Quantitative analysis of contaminants in ultrapure gases at the parts-per-trillion level using atmospheric pressure ionization mass spectroscopy

Abstract

Atmospheric pressure ionization mass spectroscopy (APIMS) has been demonstrated as a quantitative analysis technique for gas phase contaminants such as moisture, oxygen, carbon dioxide, and methane in ultrapure bulk gases (nitrogen, argon, and hydrogen). Previous analytical techniques typically provide quantitative analysis above 10–100 ppb detection limits. The APIMS technique offers true sub-parts-per-billion capability, with detection limits as low as 1 ppt. The APIMS works by ionizing a small fraction of the matrix gas in a corona discharge plasma. These matrix gas ions then transfer their ionic charge to impurities through collisions. This transfer of charge is usually thermodynamically favored due to the relatively high ionization potential of the matrix gas. The result is preferential ionization of the impurities, leading to a sensitivity increase of as much as 106 over standard electron impact mass spectrometers. The APIMS is calibrated by mixing parts-per-billion and parts-per-trillion level standard gases using a double-dilution blending system of unique design. Nonlinear calibration relationships based on models of plasma reaction kinetics have been developed to successfully account for nonlinearities seen in the calibration data. A selective moisture removal technique has been developed to eliminate interference effects due to unwanted moisture in the plasma reactor during analysis for other contaminants.