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Title: Matrix-assisted ultraviolet laser desorption/ionisation mass spectrometry of macromolecules
Author: Chan, Tak-Wah Dominic
ISNI:       0000 0001 3527 1132
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1992
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The process of ion formation by matrix-assisted ultraviolet laser desorption/ionisation has been studied, using a time-of-flight mass analyser designed and constructed during the course of the present study. Solid matrices and a liquid matrix have been examined. For solid matrices, the signal reproducibility from one sampling position to another has been found to be generally poor. Raman microscopy and scanning electron microscopy studies of the solid samples have shown that matrix-analyte mixing depends critically on the drying conditions. In the case of nicotinic acid as the matrix, nitrogen-purging during the drying process has been found to improve the matrix-analyte mixing. Protein molecule-ion signals have been registered over a wide range of matrix and analyte molar ratios. A near-elimination of matrix-ion formation has been observed at a relatively low matrix-to-analyte ratio for nicotinic acid matrix, but not for sinapinic acid matrix. For the liquid matrix (3-nitrobenzyl alcohol), the signal reproducibility has been found to be excellent. A typical sample could tolerate hundreds of laser shots. A high degree of molecular clustering has been observed for protein-ions at relatively high protein loadings (-100 pmol). 11 has been found that there is a strong dependence of molecule-ion signals on the extraction field. Strong fields have tended to suppress the formation of protein molecule-ions. Initial velocities for both positive and negative protein molecule-ions have been determined to be - 800 ms'1 within the experimental uncertainties. The kinetic energies of the protein molecule-ions after full-acceleration have been determined using a grid-electrode to which a retardation potential was applied. Pronounced energy deficits have been observed. Sample-stage potential and the molecular mass of the analyte have been found to affect the magnitudes of energy deficits. Several factors affecting charging processes in MALDI have been studied. Solution-phase acid/base properties and gas-phase proton affinities has been found not to correlate with ionisation efficiencies in MALDI. A number of synthetic polymers have been studied using MALDI. Only polymers with polar functional groups have been analysed successfully. Based on the present understanding of the desorption / ionisation processes, it is proposed that ionisation rather than the desorption process is the limiting factor in the formation of gaseous molecule-ions from non-polar polymers. The use of an aprotic matrix, it is proposed, should enhance the ionisation of non-polar molecules through electron transfer.
Supervisor: Not available Sponsor: Rosaryhill School (Hong Kong) ; Shell Research Ltd
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry