MATRIX-ASSISTED LASER-DESORPTION IONIZATION TIME-OF-FLIGHT MASS-SPECTROMETRY OF KEROGEN EXTRACTS - EFFECT OF INTERACTIONS BETWEEN SAMPLE STRUCTURE AND MATRIX

This paper describes experiments carried out within the framework of e xamining capabilities of MALDI-TOF-MS in the characterisation of fossi l fuels and related materials, The work has focused on the effect of i nteractions between sample structure and matrix composition on the qua lity of MALDI spectra Solvent extracts from six kerogens have been exa mined by MALDI-MS in the presence of three matrices of different compo sition and in the absence of matrix (laser ablation), The kerogen extr acts have also been characterised by size exclusion chromatography (SE C) and UV-fluorescence spectroscopy (UV-F),In the presence of the thre e matrices (sinapinic acid, 2,5-dihydroxybenzoic acid and alpha-cyano- 3-hydroxycinnamic acid), spectra of extracts from the three geological ly 'younger' kerogens showed larger molecular mass distributions compa red to spectra of extracts from the corresponding 'older' kerogens. Th is result directly contradicted trends observed in the absence of matr ix, where extracts from 'older' kerogens showed larger molecular mass distributions. Results from SEC and UV-F agreed with trends observed i n MALDI-spectra acquired in the presence of matrix, particularly sinap inic acid, By contrast, we recently reported that high mass signal in laser ablation mass spectra of the pyridine-insoluble fraction of a co al tar pitch, obtained in the absence of matrix, closely matched signa ls obtained in the presence of sinapinic acid, Compared to the highly condensed aromatic structure of the coal tar pitch, the kerogen extrac ts (of considerably higher oxygen content) appear less able to readily absorb incident laser radiation (337 mn) in the absence of externally added matrix, The argument is consistent with apparently more efficie nt absorption of incident laser radiation by the geologically 'older' samples, when no matrix was used, Structural characteristics suggest t hat the geologically 'older' samples would be better able to act as 's elf-matrix', Thus, differences in structure and composition of the sam ples appear to go some way towards explaining differences between tren ds observed in the presence/absence of matrix, Dependence of MALDI spe ctra on molecular structure has also been found for two sets of biomas s derived samples, Considerably more work appears warranted to determi ne MALDI-MS parameters appropriate to the analysis of the more oxygena ted, lower rank, samples. (C) 1997 by John Whey & Sons, Ltd.