Search

The review is devoted to the capabilities of gas chromatography–mass spectrometry in the analysis of high-purity volatile hydrides of naturally occurring and isotopically enriched compositions. Problems related to experimental techniques, chromatographic separation, identification, and quantitative determination of impurities are discussed. The application of this method has significantly expanded data coverage on the nature, number, and limiting possibilities of the determination of impurities in volatile inorganic hydrides. The identified impurities are compounds of various classes. Among them are atmospheric gases; saturated, unsaturated, and aromatic C1–C9 hydrocarbons; chlorine- and fluorine-containing C1–C4 hydrocarbons; homologues and alkyl derivatives of hydrides; Si2–Si4 siloxanes; fluorosiloxanes; oxygen-containing hydrocarbons; sulfur-containing substances; and hydrides of other elements. Gas chromatography–mass spectrometry analysis made it possible to detect the presence of impurities in isotopically enriched silicon and germanium hydrides, which are their molecular isobars. The use of gas chromatography–mass spectrometry in the analysis of high-purity inorganic hydrides makes it possible to reach the detection limits of impurities of 10–8–10–5 mol %.

The impurity composition of arsine is studied by gas chromatography–mass-spectrometry. Impu-rities of permanent gases; carbon dioxide; hydrides; saturated, unsaturated, and aromatic hydrocarbons C1–C6; halogenated hydrocarbons; sulfur compounds; and arsine and diarsine alkyl derivatives are determined. The impurity level of high-purity arsine is 10–6–10–5 vol %. Impurity concentrations in arsine after synthesis and in the fractions extracted in the course of its rectification are rather high and lie in the range 10–6–0.1 vol %. The limits of detection for impurities are 2 × 10–7–2 × 10–4 vol %.

The method of chromatography–mass spectrometry is used to study the impurity composition in monosilane obtained in the reaction of magnesium silicide with ammonium chloride in liquid ammonia. To improve the reliability of the impurity identification, along with the study of pure monosilane samples, we analyzed monosilane fractions isolated upon purification by low temperature rectification. To separate the impurities of permanent gases, hydrocarbons C1–C3, volatile inorganic hydrides, disilane, and alkylsilanes, we used an adsorption capillary column GS-GasPro 60 m × 0.32 mm with a modified silica gel. To separate the homologs of monosilane, siloxanes, and alkylsilanes, we used a column 25 m × 0.26 mm, df = 0.25 μm with a polytrimethylsilylpropyne (PTMSP) sorbent. The impurities were identified by comparison of their experimental mass spectra with the NIST database. In the absence of the mass spectra of analytes in the NIST electronic database or in the case of their low coincidence with the database spectra, the identification was performed using mass spectra and retention times published in the literature. The impurities of permanent gases, carbon dioxide, hydrocarbons C1–C3, volatile inorganic hydrides, monosilane homologies, siloxanes, and alkyl silanes were identified in monosilane. Quantitative determination of the impurities was carried out in the mode of selective ion detection using the mass numbers for which the signal-to-noise ratio was maximal. Their concentrations were calculated using a method of absolute calibration by the peak areas; in the case where reference samples were absent, the concentrations were determined using the dependence of the sensitivity coefficients of their detection on the magnitude of the total ionization cross sections. The detection limits of the impurities range within 1 × 10–5–2 × 10–7 mol %. The accuracy of the analysis was verified by the method of sample size variation. The results of determination of the impurities in monosilane after synthesis, as well as in monosilane purified by low temperature rectification and in the isolated fractions with concentrated higher and lower boiling impurities, are reported.

A method of chromato-mass spectrometry is used to study the impurity composition of monosilane obtained in the reaction of magnesium silicide with ammonium chloride in liquid ammonia. To improve the reliability of the impurity identification along with the study of pure monosilane samples we analyzed monosilane fractions isolated upon purification by low temperature rectification. To separate the impurities of permanent gases, hydrocarbons C 1 – C 3 , volatile inorganic hydrides, disilane, and alkylsilanes we used an adsorption capillary column GS-GasPro 60 m × 0.32 mm with a modified silica gel. To separate the homologues of monosilane, siloxanes, and alkylsilanes we used a column 25 m × 0.26 mm, d f = 0.25 μm with a polytrimethylsilylpropyne (PTMSP) sorbent. Identification of the impurities was performed by comparison of their experimental mass spectra with the NIST database. In the absence of the mass spectra of analytes in the NIST electronic database or a low coincidence of the spectra identification was performed using data of mass spectra and retention time published in the literature. The impurities of permanent gases, carbon dioxide, hydrocarbons C 1 – C 3 , volatile inorganic hydrides, monosilane homologues, siloxanes, and alkylsilanes were identified in monosilane. Quantitative determination of the impurities was carried out in the mode of selective ion detection by the mass numbers having the maximum signal/noise ratio. Calculation of their concentrations was performed using a method of absolute calibration by the peak areas. The concentrations of the impurities in the lack of reference samples were determined using the dependence of the sensitivity coefficients of their detection on the magnitude of the total ionization cross sections. The detection limits of the impurities range within 1 × 10 –5 — 2 × 10 –7 % mol. The accuracy of the analysis was confirmed by the method of sample size variation. The results of determination of the impurities in monosilane after synthesis, in that purified by low temperature rectification, and in the isolated fractions with concentrated higher- and lower-boiling impurities.

