Separation, Identification and Characterisation of Complex Mixtures of Naphthenic acids using GC×GC/ToF-MS

Charles E. West, A. Scarlett, D. Jones and S.J. Rowland

Petroleum and Environmental Geochemistry Group, Biogeochemistry Research Centre, University of Plymouth, Drake Circus, Plymouth PL4 8AA

Presented at: 36th International Symposium on Capillary Chromatography and 9th GCxGC Symposium, