Variations in molecular mass distributions observed by matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry as a function of changes in the ion accelerating voltage and composition of the matrix have been investigated, using the pyridine-insoluble fraction of a coal tar pitch. With increasing ion extraction voltages (10, 20 and 30 kV) and in the absence of added matrix, spectra clearly showed increases in intensity, particularly at higher masses. The use of higher ion extraction voltages may be considered as providing a more complete inventory of ionized species, apparently significantly enhancing the kinetic energy imparted to larger molecular weight materials. Magnitudes of molecular masses at maximum ion intensity observed using a set of different matrices may be ordered as follows: m/z 1500 [dimethoxy-4-hydroxycinnamic (sinapinic) acid], m/z 1400 (2,5-dihydroxybenzoic acid), m/z 900 [2-(4-hydroxyphenylazo)-benzoic acid], m/z 900 (α-cyano-3-hydroxycinnamic acid), m/z 300 (9-anthracenecarboxylic acid) and m/z 300 (no matrix and a sample loading comparable with the with-matrix experiments). Maximum ion intensities, in the absence of matrix, were found at values which varied from about 300 m/z up to about m/z 1500 as sample loading varied from very low (i.e. suitable for with-matrix operation) up to a visibly black spot. In the absence of a quantitatively based criterion for distinguishing between signal and instrument noise at high mass, accurate high mass limits were not available. However, in each case, ion signals could clearly be observed at values greater than m/z 100 000. The present MALDI spectra do not necessarily show the highest molecular mass materials present in the sample. In view of the observed changes in the spectra with ion extraction voltage and matrix composition, it seems conceivable that higher voltages and/or specific matrices, other than those used in the study, may lead to the detection of larger molecules. © 1997 by John Wiley & Sons, Ltd. Received 23 December 1996; Revised 3 February 1997; Accepted 25 February 1997 Rapid. Commun. Mass Spectrom. 11, 638-645 (1997 Characterization by laser desorption mass spectrometry has considerably extended the range of molecular masses identified in coal-derived liquids. The work was initially undertaken in an attempt to provide independent confirmation for the identification of molecular masses (MMs) up to 4-6000 Da found by size-exclusion chromatography (SEC).1-5 During this preliminary stage of the study, MMs up to the limit of a LIMA (laser ionization mass analysis) instrument (m/z 12 000) were detected in liquefaction extracts and in a coal tar pitch. [6][7][8][9] In these experiments, the reflectron flight path caused loss of detection at high mass but the instrument could not be operated in linear mode. Experiments were conducted at an ion extraction voltage of 20 kV and with a defocused primary beam to prevent the breaking up of sample molecules due to the power of the laser. However, lacking a spectrum additio...

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 of 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.

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