RU2227291C2 - Process determining contents of impurities in xenon - Google Patents

Abstract

FIELD: analysis of materials. SUBSTANCE: characteristic feature of process lies in conducting concentration of impurities in region of high vacuum of mass spectrometer after metering valve by way of partial condensation of xenon at temperature of 77 K. In this case impurities present in xenon and displaying higher values of pressure of saturated vapors and lesser as compared with xenon pressure in flow enter freely ion source of mass spectrometer. EFFECT: markedly raised sensitivity and accuracy of mass spectrometric process of measurements. 3 tbl

Description

The invention relates to the field of analysis of materials, namely to methods for determining the content of the following impurities in xenon: nitrogen (N ₂ ), oxygen (O ₂ ), argon (Ar), krypton (Kr), carbon monoxide (CO), methane (CH ₄ ), perfluoromethane (CF ₄ ).

Known methods for determining impurities in xenon chromatographic method and direct (classical) mass spectrometric method.

When determining impurities in xenon (Xe) using a gas chromatograph [2, 3], various types of chromatographic columns and detectors are used. Moreover, one of the drawbacks of the method is the need to use several types of detectors to analyze all of the above compounds. For example, a thermal conductivity detector is used to analyze krypton and nitrogen, and a flame-ionization detector is used to analyze methane and carbon monoxide. For the calibration of the chromatograph, it is also necessary to use a large set of calibration mixtures (CS) of xenon with a known impurity content. All this leads to a significant increase in time spent on analysis.

A more efficient method for the determination of impurities in xenon is the direct mass spectrometric method [1], in which a xenon sample is introduced directly through a needle dispenser into the ion source of a gas mass spectrometer. Under the influence of an electron impact, a fragmentation and ionization of xenon molecules and impurities occurs in an ion source.

When determining the content of impurity compounds in xenon by a direct mass spectrometric method, the following actions are performed: they inject the analyzed gas sample Xe into the vacuum system of the mass spectrometer to the metering valve; open the metering valve and let gas into the ion source of the mass spectrometer; register the mass spectrum of the sample; the magnitude of the output signals of the mass spectrum corresponding to the pressures Xe and impurity compounds in the inlet system, calculate the molar fraction of impurity compounds according to the following formula:

where J _i is the output signal corresponding to the content of the i-th impurity, V;

K _i - coefficient of relative sensitivity for the i-th impurity;

To _Xe - coefficient of relative sensitivity Xe;

J _{Xe +} - the sum of the values of the output signals corresponding to the content of xenon isotopes: 124, 126, 128, 129, 130, 131, 132, 134 and 136 amu, V.

Table 1 shows the used coefficients of relative sensitivity of the analyzed compounds and the recorded mass lines of the mass spectrum.

The main disadvantage of the known mass spectrometric measurement method (adopted as a prototype) is the low detection limit of impurities, which, expressed in molar fractions, is 0.01% and is due to the maximum allowable pressure of the analyzed gas mixture in the ionization chamber of the mass spectrometer of the order of 1 · 10 ^{- 4} mmHg

The problem to which the invention is directed, is to increase the accuracy and sensitivity of the mass spectrometric measurement method.

The problem is solved due to the fact that in the known method for determining impurities by direct mass spectrometric method, which consists in direct input of the analyzed gas mixture through a needle dispenser into the ion source of the mass spectrometer, cryogenic concentration of impurities in the high vacuum region of the mass spectrometer behind the metering valve . Concentration is carried out by partial condensation of xenon at a temperature of 77K (temperature of liquid nitrogen). In this case, impurities having higher saturated vapor pressures and lower pressure in the flow compared to xenon freely pass into the ion source of the mass spectrometer. The degree of concentration of impurities depends on the flow of Xe through the metering valve and averages 1000-2000 times. The Xe pressure in the ion source of the mass spectrometer does not depend on the Xe flow rate through the needle dispenser and is determined by the temperature value of the part of the inlet system (concentrator) in which Xe condensation occurs. The concentrator is a copper tube, the length of the cooled part of which is 30-50 cm.

