AVS 57th International Symposium & Exhibition
    Vacuum Technology Monday Sessions
       Session VT+MS-MoA

Paper VT+MS-MoA9
Ion Residence Times for Electron-Impact Ion Sources of Mass Spectrometers

Monday, October 18, 2010, 4:40 pm, Room Laguna

Session: Gas Analysis in Vacuum and Process Applications
Presenter: M.F. Vollero, INFICON, Inc.
Authors: R.E. Ellefson, REVac Consulting
M.F. Vollero, INFICON, Inc.
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Electron impact ionization is a common method of ion production for mass spectrometers. The mass spectrum produced is affected by the electron energy and ion residence time in the ionization region. The residence time is the time between the initial direct ionization event and the extraction and focus of the ion(s) from the ionization region into the mass analyzer. During the residence time, additional ionization, fragmentation and ion-molecule reactions can occur. In closed ion sources or other high-pressure ion sources, ion-molecule reactions can be significant; the ions extracted and analyzed can exhibit a bias in inferred gas composition due to the addition or depletion of the species-related ions of interest by the ion-molecule reactions. The presence of a potential well created by the ionizing electron beam and the ion extraction potentials dictate the residence time for ions in the ion source. Models for the depth of the potential well as a function of electron emission current and ion extraction potentials are given and estimated ion residence times are calculated. Evidence for change in residence time with emission current is provided by monitoring mass 80-Ar2+ abundance relative to mass 40-Ar+ at a fixed ion source pressure for different emission currents. Other ion-molecule reactions (e.g. N2+* + N2 gives N3+ + N) and charge-exchange reactions (e.g. He+ + Ar yields He + Ar+ ) are presented together with pressure dependence and magnitude of the reactions. Recommendations are given for ion source operation to minimize biases in compositional analysis of gas mixtures.