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UBC Theses and Dissertations

Space charge effects in linear quadrupole ion traps Gao, Cong


Space charge effects in a linear quadrupole ion trap operated with mass selective axial ejection have been investigated. If too many ions are confined in an ion trap, the Coulomb repulsion between ions, or from "space charge", causes ions to shift towards higher apparent mass-to-charge ratio (m/z) in a mass spectrum. This phenomenon, called "mass shifts", can be described by a model with the assumption that the trapped ions have a Boltzmann or thermal distribution of positions in an effective potential. The two parameters in this model, the ion temperature and the effective potential, are determined in this thesis, allowing improved modelling of space charge effects with well determined parameters. Space charge effects were also studied experimentally, including the simplest case (self-space charge) and the most difficult case (space charge from a heavy ion species of high abundance). A new technique to lower mass shifts, dual-frequency excitation, was introduced and optimized. As a result, the instrument performance of linear quadrupole ion trap mass analyzers, including the intensity, limits of detection and the trap capacity, can be improved.

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