Elucidation of ion motion in a quadrupole ion trap mass spectrometer

Curtis David Cleven, Purdue University

Abstract

The quadrupole ion trap mass spectrometer has matured into a high performance instrument. To improve upon its performance, attention must be focussed on the details of the ion motion that affect the performance of the instrument. This thesis describes the use of laser tomography, pump/probe experiments, simulations, and mass measurements to elucidate the motion of an ion cloud under various conditions. Mass measurements display non-random errors which are attributed to local space charge conditions. Each ion experiences a degree of charge interaction which depends on (i) the total number of charges in the trap, (ii) the number of ions of the same m/z value, and (iii) the abundances and mass differences of neighboring ions. The net field formed by the sum of these ion/ion interactions produces delayed ion ejection. The different environments experienced by calibrant and analyte ions constitute a source of error in external calibration methods. Photodissociation of trapped ions was used as a tool to determine the effects of space charge on spatial distributions of the ion clouds. Tomography experiments show that the ion cloud expands significantly in the radial dimension as the number of trapped ions is increased. This expansion correlates with an increasing error in mass assignment due to delayed ion ejection. Furthermore, both effects appear to be related to the occurrence of compound-dependent effects on ion ejection. The molecular ions of nitrobenzene and n-butylbenzene, and the benzoyl cation each display different mass shifts which correlate with differences in the magnitudes of their radial distributions. A modified geometry is shown to be a means of controlling the compound-dependent positional distributions as well as the corresponding mass shifts. Other experiments utilized a DC pulse to manipulate the ion cloud so that the ion motion could be characterized. The unique characteristic of DC activation is that it forces coherence among all ions. After activation, laser probes were used to characterize the frequency content and internal energies of the ions. The measured secular frequencies, f$\sb{\rm z}$, agree well with those predicted by calculation, and these frequencies can be used to study the dependencies of the ion motion on operating parameters.

Degree

Ph.D.

Advisors

Cooks, Purdue University.

Subject Area

Analytical chemistry

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