Positive-Ion Electrospray Ionisation Mass Spectrometry of Acetone- and Acetonitrile-Soluble Fractions of Coal-Derived Liquids

The acetone-soluble fraction of a coal tar pitch has been examined using positive ion electrospray ionisation (ESI), by infusion into the ESI stream. The acetonitrile-soluble fractions of a coal tar pitch, a coal digest and a low temperature coal tar have also been studied by high performance liquid chromatography/ESI mass spectrometry. In contrast to positive-ion ESI of proteins, which gives rise to multiply charged ions, this application to fossil fuels, petroleum asphaltenes and humic acids, appears to only give singly-charged ions. The major components of these samples, polycyclic aromatics that could be detected by gas chromatography/mass spectrometry, formed no ions in positive-ion ESI. The only ions detected were formed from azaarenes. Ions up to only m/z 500 or so were detected, although these fractions are known to contain higher-mass species, up to at least several thousand mass units. Fractions of the three samples [coal tar pitch, coal digest and a low temperature coal tar], insoluble in acetonitrile gave no satisfactory ESI spectra. The nitrogen contents of the fractions indicate that azaarenes become more prominent with increasing mass. However, these larger species could not be observed in the ESI mass spectra. Only some of the minor components of these samples could be observed. Size-exclusion chromatography and matrix-assisted laser desorption/ionisation mass spectrometry both indicate considerably larger molecules than those found by ESI-MS.