Supercritical fluid chromatography for the analysis of oxygenated polycyclic aromatic compounds in unconventional oils

Abstract

Unconventional oil feeds can be rich in oxygenated organic compounds that will negatively affect the fuel properties if they are not removed during refining. In this study, supercritical fluid chromatography (SFC) was utilised for the combined analysis of polycyclic aromatic hydrocarbons (PAHs) and oxygenated polycyclic aromatic compounds (OPACs). One objective was to chromatographically separate PAHs from OPACs; another to reach a high peak capacity, improved peak shapes and high signal-to-noise ratios (S/N) for OPACs. These objectives were set to establish a non-target analysis method for oxygenated compounds in unconventional oils by SFC hyphenated to a UV detector and a quadrupole time-of-flight mass spectrometer (QTOF-MS) with negative electrospray ionisation (ESI−). Highest peak capacities were observed with a 2-picolylamine column with methanol as modifier, however, a better resolution and S/N were obtained with ethanol and 0.1% formic acid. The elution order for OPACs on all columns followed mainly the polarity of the analytes: furans < aldehydes ≤ ketones < phenols ≤ carboxylic acids. Best separation between PAHs and OPACs was achieved with the ethylene-bridged silica column. The optimised SFC-UV-ESI−-QTOF-MS method was tested on a coal tar middle distillate and a pyrolysis oil where a number of homologous series (e.g. hydroxy-naphthalenes and –benzaldehydes) was tentatively identified.