CHROMATO-MASS-SPECTROMETRIC STUDY OF THE IMPURITY COMPOSITION OF HIGH-PURITY MONOSILANE OBTAINED FROM MAGNESIUM SILICIDE

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 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 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 C₁ – C₃, 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.

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