Coal tar analysis by mass spectrometry — a comparison of methods

Abstract

The analyses of coal-derived hydropyrolysis tar fractions by a variety of mass spectrometric methods are compared, to identify common ground in the analytical results. Methods compared include high performance liquid chromatography interfaced with mass spectrometry (m.s.) by moving belts and a particle beam, low and high voltage solids injection probe to 350 °C, desorption chemical ionization (d.c.i.) using isobutane, field ionization (f.i.), field desorption (f.d.) and fast atom bombardment (f.a.b.). Aromatic and asphaltenic fractions of two hydropyrolysis tars are compared. The tars were produced at relatively low temperatures under gentle conditions. The results show that for the aromatics, the probe methods, solids injection, d.c.i. and f.i. all show similar masses and mass ranges for the aromatics ; differences reflect differences in sensitivity and conditions of ionization. F.d. involves ionization at a surface followed by desorption of the ion, and signals of different components are seen. For the asphaltenes, some components, mainly multi-hydroxyl derivatives of aromatics, are identified by probe methods (solids injection and f.i.m.s.) but f.d. again detects different material. F.a.b. produces evidence of the structure of the molecules detected — small aromatic and heterocyclic groups loosely linked together, rather than the very large, condensed aromatic structures expected of a high temperature coal tar. Further definition of the molecular structure of asphaltenes requires an improvement in separation techniques.