The Measurement of Very-Low Partial Pressures.

Abstract

The measurement of partial pressures in the ultra high and extreme high vacuum ranges (below about 10-6 Pa) is important for residual gas analysis and the measurement of low-pressure constituents (both contaminants and deliberate additions) in higher-pressure process gases. Mass-spectrometer type instruments-partial pressure or residual gas analyzers (PPAs and RGAs) -are generally used for this purpose. Typically, these instruments consist of an electron-impact ionizer, a quadrupole mass filter, and an ion detector. This paper discusses these elements and other factors that determine the very-low-pressure performance of these instruments-detection limits, mass resolution, and stability and pressure-dependence of the sensitivity. Particular attention is paid to ion-source design and operating parameters, the coupling of ions between the ionizer and the mass analyzer, and the influence of the total pressure on partial pressure sensitivities.