Molecular mass distributions of coal-tar pitch fractions separated by planar chromatography (P-C) have been compared by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry, carried out in the absence of added matrix. Solvent pairs used in the P-C separation were pyridine-acetonitrile, pyridine-N,ZVdimethylformamide and tetrahydrofuran-toluene. Molecular complexity and average molecular masses were found to increase with decreasing mobilities of the fractions. UV-fluorescence spectroscopy showed shifts of peak intensities to longer wavelengths and decreasing quantum yields, suggesting the presence of progressively greater concentrations of large polynuclear aromatic systems with decreasing mobility of samples in P-C. Fractions immobile in pyridine gave MALDI mass spectra of low intensity, indicating that larger molecules are less easily desorbed or ionized. The observed similarity of upper-mass limits of planar-chromatographic fractions of different mobility is thought to suggest the presence of relatively large-molecular mass (MM) materials of variable polarity in coal-tar pitch. The results were consistent with earlier investigations showing the presence of high molecular mass materials in coal-derived liquids. MM-distributions determined by size exclusion chromatography (SEC) were also observed to increase with decreusing mobility of the fractions on P-C plates. This qualitative agreement with results from MALDI-MS indicates that reported shifts of polar molecules to shorter elution times in SEC (i.e. to apparently larger-MMs) were not of a sufficient magnitude to distort the relative ordering of MM-distributions of PC-fractions observed by SEC. A number of problems relating to the refinement of MALDI-MS determinations on complex mixtures are discussed.
Recent attempts to reconcile molecular mass (MM) distributions identified in coal-derived liquids by laser desorption-based mass spectrometric techniques and by size exclusion chromatography (SEC) have passed through several stages.' Briefly, the first confirmation of the order of magnitude of MMs found by SEC, using tetrahydxofuran (THF) as mobile phase:-6 has been arrived at during work with a laser-ionization mass analysis instrument (LIMA)? Depending on the nature of the sample, clustered peaks were observed between mlz 500 and m / z 3000, with traces of material extending to the detection limit of the instrument (mlz 12 000). Follow-up work with a matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer (using sinapinic acid as matrix), on a set of coals, coal derived tars and extracts and demineralized kerogens*-" extended the observed MM-ranges to well beyond values observed by SEC in THE Depending on the nature of the sample, MM-ranges of the clusters of peaks were observed between mlz 1000 and mlz 6000. The upper mass limits of the trailing edges of trace material were found to extend to very high mlz values, reaching, in the case of the coal-tar pitch, almost to mlz 200 000.
The range of coal-derived materials that is soluble in the SEC-mobile phase can be considerably extended by using N M P (1 -methyl-2-pyrrolidinone) in preference to THF and pyridine." Our recent comparison of SEC in THF and in N M P confirmsI2 l3 earlier observations by Lafleur and Nakagawa" that a large fraction of coal extracts appears at Author for correspondence.