Ultimate limits in the gas composition determination within small sealed volumes by quadrupole mass spectrometry

Abstract

Miniaturization of modern sealed vacuum devices and higher demands for their stable operation require accurate determination of the gas composition in the early stage of their operation as well as after a long operational period. Among a few highly sensitive gas methods, quadrupole mass spectrometry seems to be the most appropriate one for this task as gas amounts are well below 1·10−4 mbar L. In this paper, a new approach, how to prepare a quadrupole mass spectrometer for routine quantitative analysis of small gas amounts, is described. In the first stage, it was calibrated by an innovative in-situ procedure using three different gases: nitrogen, argon and neon. Each gas was admitted into a chamber with a precisely determined volume equipped by a capacitance manometer. By opening the variable leak valve, ion current versus flow rate dependence was determined over three orders of magnitude. Non-linear response at very low flow rates was detected. In the next stage, gas quantities from 3·10−5 mbar L to 6·10−7 mbar L of pure gas were admitted, which proved that by numerical correction of non-linearity, the achieved accuracy of gas quantity determination could be noticeably improved. Rather poor results were on the contrary obtained when synthetic gas mixtures were analysed. Additional calibrations with gas mixtures revealed a strong interference provoked by argon. It enhanced the instrument's sensitivity for nitrogen for a factor of up to 2.3, depending on argon fraction. Considering this effect, further corrections led to a substantially improved accuracy of the nitrogen fraction determination in small gas amount mixtures. In general, quantification of very small gas amounts needs a careful analysis of all contributions influencing the accuracy, as the number of data points is limited and mostly recorded in the non-linear mode of the instrument's operation.