Mass Spectrometer Sample Line Diffusion Processes

Abstract

We observed transients during the isotopic analysis of carbon dioxide, which could not be attributed to mixing processes because of their asymmetry, and to explain these we were led to consider the effects of diffusion on gas flow in the sample line. In particular, we investigated the propagation by diffusion of a concentration anomaly against a steady gas flow. If fractionation of isotopic(or chemical)species at the leak is to cause a change with time of the composition of the gas in the sample reservoir, then the effects of the fractionation must diffuse backwards as far as the gas reservoir.Using a one-dimensional Green's function, we obtained explicit analytical functions for the steady state concentration variation along a connecting tube between the leak and the reservoir, and for the composition variations with time at the leak. It is shown that the isotopic composition in the sample reservoir need not vary with time, even with a fractionating leak, provided that bulk gas velocity in the connecting tube is large enough. It is also shown that the diffusion process in combination with a fractionating leak can lead to the type of transients we observed.