of the sample under study and subsequent increase in the amount of condensate on the walls of the condensation unit to increase the signal level in the atomization mode makes it possible to avoid repeated replacement of small portions of the sample in the evaporation zone, which requires disassembly and new assembly of units. This reduces a large amount of time for carrying out the entire experiment and saves the electricity required for heating during evaporation of the sample.

Information sources

1. Gilmutdinov A.X., Nagulin K.Yu. A method for elemental analysis of a substance and devices that implement it. Russian patent No. 2370755 (2009).

2. Gilmutdinov A., Sperling M, Welz V. Electrothermal atomization means for analytical spectrometry. US Pat. 1999. No. 598, 19, 12.

3. Zakharov Yu.A., Kokorina O.B. Method of spectral analysis. Russian patent. No. 2273842 (2006).

4. Oreshkin V.N., Vnukovskaya G.L., Tsizin G.I. Improving the metrological characteristics of direct AA and AF determination of Ag, Bi, Cd, Tl in sea and river suspended matter and concentrates (after sorption from waters). Geochemistry 1998. No. 1. With. 108-111.

5. Oreshkin V.N. Electrothermal two-stage atomizer for analytical spectrometry. Russian patent No. 199394 (2020).

6. Technical parameters of graphites of the MPG type (MPG-6,7,8) [https://gigabaza.ru/doc/32233].

7. Karpov Yu.A., Savostin A.P. Methods of sampling and sample preparation. M.: BINOM Knowledge Laboratory. 2003. 248 p.

Claims (8)

1. A method for atomic absorption determination of elements with electrothermal atomization, including the selection of analysis conditions, sample preparation, assembly of a graphite block of an electrothermal atomizer from units designed to form zones of evaporation, condensation, atomization, carrying out the first and second stages of analysis and obtaining results, according to the characteristics of which determine the elemental composition of the sample, characterized in that at the stage of preparing and loading the sample, its total mass from 1 to 25 mg is placed in an evaporation unit and divided into parts with a small volume in the working areas of the container channels, which contains from 5 to 30 independent channels with equal internal volumes of the working zones, which makes it possible to create similar conditions for thermal decomposition, ashing and fractional evaporation of 5 to 30 parts of the sample during the analysis process, where the porous structure of the container material and the large lateral surface of the working zone of a separate channel in relation to the area of its entrance into the working area provides the opportunity to reduce emissions of sample fractions, where the container, when assembling the electrothermal atomizer block, is installed in the working volume of the evaporation unit. 2. The method according to claim 1, characterized in that at the preparation stage the sample is crushed and a particle size of less than 0.1 mm is selected. 3. The method according to claim 1, characterized in that when the container channels are filled, the evaporation unit with the channel and the sample is shaken. 4. The method according to claim 1, characterized in that at the first stage the temperature of the evaporation unit, which includes a multi-channel container with the sample, is gradually increased to values from 1700 to 2200°C during a heating time of 20 to 60 s. 5. The method according to claim 1, characterized in that at the second stage, while heating the evaporation zone continues, the condensation zone is pulsedly heated and, with some advance, the atomization zone to a temperature of 1600 to 2100°C for a heating time of 4 to 8 s. 6. A device for determining chemical elements in powdered organomineral samples by atomic absorption analysis with electrothermal atomization, consisting of an atomic absorption spectrometer (9), power supplies (11), an electrothermal atomizer unit, including functionally related units (1, 4, 6 ), intended for the formation of zones of evaporation, condensation and atomization, characterized in that the unit for forming the evaporation zone (1) is made with the possibility of installing a multi-channel container (2) and with the possibility of placing small volumes of the test sample in its channels (3), while the multi-channel the container contains from 5 to 30 independent through channels (3), made vertically and parallel to each other, where each of the channels provides the possibility of forming an independent zone of evaporation of a small mass of the sample, where the total volume of the through channels (3) of the container (2) provides the possibility of placing a total sample mass from 1 to 25 mg, while the working area of each of the 5-30 channels has a large ratio of the area of the lateral surface of the working area of the container channel relative to the area of the channel inlet, the value of which ranges from 30:1 to 160:1, with In this case, the container is made of porous gas-permeable graphite, which allows for uniform heating and evaporation of small doses of samples placed in the channels, and further reduces the level of particle emissions from narrow channels. 7. The device according to claim 6, characterized in that graphite, which is part of the group consisting of MPG-6, MPG-7, MPG-8, and preferably with a pyrolytic coating, is used as the material for forming the zones of evaporation, condensation and atomization. 8. The device according to claim 6, characterized in that the container material is graphite, which is included in the group consisting of MPG-6, MPG-7, MPG-8.