Comparison of the fractionation of a coal tar pitch by solvent solubility and by planar chromatography

Abstract

Two methods of fractionation of a coal tar pitch, one based on solvent solubility separation and the other on preparative thin layer chromatography were compared for effectiveness of the separation. The fractions were characterised using size exclusion chromatography (SEC) and UV-fluorescence spectroscopy (UV-F). Fractionation by planar chromatography was found to give a better separation and to use less solvent than the solvent separation method. Both SEC and UV-F clearly showed less overlap in fractions separated by planar chromatography, compared to solvent fractionation. The cleaner separation of the heaviest fraction is particularly relevant for the mass spectroscopic investigation of large molecular mass materials, where the presence of lighter materials was observed to suppress high-mass signal. The close agreement observed between SEC and UV-F is consistent with previously reported correlations for coal tar pitches. However, the relevant SEC work was performed using pyridine as the mobile phase. We have found that 10%–15% of coal tar pitches cannot be dissolved in pyridine whilst total dissolution of coal tar pitch may be achieved using NMP. The calibration of analytical columns operated with NMP would be expected to lead to more accurate evaluations of molecular masses in these complex mixtures.