FRACTIONATION BY PLANAR CHROMATOGRAPHY OF A COAL-TAR PITCH FOR CHARACTERIZATION BY SIZE-EXCLUSION CHROMATOGRAPHY, UV FLUORESCENCE AND DIRECT-PROBE MASS-SPECTROMETRY

A coal tar pitch, extensively studied by other techniques, has been fractionated by planar chromatography using successive development in tetrahydrofuran, chloroform-methanol (4:1, v/v), toluene and pentane. Pitch fractions were distinguished by relative retention. The fractions were characterised in solution by size-exclusion chromatography, UV-fluorescence emission spectroscopy and as the solid on silica, by direct solid-probe mass spectrometry. The method has led to structural information not readily available by direct characterisation of the original mixture itself: (i) A progressive shift of UV-fluorescence spectral maximum intensity and molecular mass with decreasing mobility of the fractions on the planar chromatographic plate, suggesting progressively larger polynuclear aromatic systems; (ii) Nitrogen containing species in the pitch separated from the polynuclear aromatic hydrocarbons which form the bulk of the sample. Neutral and basic nitrogen fractions were defined relative to standards and molecular mass numbers in probe mass spectrometry; (iii) Mass ranges of nitrogen components in these fractions exceed those found previously by GC-MS of the total pitch or its fractions. (iv) Size-exclusion chromatography of the fractions suggests that polar materials are not simply separated by size.