Magnetically enhanced quadrupole mass spectrometer

Maher, Simon Magnetically enhanced quadrupole mass spectrometer. Doctor of Philosophy thesis, University of Liverpool.

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Abstract

The Quadrupole Mass Filter (QMF) is the key component of a Quadrupole Mass Spectrometer (QMS) first described by Nobel laureate Wolfgang Paul and co-worker Helmut Steinwedel in 1953. The QMS remains a widely used analytical tool for separating charged species in space according to their mass-to-charge ratio, with an extensive range of applications in both industry and research. In the original work of Paul and Steinwedel much emphasis was placed on the fact that the QMF separates ions due to the quadrupolar electric field (comprising dc and ac components) and without the use of magnetic fields. In the present thesis both experimental and theoretical work is presented, to demonstrate that the application of a magnetic field to a QMS enhances device performance. Theoretical work has focussed on the analytical investigation of the Mathieu equation for this particular case. This has led to an extensive simulation study of a QMS under the application of static and dynamic magnetic fields. A method is described that enables the three-dimensional (3D) fields of a simple QMF to be determined to a high accuracy. The technique produces accurate field values and was used to investigate fringe field effects particularly at the entrance to the QMF where they can often be detrimental to performance. Moreover, a two-dimensional (2D) software model was used to study the behaviour of the QMF and modified to incorporate the effect of applying a magnetic field. The software model is capable of accurate simulation of spectra allowing the user to specify QMF dimensions and various applied input signals including, electrode geometries, dimensions and operating parameters. A new performance method has been developed that allows the effective stability diagram of the QMS to be mapped numerically for given operational conditions. This method generates an effective stability diagram which enables instrument behaviour to be predicted and provides a means for optimising performance. Using this method the stability diagram has been effectively mapped for a QMS with a static transverse magnetic field applied in zone 1. This provides an insight into the fundamental workings of this device therefore allowing operating parameters to be optimised accordingly. The ability to scan a range of ion masses and provide uniformity of the effect of the magnetic field is a necessary requirement for a viable mass spectrometer instrument. The proof-of-concept for this novel instrument is demonstrated using a dynamic magnetic field. In order to implement the magnetic field with the conventional scanning method a Helmholtz coil electromagnet was used; this allowed the magnetic field intensity to be altered by varying the current. A custom current ramp was designed and built to operate the Helmholtz coil. This was manually synchronised with a commercial quadrupole mass spectrometer to provide a constant peak width mass scan demonstrating the proof-of-concept for a magnetically enhanced QMS. A magnetically enhanced QMS is a novel instrument that has improved performance with more powerful monitoring and sensing capabilities than conventional QMS instruments available hitherto. One of the challenges of developing and deploying portable MS is that of reducing the size (to render the MS system portable), whilst at the same time maintaining instrument performance in terms of resolution, sensitivity and mass range. Such an instrument in deployment should find a range of applications.

Item Type:	Thesis (Doctor of Philosophy)
Additional Information:	alt_title: Magnetically Enhanced QMS Date: 2013 (completed)
Depositing User:	Symplectic Admin
Date Deposited:	31 Jul 2014 09:38
Last Modified:	17 Dec 2022 01:19
DOI:	10.17638/00017333
Supervisors:	Taylor, Stephen
URI:	https://livrepository.liverpool.ac.uk/id/eprint/17333