RU2444720C1 - Method of measuring trace elements in air stream - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to analytical methods of measuring impurities in a gas, which is based on converting impurity molecules into aerosol particles and subsequent detection of the aerosol, and can be used in high-sensitivity gas analysers used to solve environmental problems, as well as in diagnosis of diseases by analysing exhaled air. The method of measuring trace elements in an air stream involves converting impurity molecules into a condensation nucleus. The nuclei undergo two-step enlargement to size of aerosol particles in supersaturated vapour of the detecting substances in thermodiffusion-type condensation devices and light-scattering of the obtained aerosol is measured. To perform conversion, the stream is heated to 200-600°C, and the detecting substance used at the first enlargement step is semi-volatile organic acids with ionisation constant in water of 4·10^-2-2·10^-1.

EFFECT: high sensitivity and selectivity of detecting ammonia and organic amines in atmospheric air.

1 tbl, 6 dwg

Description

The invention relates to analytical methods for measuring impurities in a gas, based on the conversion of impurity molecules into aerosol particles and subsequent registration of an aerosol, and can be used in highly sensitive gas analyzers necessary for solving environmental problems, as well as in problems of diagnosing diseases by analyzing exhaled air.

A known method for determining microimpurities in a gas, including converting the target component molecules into molecular-size condensation nuclei (MOX) in a converting device, followed by a two-stage enlargement of MOX in condensation devices of a mixing type, first in supersaturated vapors of a highly refractory substance that specifically interacts with MOX (the so-called "manifestation "), Then in supersaturated vapors of diisobutyl phthalate (" enlargement ") with the formation of a monodisperse aerosol and registration of the resulting aerosol Nola by the nephelometric method (Y.I. Kogan, USSR AS, No. 188132, Cl G01N 15/12, 1961).

This method allows you to determine some organometallic compounds in clean, dry air with a sensitivity of up to 10 ^-7 mg / m ³ .

The disadvantage of this method is its relative complexity associated with the need to accurately maintain the constancy of many parameters that affect the coefficient of conversion of the concentration of the target component into the photocurrent of the nephelometer: three carrier gas flows: the main flow and flows through the evaporators of the condensation devices to create a supersaturation (developing and enlarging supersaturation substances in this method is carried out by mixing a hot stream that carries saturated steam, with a cold stream that carries MOX), four ex temperature evaporators and mixers developing and coarsens condensing devices.

Another disadvantage is the high consumption of the carrier gas (2 ÷ 20 dm ³ / min) associated with the proposed method for creating supersaturation, which does not allow the use of preliminary accumulation and gas chromatographic separation of the sample. This, in turn, leads to low selectivity in determining the target components.

The closest to the proposed technical essence and the achieved result is the prototype method for determining microimpurities in gas, which includes converting the target component molecules into molecular-size condensation nuclei (MOJCs) in a converting device, followed by a two-stage enlargement of molecular masses in thermodiffusion condensation devices, first in supersaturated vapors a highly non-volatile substance that specifically interacts with MOX, then in supersaturated diisobutyl phthalate vapors with monodisperse aerosol and registration of the obtained aerosol by the nephelometric method (Kanzhetsian R.A., Katelevsky V.Ya. and Bakunin G.G., Patent R.F. No. 2061219, class G01N 15/00, Bull. No. 15, 1996 )

In this method, the flow rate of the carrier gas is 100 ÷ 500 cm ³ / min, which allowed the use of gas chromatographic separation of the sample and selectively determine in the atmospheric air some organometallic compounds with very high sensitivity.

The disadvantage of the prototype method is the low sensitivity and selectivity, as well as the non-linearity of the response of the detector when determining organic amines in air, which is associated with the use of pH neutral compounds (amino acid or ester) as a developing substance and photo exposure to convert the molecules of the determined components into condensation nuclei.

The objective of the invention is to increase the sensitivity and selectivity of the determination of atmospheric ammonia and organic amines.

The problem is solved by the proposed method for measuring microimpurities in an air stream, including converting impurity molecules into condensation nuclei by heating the analyzed stream to 200–600 ° C, followed by two-stage enlargement of the nuclei to the size of aerosol particles in thermal diffusion condensation devices, first in a supersaturated pair of hardly volatile organic acid with value ionization constants in water 4 10 ^-2 -2 · 10 ^-1, and then a supersaturated vapor of diisobutyl phthalate and light scattering measurement obtained AER sol.

The difference of the proposed method from the prototype is that the conversion of impurity molecules into condensation nuclei is carried out at a flow temperature of 200 ÷ 600 ° C.

