Sequential-impact mass spectrometry; pressure dependence of ion currents from trapped-ion sources

Abstract

Positive ions can be trapped by electron space–charge fields in the ion-source of a mass spectrometer resulting in electron–ion collision sequences. With high electron current density (~0.5 A cm−2) and potential barriers to prevent ion escape along the axis of the electron beam, the trapping time can be increased sufficiently so that collision sequences with as many as 12 steps are observed.A simple model of the behavior of a trapped-ion source has been developed and solutions for the ion currents as a function of pressure obtained. The predictions of the model are compared with experimental results for neon and argon and approximate agreement obtained. Approximate values of cross sections for ionization and excitation of ions can be obtained by fitting the predictions of the model to the measured ion current vs. pressure curves.