Molecular mass distributions and structural characterisation of coal derived liquids

Calibration data for size exclusion chromatography and results from the analysis of MALDI-TOF mass-spectra of coal derived liquids have been presented. The work provides the means for obtaining much more quantitative information from these two techniques than has hitherto been possible. The polystyrene based calibration has been matched against elution times of a wide range of model compounds (PAH, azaarenes, other nitrogen bearing compounds, several dyes, polars and numerous oxygenated compounds), covering a molecular mass range up to 1086 u. Compounds in all groups appeared to elute with a predominantly size dependent mechanism. With the exception of the somewhat atypical fullerene mixture, none of the model compounds eluted at times so early as to be detected in the excluded region of the chromatogram (i.e. near 10.5 min). Several semi-quantitative methods have been used to establish the upper mass limit of corresponding MALDI-TOF mass-spectra, which can be safely considered as representing signal. Spectra of a coal tar pitch and its pyridine insoluble fraction were analysed. The highest estimates of the high mass limits based on the calculation of number and weight average parameters was >300,000 u. A conservative estimate obtained with a new method based on subtracting multiples of the standard deviation from the signal gave 42,000 and 95,600 u, for the two samples, respectively. Fractions of the same coal tar pitch (separated by planar chromatography) have been characterised, to test for changes in structural features with changing MM-distributions. SEC and MALDI-TOF-MS showed decreasing MM-distributions with increasing mobility in planar chromatography. However, C-13 NMR and pyrolysis-GC-MS showed that the immobile fraction contained greater proportions of aliphatic material compared to more mobile fractions. The mobile material consists of the aromatic systems normally associated with coal tar. The relatively immobile (larger-MM) fractions consisted of aromatic systems too large to elute through the chromatographic column tin pyrolysis-GC-MS) and linked together by aliphatic chains, which were released on pyrolysis and detected as major pyrolysis products. (C) 2000 Elsevier Science Ltd. All rights reserved.