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The ion conveyor : an ion focussing and conveying device for mass spectrometry

By Matthew Clive Gill


The advent of electrospray ionisation (ESI) has been a major factor driving the\ud revolution which has taken place in the field of biological mass spectrometry over\ud the past twenty years. In this thesis, efforts to improve electrospray ionisation in\ud two respects are detailed. One is the transmission of ions from ambient pressure\ud into the high vacuum of a mass spectrometer. The other is control and influence\ud over making and breaking of bonds, in particular long and weak bonds, during\ud this journey into the mass spectrometer.\ud These improvements to electrospray ionisation have been sought through the\ud development, theoretical analysis and application of the ion conveyor, which is an\ud electrodynamic ion-transport device. The ion conveyor as reported here is a\ud device consisting of stacked ring-electrodes intended for operation within the first\ud and second differential-pumping regions of a mass spectrometer with an\ud electrospray ionisation source. It is demonstrated how the ion conveyor achieves\ud aspects of ion trapping and ion transport through the application of one or more\ud radiofrequency waves to the stacked ring-electrodes. The operating principles of\ud the device are detailed, and related technologies are described. Theoretical\ud simulations have been used to explore for explanations for the behaviour of the\ud device and to make predictions as to the optimum performance of the device.\ud The first experiments with an ion conveyor and subsequent experiments with two\ud evolved designs are described. The operation of the device has been\ud investigated with three different types of mass spectrometer, each with its own\ud design of electrospray ionisation source. These designs are described in detail,\ud and the responses of the ion conveyor to various operating parameters have\ud been characterised. A major finding was that the transmission of the ion conveyor\ud remains significantly high in the absence of all applied potentials. This discovery\ud reveals and emphasises the importance of mechanical forces to ion transport\ud through the ion conveyor.\ud A wide variety of analytes have been used in experiments to characterise the\ud device. The mass spectra of vancomycin hydrochloride presented are of\ud particular note, because they exhibit unusually intense peaks corresponding to\ud the doubly charged dimer-species. The dimer of vancomycin plays the central\ud role in long-standing theories of anti-bacterial action, but has not been observed\ud in previous studies of vancomycin by mass spectrometry.\ud The experimental results support the view that the ion conveyor is a useful\ud technique for the effective transport of ions through the differential-pumping\ud stages of an electrospray ionisation source. The results suggests that the device\ud could be developed through appropriate manipulation of ion-neutral collisions in\ud the higher-pressure regions to preserve and transmit delicate non-covalently\ud bound species and facilitate their accurate measurement by means of mass\ud spectrometry

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