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Studies of the effects of the amplitude and phase of the RF potential on the trapping process in laser desorption quadrupole ion trap mass spectrometry

University of British Columbia

The thesis is concerned with elucidating the mechanism by which ions are contained by the quadrupole field in laser desorption ion trap mass spectrometry (LD-ITMS). In the past, the process by which ions are trapped has been poorly understood. The several models that have been proposed to explain the dependence of trapping efficiency on the amplitude (q[sub z]) and phase of the RF potential are inconsistent with each other. Previous experimental studies of the trapping process have been hindered by poor reproducibility in the ion production from standard laser desorption samples. A new sample type, electrodeposited polythiophene films, has been developed to resolve this problem. These films have been shown to be capable of providing consistent ion production over many thousands of laser shots. Reliable data have been acquired for the first time showing the dependence of mass spectral signal intensity on phase and qz. The signal intensity for laser desorbed molecular ions was found to be inversely related to q[sub z]. Contrary to previous assertions, this trend is not due to the inability of desorbed ions to penetrate a static "potential barrier" that has a magnitude proportional to q[sub z]. Rather, experimental evidence indicates that collision activated dissociation at high RF potentials is a dominant process that reduces the signal intensity from molecular ions, while increasing those of their lower mass fragments. New experimental data also demonstrate the dependence of LD-ITMS sensitivity on phase. The range of phases over which ions may be trapped, as well as the value of the optimum phase for trapping, depends on q[sub z] and the number of ions produced per laser shot. It is believed that these dependencies are related to the effects of Debye shielding, which are reduced if fewer ions are produced, or if q[sub z] is raised. Presently, it is impossible to determine a priori the optimum value of the phase angle for trapping ions, because the effects of Debye shielding will vary unpredictably over the course of an experiment. Computer simulations utilizing SIMION 6 were also undertaken, to gain further understanding of the experimental data.

Thesis/Dissertation

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2009-05-28

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http://hdl.handle.net/2429/8440

Chemistry

University of British Columbia

UBCV

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