Species enrichment due to Mach-number focusing in a molecular-beam mass-spectrometer sampling system

Abstract

The analysis and measurements reported here emphasize the need to take into account species enrichments due to Mach-number focusing when sampling gas mixtures using a molecular-beam mass-spectrometer sampling system. Depending upon source conditions and system geometry, the enrichment of a given species may vary from unity to a value which is of the order of magnitude of the ratio of the given-species mass to the major-species mass. Procedures for handling effects of Mach-number focusing on species enrichments are presented. These procedures are more general than previous procedures in that they accommodate, as needed, (a) flow divergence upstream from the skimmer and (b) transition to free-molecule flow (for one or more of the several species) upstream from the skimmer. The measurements support a procedure for handling effects of pressure diffusion near the sampling source which was suggested earlier. An empirical curve for handling modest effects of skimmer interference is included.