UBC Theses and Dissertations
A new tandem quadrupole mass analyzer Du, Zhaohui
The thesis describes an inductively coupled plasma mass spectrometer system with a quadrupole operated in the third stability region with Mathieu parameters (a, q) « (3, 3). The transmission-resolution curves for ions from three elements, Mg, Co and Ga with operation in the first and the third stability region were compared. A limiting resolution of 4000 was obtained with Co (compared to 800 for the first region). For the same resolution (less than 800), the sensitivity with operation in the first region is about 2-7 times higher than that of the third stability region at the upper tip (M tip). It was also found that operation at the upper tip usually gave better transmission for the same resolution and a higher limiting resolution than operation at the lower tip (S tip). High abundance sensitivity (>107) was obtained at medium resolution (R[sub ½] = 500 to 1000) for operation at the upper tip of the third region. The work done with a novel tandem quadrupole mass analyzer comprises most of this thesis. The tandem analyzer consisted of two quadrupoles in series. Each was operated as a mass analyzer. The resolution in each quadrupole was usually low and a mass offset was introduced between the two quadrupoles to give a mass spectrum. The resolution of the tandem mass analyzer was changed by changing the mass offset between the two quadrupoles. It was found that the sensitivity could be increased by placing the two quadrupoles as close together as possible (usually 2mm spacing) without an intervening ion lens, and with the rf voltages phase locked. A phase shift between the rf voltages applied to the two quadrupoles also improved the sensitivity. A comparison of the transmissionresolution curves, with the quadrupoles operated in both the first and third stability regions showed that for the same resolution, the sensitivity of the tandem quadrupole was comparable to that of a single quadrupole for the same region. Operation in the third stability region is advantageous for elemental analysis because the tandem analyzer exhibited better abundance sensitivity on both the low and high mass sides of a peak and better peak shapes under conditions where a single quadrupole produced a long peak tail or peak structure. Scattering losses in the tandem analyzer were half of those in a single quadrupole. Tests with a low mechanical quality tandem quadrupole analyzer showed better resolution than a single low quality quadrupole operated in the third stability region for atomic ions (mle < 250). It also provided better resolution than a single low mechanical quality quadrupole for molecular ions (m/e = 609) with an electrospray ion source in the first stability region.
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