Effect of polydispersity on the characterization of coal-derived liquids by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: inferences from results for mixtures of polystyrene molecular mass standards

Abstract

Seven polystyrene molecular mass standards have been run in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) singly, in pairs, in groups of three and as a mixture of the seven distinct samples. All standards had polydispersities or near unity. When run singly or in pairs, MALDI-spectra of all standard polymer samples could be observed clearly and showed approximately equal areas. Mixtures of multiple standards have been run at variable ion extraction voltages: when the voltage was reduced below 25 kV, low molar mass samples tended to predominate in the spectra, at the expense of the higher molar mass standards. Peaks due to the high mass standards were still observable above the noise level, but (for equimolar mixtures) areas under the peaks were no longer comparable. With the full set of seven samples, discrimination against intensities of the higher mass standards could be clearly observed, even at the highest ion extraction voltage available (30 kV). With increasing ion extraction voltage the MALDI-spectra of a coal tar pitch and its tetrahydrofuran-insoluble fraction showed increasing intensities of higher mass ranges. Due to the wide polydispersity of coal derived samples, MALDI-spectra would be expected to severely underestimate the proportion of high mass materials present. Further work on mixtures of different types of polymer standards would clearly be useful; for coal derived liquids, more extensive fractionation than that used here would seem advisable in order to reduce sample polydispersity. © 1997 John Wiley & Sons, Ltd.