Molecular mass determinations in coal-derived liquids by MALDI mass spectrometry and size-exclusion chromatography

Abstract

This study examined the effect of changes in instrument-related parameters on mass spectra obtained by MALDI-m.s.: ion-extraction voltage, laser power and sample loading. Results were compared with those of size-exclusion chromatography (s.e.c.) using 1-methyl-2-pyrrolidinone as mobile phase. A coal tar pitch and its pyridine-soluble and pyridine-insoluble fractions were used as samples. MALDI-mass spectra were obtained with no added matrix, since the sample absorbed strongly at the laser 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 material was indicated by s.e.c. Increasing the ion extraction voltage served only to accelerate species already ionized, without otherwise disturbing the sample: the use of higher ion extraction voltages must be therefore 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 were indications that for both these parameters, the results might no longer improve (and might eventually deteriorate) beyond optimum values, which appear to be sample-dependent. The low-power spectra revealed more structural information at low masses, where series of homologous peaks could be observed, compared with the spectra at high power.