Identification of large molecular mass material in high temperature coal tars and pitches by laser desorption mass spectroscopy

Molecular masses of up to 12 000 Da have been identified by laser desorption mass spectroscopy (l.d.-m.s.) in a number of coal tars and extracts. Observed molecular masses were greater than any hitherto detected in coal-derived liquids; fractions of high temperature coke oven tars have been used for verification of initial findings. Low molecular mass materials have been analysed using similar experimental procedures, to confirm the absence of laser or other apparatus derived high molecular mass clusters. Spectra extending to 12 000 Da show an envelope between 1000 and 3000 Da and a low molecular mass envelope between 12 and 400 Da, the latter apparently indicating the presence of carbon clusters. At high laser power densities, the 1000 to 3000 Da envelope was observed to lose intensity, with parallel gain in intensity of the low mass carbon cluster envelope, suggesting increased breakdown of substrate molecules under increasing laser power density. Comparisons with results from g.c.-m.s. and probe m.s. are presented, showing masses up to 600 Da. Limited structural information about the large molecular mass materials can be inferred from the present results, suggesting the presence of small aromatic groups with mass differences corresponding to methyl groups, ethylene bridges and benzo groups. © 1993.