MATRIX-ASSISTED LASER-DESORPTION IONIZATION MASS-SPECTROMETRY OF PITCH FRACTIONS SEPARATED BY PLANAR CHROMATOGRAPHY

Molecular mass distributions of coal-tar pitch fractions separated by planar chromatography (P-C) have been compared by matrix-assisted lase r 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,N-dimethylformamide and tetrahydrof uran-toluene. Molecular complexity and average molecular masses were f ound to increase with decreasing mobilities of the fractions. UV-fluor escence spectroscopy showed shifts of peak intensities to longer wavel engths and decreasing quantum yields, suggesting the presence of progr essively greater concentrations of large polynuclear aromatic systems with decreasing mobility of samples in P-C. Fractions immobile in pyri dine 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-molec ular mass (MM) materials of variable polarity in coal-tar pitch. The r esults were consistent with earlier investigations showing the presenc e of high molecular mass materials in coal-derived liquids, MM-distrib utions determined by size exclusion chromatography (SEC) were also obs erved to increase with decreasing mobility of the fractions on P-C pla tes. This qualitative agreement with results from MALDI-MS indicates t hat 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 obs erved by SEC. A number of problems relating to the refinement of MALDI -MS determinations on complex mixtures are discussed.