A possibility of the use of capillary adsorption columns in combination with mass-spectrometric detection was investigated with the aim to determine the content of impurities in high-purity phosphine. The effect of the volume of the main substance on separation and determination of impurities was studied. This effect mostly manifests itself with respect to impurities closely located to the front of phosphine. During the change of pressure of the introduced phosphine from 0.1 to 1 atm. the relative retention time of carbon dioxide decreases by 1-5 %. In case of impurity compounds with the retention factor not more than 0.5 the retention time does not depend on the volume of the introduced phosphine. The effect of displacement of impurities is accompanied by the increase in efficiency of a column. In substances eluting after phosphine, e.g., arsine, at pressure input of a sample more than 0.1 atm. there is almost a 10 times decrease is observed in efficiency of the column. The limits of detection of substances of 5∙10 -8 -1∙10 -5 vol. % were attained. They are the best values among those available in literature and for some impurities they by 2-1000 times lower. The content of 35 impurities was detected using the calibrating dependencies and estimates of sensitivity on the values of the total ionization cross sections of the determined substances. The content of impurities was determined in the samples of purified phosphine and in the fractions extracted from the top and the bottom part of rectification column. It was found that carbon dioxide, arsine and propylene are the mostly hard-to-remove impurities. Keywords: phosphine, impurities, gas chromatography–mass spectrometry, adsorption capillary columns, detection limit, trueness (Russian) DOI: http://dx.doi.org/10.15826/analitika.2013.17.4.011 V.A. Krylov 1,2 . O.Iu. Chernova 1 , A.Iu. Sozin 1 , A.P. Kotkov 1,3 , G.V. Pushkarev 2,3 1 G.G. Devyatykh Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences, Russia, Nizhny Novgorod 2 N.I. Lobachevsky Nizhny Novgorod State University, Russia, Nizhny Novgorod 3 FGUP NPP “Saljut”, Russia , Nizhny Novgorod

The procedure of gas chromatography/mass spectrometry determination in arsine of high purity was described for impurities of permanent gases, carbon dioxide, C1–C5 hydrocarbons, and hydrides of groups 4–6 of Mendeleev’s periodic table of elements. The detection limits of impurities took values from 2 × 10−5 to 2 × 10−7 mol %.

We have developed analytical techniques for the determination of impurities in isotopically enriched 28SiH4 and 28SiF4. The impurities in SiF4 were first determined by IR spectroscopy, and those in SiH4, by gas chromatography/mass spectrometry. High-sensitivity determination of organic impurities in SiH4 and SiF4 was performed by gas chromatography. SiF4 was found to contain C1–C4 hydrocarbons, hexafluorodisiloxane (Si2F6O), hydrogen fluoride, trifluorosilanol (SiF3OH), fluorosilanes, water, and carbon oxides. The impurities identified in SiH4 include C1–C4 hydrocarbons, disilane (Si2H6), inorganic hydrides, Si2H6O, alkylsilanes, and fluorinated and chlorinated organics. The detection limits of IR spectroscopy were 3 × 10−3 to 5 × 10−5 mol %, those of gas chromatography/mass spectrometry were 8 × 10−6 to 10−8 mol %, and those of gas chromatography were 6 × 10-6 to 2 × 10−7 mol %.

The aim of this work was the development of an as-simple-as-possible instrument for trace moisture concentration measurements in high-purity ammonia. A near-infrared diode-laser-based instrument has been applied to measure the humidity in a process of on-line detection of water in ammonia during industrial purification. The results of water concentration measurements were compared with alternative techniques (primarily dew-point detection) and good agreement was achieved. The long-term sensitivity of such a diode-laser-based instrument was estimated to be 5 ppm. The calculation of the water concentration from measurements of the integrated volume of water contained in the heavy fraction, extracted during the purification process, yields an even lower detection limit of less than 0.1 ppm, depending on the initial ammonia purity.

It is shown that the results of determining methane in high-purity arsine by reaction gas chromatography can be overestimated by more than two orders of magnitude. This overestimation is caused by the formation of methane as a side product of the reaction. Cryogenic preconcentration at the injection stage is proposed to improve the accuracy of determination of methane, and the cryofocusing of impurity hydrocarbons is proposed to improve separation. Detection limits of (3–10) × 10–6 vol % are achieved for C1–C5 hydrocarbons.