Comparison of electrospray ionisation mass spectrometry with matrix-assisted laser desorption ionisation mass spectrometry and size exclusion chromatography for the characterisation of polyester resins

A series of 17 polyester paint resins have been analysed by electrospray ionisation (ESI) and matrix-assisted laser desorption (MALDI) mass spectrometry to compare the structural and molecular weight data derived from each technique. Optimisation of sample preparation and laser power were found to be important factors in obtaining constant molecular weight distributions by MALDI. With ESI, we have shown previously that the voltage applied to the sampling cone (or orifice) has a major influence on the observed ion distributions. Using sampling cone voltages at which ion currents owing to the polymer were at a maximum, number average (Mn) and weight average (Mw) molecular weight values estimated by ESI were similar to, or slightly higher than those determined by MALDI. The use of size exclusion chromatography (SEC) for fractionation of polyester samples prior to ESI or MALDI analysis, however, showed that both techniques significantly underestimate the average molecular weights of the polyesters. This is consistent with other studies that have shown that ESI and MALDI are unsuitable for determining molecular weights of polydisperse polymers. Differences in the relative abundances of branched and cyclic species in ESI versus MALDI mass spectra were also noted. These differences were accentuated in the data obtained for the SEC fractions which showed reduced sensitivity for branched species in ESI (especially at higher masses) and some discrimination against masses <500 Da in MALDI. These data indicate that the different sample preparation and/or ionisation processes in MALDI and ESI may result in enhancement/suppression of closely related species to differing extents. Hence, where possible, it would be useful to employ both techniques to ensure complete characterisation of complex polymer samples.