Concentration enrichment in the ion source of a pulsed electron beam high pressure mass spectrometer

Abstract

The magnitude of concentration enrichment of methyl iodide in methane, helium, and argon buffer gases within the ion source of a pulsed e-beam high pressure mass spectrometer (PHPMS) is characterized here by a theoretical model and by kinetic measurements of the gas phase ion/molecule reaction, F − + CH 3I → CH 3F + I −, at 150°C. A relatively simple PHPMS ion source is used in which the magnitude of enrichment is related only to the ventilation of individual molecules out of the ion source by passage through its ion exit and electron entrance slits. It is shown that significant enrichment does occur under typical operating conditions of this source and that the magnitude of enrichment is strongly dependent on the choice of ion source pressure. These observations are explained in terms of the flow conditions thought to exist in the ion source. Recommendations for improving the accuracy of future kinetic and equilibrium measurements by the PHPMS are provided.