Another difference is that as a detecting substance in the first stage of enlargement, volatile organic acids with an ionization constant in water of 4 · 10 ^-2 -2 · 10 ^{-1 are used} .

The invention is illustrated by the diagram (figure 1) and graphs (figure 2-6).

Figure 1 presents a diagram of the measurement of trace amounts of amines in air according to the proposed method, which adopted the notation:

1, 13 - cost drivers;

2, 11, 14 - combined filter for the absorption of vapors and aerosols;

3 - crane switch;

4 - aerosol filter;

5 - gas chromatographic column;

6 - thermal converter;

7 - block two-stage enlargement of the condensation nuclei, including:

8 - showing a condensing device;

9 - enlarging condensation device;

10 - aerosol nephelometer;

12 - recorder;

15 - flow meter (rotameter);

16 - capacity with a desiccant;

17 - flow meter controller;

18 - hub.

The proposed method for determining the trace amounts of amines in air is as follows (see figure 1). At the beginning of the measurement cycle, the micro-supercharger (1) for a certain time sucks the air to be analyzed through an aerosol filter (4), a concentrator (18) and a combination filter (2). At the same time, pairs of substances present in the air, including target components (amines), are adsorbed on the concentrator. The masses m _{i of the} accumulated substances are determined by the formula (1):

,

,

where C _i is the concentration of the I-th component in the analyzed air;

V is the volume of the sample taken;

v - air consumption during sampling;

τ is the sampling time.

Then, with the help of a switch valve (3), the concentrator is connected to the carrier gas line, which consists of a micro-blower (13), a filter (14), a tank with a dryer (16), a flow regulator (17), a gas chromatographic column (5), and a thermal converter ( 6), an enlargement unit for condensation nuclei (7), consisting of a developing condensation device (8) and an enlarging condensation device (9), and a photoelectric aerosol nephelometer (10) with a recorder (12).

After a short pause, in order to calm the flows, a short-term (within several seconds) heating of the concentrator is carried out, as a result of which the accumulated products are desorbed into the carrier gas stream. Further along the flow in the gas chromatographic column (5), the separation of the sample components occurs. Then, in the thermal converter (6) at a temperature of 200 ÷ 600 ° C, as a result of incomplete oxidation by atmospheric oxygen, the amino compounds are converted to condensation nuclei.

Then, in the developing condensation device (8), under the influence of supersaturated vapors of a hardly volatile strong organic acid with an ionization constant of 4 × 10 ^-2 -2 · 10 ^-1 , primary condensation nuclei enlarge to sizes ~ 0.01 μm. In the next condensation device (9), a monodisperse aerosol with a particle radius of ~ 0.25 μm is formed in a supersaturated pair of diisobutyl phthalate, which then enters the “torch” of the aerosol nephelometer (10) to measure its light scattering, characterized by the value of photocurrent I, which is recorded by the recorder (12 ) The nephelometer torch is blown by a stream of air cleaned by means of a filter (14) from aerosol impurities, measured by a rotameter (15). At the outlet of the nephelometer, the air is cleaned of aerosol impurities and the vapors of the detecting substances with a gas mask (11).

A chromatogram of the desorption products from the concentrator will be displayed on the tape of the recorder.

The peak areas of the components S _i or their heights H _{i are} proportional to the masses of accumulated substances m _i , and in accordance with formula (1) and their concentrations in air C _i :

Calibration of the gas analyzer (ie, determination of the constants A or A ¹ in formulas (2) using the proposed method is carried out by supplying to the gas analyzer inlet a steam-air mixture containing known micro-concentrations of amine obtained by diluting its saturated steam a known number of times. Saturated steam sensor thermostated with an accuracy of ± 0.1 ° C.

Figure 2-5 presents examples of determination using the proposed method of nicotine vapor in the air. In all cases, for gas chromatographic separation, a polycapillary column with an OV-1301 silicone phase was used, the carrier gas flow rate of 200 cm ³ / min.).

Figure 2 corresponds to the analysis of air with a concentration of nicotine vapor equal to 2.5 · 10 ^-5 mg / DM ³ ; the developing substance is oxalic acid; conversion temperature 540 ° C; column temperature - 60 ° С (the nicotine peak is marked with an asterisk).

Figure 3 presents the definition of nicotine vapor using picric acid as a developing substance; conversion temperature 540 ° C; column temperature - 61 ° C; the concentration of nicotine in the air is 5 · 10 ^-5 mg / DM ³ .

Figure 4 corresponds to the analysis of an air sample (1 dm ³ ) taken over tobacco (the developer is picric acid, the conversion temperature is 555 ° C, the column temperature is 61 ° C). Peak 343 s corresponds to nicotine, and 430 s, apparently, corresponds to anabazine.

