| Popis: |
Advances in miniature and portable quadrupole mass spectrometry (QMS) have resulted in smaller instruments but often with reduced sensitivity, and this can restrict the applicability of portable MS for in-field applications. Sensitivity is influenced by the three-dimensional (3D) fringe field effects on the ion transmission characteristics of the Quadrupole Mass Filter (QMF). There is a requirement for a greater understanding of these effects. Particularly, the influence of the gap from the ion source to QMF entrance (source gap) on fringe field length, and its effects on ion transmission, are significant and little explored. The work presented in this thesis analyses the influence of fringe fields on ion transmission using a recently developed 3D QMF simulation model. In conjunction with simulations, experiments were carried out, using a single filter quadrupole mass spectrometer, to examine transmission characteristics across a range of common gases. To achieve this, an approach originally proposed by Ehlert was enhanced with a novel method for calibrating absolute transmission curve. Experimental results are compared with the predictions of the simulation model, which is used to study the significance of the source gap on fringe field effects. The effects of axial velocity in the fringing field region at the QMF entrance are also numerically simulated for multiple ion trajectories. The analysis of fringe fields shows a complex pattern of ion acceptance based on the initial ion entry conditions. The effect on ion transmission of the initial phase angle at which the ion is injected into the QMF has been shown to be of less importance than was predicted by the two-dimensional (2D) QMF model. Experimental measurements confirm a peaked transmission curve and agree well with the predictions of simulations. Different methods for determining fringe field length are discussed. Further analysis demonstrates the influence of the physical source gap length on both the length and intensity of the fringe field at the QMF entrance, and thereby ion transmission characteristics. For the first time, it is shown that, the source gap can be tailored for optimal transmission at the mass range of interest. Axial velocity perturbations are visualised in the fringe field region for different source gaps. The results presented are of importance to those within industry and academia working on the design of miniature and portable QMS targeting specific applications. The use of portable MS in the detection and monitoring of volatile organohalogens is demonstrated as an important environmental application. The wider significance of this work is that it allows QMS transmission efficiency (and hence instrument sensitivity) to be maximised for given instrument parameters and/or analytes (m/z values). |
