MOLECULAR-MASS DETERMINATIONS IN COAL-DERIVED LIQUIDS BY MALDI MASS-SPECTROMETRY AND SIZE-EXCLUSION CHROMATOGRAPHY

This study examined the effect of changes in instrument-related parame ters on mass spectra obtained by MALDI-m.s.:ion-extraction voltage, la ser power and sample loading. Results were compared with those of size -exclusion chromatography (s.e.c.) using 1-methyl-2-pyrrolidinone as m obile phase. A coal tar pitch and its pyridine-soluble and pyridine-in soluble fractions were used as samples. MALDI-mass spectra were obtain ed with no added matrix, since the sample absorbed strongly at the las er wavelength, 337 nm. Higher extraction voltages (up to 30 kV) led to the observation of higher masses in the whole pitch and its pyridine- insoluble fraction, where the presence of higher-molecular-mass materi al was indicated by s.e.c. Increasing the ion extraction voltage serve d only to accelerate species already ionized, without otherwise distur bing the sample: the use of higher ion extraction voltages must be the refore be considered as providing a more complete inventory of already ionized species. Increasing the laser power level and sample loading on the target led to similar changes in the spectra, although there we re indications that for both these parameters, the results might no lo nger improve (and might eventually deteriorate) beyond optimum values, which appear to be sample-dependent. The low-power spectra revealed m ore structural information at low masses, where series of homologous p eaks could be observed, compared with the spectra at high power. (C) 1 997 Elsevier Science Ltd.