UBC Theses and Dissertations
Mass analysis with quadrupoles with added multipole fields Xiao, Zilan
Conventional mass analysis, mass analysis with islands of stability, and mass selective axial ejection were investigated using linear quadrupoles with added hexapole fields. Experiments agree with computer simulations. When the resolving dc is applied, so that the Mathieu parameter a>0, conventional mass analysis is possible. However, the transmission and resolution are low, and there is peak structure. When a<0, only very low resolution (80) and transmission are seen. With both a>0 and a<0, the use of islands of stability for mass analysis greatly improves peak shape, resolution, and in some cases, transmission. Mass selective axial ejection (MSAE), has also been investigated with linear quadrupoles with added hexapole fields. Trapped ions were ejected using dipole excitation with various ejection parameters: ejection q, excitation amplitude, excitation between the x or y rods, scan speed and scan direction. At high ejection q (q=0.73), excellent peak shapes and resolution up to 4500 were obtained. Scan speeds up to 4000 Th/s were possible. The optimal combination of specific rod set, scan direction, and dipole excitation rod pair was determined. MSAE at lower ejection q was also investigated and found to give lower resolution and spurious peaks on the low mass side of the peaks. Simulations were used to investigate mass analysis in islands of stability with linear quadrupoles with added octopole or hexapole fields. With both a>0 and a<0, the best resolution and peak shape are obtained with operation at the upper tip of the uppermost island formed with the auxiliary excitation. The improvement of resolution and peak shape with a<0, confirms that the use of islands generally can provide a method for mass analysis when added multipole fields otherwise prevent mass analysis. The possibility of operating conventional quadrupole as a mass filter and ion trap as Q3 in a triple quadrupole system is limited because of its low fragmentation efficiency at low pressure. Quadrupoles with added multipoles give greater fragmentation efficiency at 1x10⁻⁴ Torr. The experiments and simulations here suggest that quadrupoles with added hexapole fields might be good candidates where Q3 must be operated as both a trap and mass filter.
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