Abstract
RationaleRecent trends towards miniature and portable quadrupole mass spectrometry (QMS) entail challenges in instrumental sensitivity, which is influenced by 3D fringe field effects on ion transmission in the Quadrupole Mass Filter (QMF). The relationship of these effects with the gap from the ion source to the QMF entrance (source gap) is significant and little explored. We examine transmission characteristics experimentally and use the results to test the predictive accuracy of a recently developed 3D QMF simulation model. The model is then applied to directly investigate optimal transmission m/z ranges across multiple source gaps.MethodsA portable single filter quadrupole mass spectrometer is used to analyse transmission characteristics across a range of common gases. We use an experimental approach originally proposed by Ehlert, enhanced with a novel method for absolute calibration of the transmission curve. Custom QMF simulation software employs the boundary element method (BEM) to compute accurate 3D electric fields. This is used to study the effects of the source gap on transmission efficiency.ResultsExperimental findings confirm a centrally peaked transmission curve; simulations correctly predict the optimal transmission location (in m/z) and percentage, and extend the experimental trend. We compare several methods for determining fringe field length, demonstrating how the size of the physical source gap influences both the length and the intensity of the fringe field at the QMF entrance. A complex relationship with ion transmission is revealed in which different source gaps promote optimal transmission at differing m/z ranges.ConclusionsThe presented results map the relationship between the source gap and transmission efficiency for the given instrument, using a simulation method transferrable to other setups. This is of importance to miniature and portable quadrupole mass spectrometers design for specific applications, for the first time enabling the source gap to be tailored for optimal transmission in the desired mass range.
Highlights
The Quadrupole Mass Filter (QMF) is widely used in analytical instruments, with applications from biomedical science to environmental monitoring.[1,2] There is an increasing trend towards the development of small and miniature portable quadrupole mass spectrometers for point of use applications.[3-10]
The 'transmission efficiency' of a QMF refers to the proportion of target ions which, having left the source, are successfully transmitted through the QMF.[12]
Gibson et al have demonstrated that the Boundary Element Method (BEM) is both an accurate and a computationally efficient method of calculating QMF 3D electric field values.[27]
Summary
The Quadrupole Mass Filter (QMF) is widely used in analytical instruments, with applications from biomedical science to environmental monitoring.[1,2] There is an increasing trend towards the development of small and miniature portable quadrupole mass spectrometers for point of use applications.[3-10]. The Quadrupole Mass Filter (QMF) is widely used in analytical instruments, with applications from biomedical science to environmental monitoring.[1,2]. There is an increasing trend towards the development of small and miniature portable quadrupole mass spectrometers for point of use applications.[3-10]. To retain performance with small or micro footprint instruments, highly accurate modelling of QMF behaviour for various design geometries is advantageous since it is capable of streamlining experimental prototype testing. The phrase 'ion transmission' is used to refer to the peak maxima of mass scans. The 'transmission efficiency' of a QMF refers to the proportion of target ions (i.e. ions having m/z values corresponding to the peak maximum of the mass scan) which, having left the source, are successfully transmitted through the QMF.[12]. Analysis of QMF behaviour assuming an ideal quadrupole field (that is, a field resulting from infinite length electrodes, referred to as '2D') takes electric field components in all
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