Year

1996

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

Synthetic polymers have traditionally been difficult to analyse directly by mass spectrometry due to their high molecular weight and involatile nature. In recent years electrospray ionisation mass spectrometry (ESI-MS) has allowed the detection of intact molecular species with masses up to 150,000 Da, and although a host of biomolecular applications has appeared, few polymer applications have been reported.

In this study, a range of polyester resins was examined by ESI-MS. These resins are major constituents in paints used in the manufacture of coated metal products. The effects of solvent systems, the addition of alkali metal salts and the applied cone potential were examined to determine the optimum conditions for analysis of these resins by ESI-MS. Characterisation of these resins has identified a range of structural features such as branching, cyclic oligomers and products of transesterification. Initial structural assignments were confirmed by tandem mass spectrometry. Samples in which ESI-MS showed the presence of a high proportion of cyclic oligomers were also analysed by Fourier transform infrared and nuclear magnetic resonance spectroscopy, and these data confirmed the ESI-MS results. The propensity for cyclisation of various monomer combinations during resin synthesis observed in the ESI mass spectra was also correlated with probability functions of end-to-end distances calculated by molecular modelling.

Identification of individual polyester species potentially allows the calculation of average molecular weights from the ESI mass spectra of the polyester resins. The validity of these results, however, was questioned upon observation of changes of relative peak heights with variations in sampling cone potentials. The influence of cone potential on the ESI-MS response of individual polyester, polyethyleneglycol and kemptamide ions of varying charge and mass was studied in some detail. These effects were correlated with a mathematical model of gas dynamics and ion mobilities of ions in the expansion region of the ESI source, to reveal several mass and charge-dependent effects which influence the optimum sampling cone potential according to their m/z values. The conclusion of this work is that average molecular weight data cannot be determined directly from the ESI mass spectra of complete polymer samples. The magnitude of this effect is naturally accentuated for polymer samples in which the oligomers span a greater mass range as is the case with the polyesters studied here.

Matrix-assisted laser desorption ionisation mass spectrometry (MALDI-MS) was also used to characterise the polyester resins. Similar species were detected by MALDI- and ESI-MS, although minor variations in the relative abundances observed for different structures were observed which may be indicative of the differences in ionisation mechanisms between the two techniques. Similar mass distributions were also obtained, however the higher masses were observed as doubly-charged species with ESI-MS. Fractionation of the polyester samples by size exclusion chromatography prior to analysis by ESI- and MALDI-MS allowed the detection of higher masses than were observed in the spectrum of complete samples, although several high mass fractions were not detected, presumably due to the limits of sensitivity of both techniques. The mass spectra of individual fractions were used to construct a calibration curve from which average molecular weight values were determined, thus mitigating the effects of polydispersity on the determination of average molecular weights.

Finally, cross/inkers, complete paint formulations and coating extracts were also analysed by ESI-MS to investigate the utility of this technique for characterisation of a wider range of samples relevant to the steel coatings industry

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