Figure 5 presents the chromatogram of an air sample (3 dm) exhaled by a smoker several minutes after smoking (conversion temperatures and columns, respectively 555 and 61 ° C; developer is picric acid). Peaks with a retention time of 302, 355 and 455 seconds are always present in the exhalations of smokers; the last two correspond to nicotine and anabazine.

Figure 6 presents chromatograms of air samples containing impurities of decane and triethylamine. Column temperature 30 ° C; carrier gas flow rate 150 cm ³ / min. The solid line corresponds to the proposed method (conversion temperature 200 ° C, the developer is picric acid, the concentration of decane is 10 ^-1 mg / dm ³ , triethylamine is 3 · 10 ^-6 mg / dm ³ ), and the dashed line corresponds to the prototype (photoconversion with light from a mercury lamp , the developer α-aminoisovalerianic acid, the concentration of decane and triethylamine are equal to 10 ^-1 mg / DM ³ ). The number 1 indicates the peak of the dean, the number 2 - triethylamine.

Comparative characteristics of the proposed method and prototype are presented in the table.

Table The detection limits of ammonia and amines in the air using the proposed method and the prototype method Developer Ionization constant The limits of detection, mg / DM ³ Nicotine Triethylamine Triisoamylamine Ammonia Oxalic acid 5 · 10 ^-2 1 · 10 ^-6 1 · 10 ^-7 7 · 10 ^-8 3 · 10 ^-6 Mellitic acid 4 · 10 ^-2 1 · 10 ^-7 5 · 10 ^-8 3 · 10 ^-8 2 · 10 ^-6 Picric acid 2 · 10 ^-1 2 · 10 ^-6 1 · 10 ^-7 2 · 10 ^-8 3 · 10 ^-6 2,4,6-trioxybenzoic acid 2 · 10 ^-2 ~ 1 · 10 ^-3 ~ 1 · 10 ^-3 ~ 1 · 10 ^-3 ~ 1 · 10 ^-3 α-aminoisovaleric acid (prototype) 3 · 10 ^-11 1 · 10 ^-2 1 · 10 ^-2 1 · 10 ^-2 1 · 10 ^-2

As can be seen from the graphs (figure 2-6) and table, the proposed method in comparison with the prototype method has the following advantages:

- it allows you to determine amines in the air with a sensitivity of 3000-500000 times higher than in the prototype method [cf. the detection limits (Ob) of amines in the first three rows of the table corresponding to the proposed method, with the Ob values in the last row corresponding to the prototype method];

- the selectivity of the determination of amines with respect to other classes of organic compounds present in atmospheric air, for example, hydrocarbons, in the proposed method is several orders of magnitude higher than that in the prototype method (see chromatograms in Fig.6, of which it is clear that the proposed method it allows triethylamine to be detected against a background of a 30,000-fold excess of decane, whereas in the prototype the decan interferes with the determination of triethylamine in a comparable concentration).

With respect to the claimed range ionization constants K _and, with the acid value _and K is greater than 2 × 10 ^-1 (some arylsulfonic etc.) are sensitive to moisture (blur even in a relatively dry air is used as carrier gas) and work unstable in time. And when using acids with _a K value less than 4 x 10 ^-2 (cm-2,4,6 trioksibenzoynuyu acid Table..) - detection sensitivity drops sharply amines.

Going beyond the stated temperature range (200-600 ° C) leads to:

- at a temperature below 200 ° C to a decrease in the sensitivity of the determination of hardly volatile amines such as nicotine due to the strong broadening of its GC peak,

- at temperatures above 600 ° C, both the sensitivity and selectivity of the determination of amines decrease due to the strong increase in the background of the thermal converter.

Thus, the proposed method allows to selectively determine in the air ammonia and amines with a detection limit of 3 · 10 ^-6 -2 · 10 ^-8 mg / DM ³ . The method can be used in drug search tasks, to control pollution of the atmosphere, soil and water with toxic amino compounds, for example, asymmetric dimethylhydrazine, and also in the diagnosis of certain diseases by analyzing expired air.

Claims (1)

A method of measuring microimpurities in an air stream, including converting impurity molecules into condensation nuclei, subsequent two-stage enlargement of the nuclei to the size of aerosol particles in supersaturated pairs of detecting substances in thermal diffusion type condensation devices, and measuring the light scattering of the obtained aerosol, characterized in that the stream is heated to 200 to convert ÷ 600 ° C, and as a detecting substance in the first stage of enlargement, volatile organic acids with a value of m ionization constants in water 4 · 10 ^-2 · 10 -2 ^{to -1.}

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