The implementation of this method for determining impurities in xenon has become possible due to two factors due to the physical properties of xenon and analyzed impurities. First, the xenon pressure at 77 K is ~ 0.5 Pa (~ 3 · 10 ^-3 mm Hg) and is the "working" pressure in the gas mass spectrometer inlet system behind the needle valve. Secondly, the analyzed impurities have lower boiling points in comparison with xenon (Table 2) and, therefore, higher saturated vapor pressure at a given temperature, which makes their passage through the concentrator unobstructed.

Determination of the molar fraction of impurities is carried out similarly to the method described in [4], by the relative method using calibration mixtures (CS) with a known content of impurities. In this case, a sequential inlet is carried out through the concentrator KS and samples.

The molar fraction of impurities (C $\genfrac{}{}{0ex}{}{\text{n}}{\text{i}}$ ) in a xenon sample in percent is determined by the formula

where c $\genfrac{}{}{0ex}{}{\text{cop}}{\text{i}}$ - the molar fraction of the i-th impurity in the COP,%;

I $\genfrac{}{}{0ex}{}{\text{n}}{\text{i}}$ - the value of the output signal of the measuring system of the mass spectrometer, proportional to the proportion of the i-th impurity in the sample Xe, B;

I $\genfrac{}{}{0ex}{}{\text{cop}}{\text{i}}$ - the value of the output signal of the i-th impurity in KS, V;

P _n ; P _cc - sample pressure Xe and KS before the metering valve, Pa.

For those impurities that are not contained in the COP, the calculation of the molar fraction is carried out according to the formula using the coefficients of relative sensitivity (Table 1):

where K _i , K _j is the coefficient of relative sensitivity for the i-th and j-th impurities.

When determining the content of impurity compounds in xenon by a mass spectrometric method with cryogenic concentration of impurities, the following actions are performed: the analyzed gas sample Xe is introduced into the vacuum system of the mass spectrometer to the metering valve; open the metering valve and let gas through the hub into the ion source of the mass spectrometer; register the mass spectrum of the sample and record the pressure of the sample in front of the metering valve; pump the vacuum system of the mass spectrometer; inject the calibration mixture into the vacuum system of the mass spectrometer to the metering valve; register the mass spectrum of the COP and record the pressure of the COP in front of the metering valve; the magnitude of the output signals of the mass spectrum corresponding to the content of impurity compounds in the sample and the calibration mixture, calculate the molar fraction of impurity compounds according to the formula 2 or 3.

Example. The molar fraction of the following impurities in the calibration mixtures of xenon was determined: N ₂ ; O ₂ ; Ar; Kr; CF ₄ ; CH ₄ . Mixtures were prepared on the basis of pure substances. The results of the determinations calculated by the formulas (2, 3) are shown in table 3.

Analysis of the data confirms that in the claimed invention in comparison with the prototype provides a higher sensitivity mass spectrometric determination of impurities: the detection limit is about ~ 2 · 10 ^-6 %; the accuracy of the mass spectrometric method is increased: the relative error of a single determination does not exceed 15%.

Sources of information

1. Xenon. Technical conditions GOST 10219-77, -M .: IPK publishing house of standards.

2. Shekhovtsov N.A. Isotopic mass spectrometry. -M .: Atomizdat, 1971.

3. Agafonov I. L., Ninth G. G. Mass spectrometric analysis of high purity gases and vapors. -M .: Science, 1980.

4. Application No. 2000124840. A method for determining impurities in uranium hexafluoride and a device for its implementation. Priority from 09.29.2000, the decision to grant a patent of the Russian Federation from 01.21.02.

Claims (1)

A method for determining the content of impurities in xenon, comprising sequentially introducing into the ion source of a gas mass spectrometer a calibration mixture with a known content of impurities and a sample with an unknown content of impurities, recording mass spectra of impurities in the calibration mixture and sample, comparing analytical signals of impurities and determining the molar fraction of impurities in a xenon sample, characterized in that the registration of mass spectra is carried out after partial condensation of xenon at a temperature of 77 K in the high vacuum region of a spectrometer for the metering valve, and impurities having lower boiling points freely passed into the ion source of the mass spectrometer.