Liquid chromatographic-mass spectrometric analysis of coal tar fractions

Abstract

Conventionally, coal-derived tars are separated by solvent fractionation into pentane solubles, asphaltenes (pentane insoluble, benzene soluble) and preasphaltenes (benzene insoluble) with further separation of the pentane solubles into saturated, aromatic and polar fractions by open-column chromatography on silica.

Normal-phase HPLC, using hexane with a gradient to dichloromethane-methanol (95 + 5) coupled to a mass spectrometer by a moving-belt interface, has indicated that the so-called aromatic fraction of a low-temperature hydropyrolysis coal tar contains some saturates (alkanes and cycloalkanes including hopanes, up to C₆₄) and an appreciable amount of polar materials which may represent 50% of the aromatic fraction, in addition to aromatics from alkylbenzenes to alkylrubicenes as extensive homologous series.

Mass spectrometry with electron impact and chemical ionisation using ammonia and isobutane has given new structural information on the polar materials present in this “aromatic” fraction. Fast atom bombardment of asphaltenes and benzene-insolubles indicated that their structures resemble those of the polar compounds and differ from those of the aromatics. Size-exclusion chromatography confirmed the relative molecular mass ranges detected by mass spectrometry, GC-MS with a capillary column proved inadequate for defining many of the aromatics. The relevance of the work to the structure of coal is considered.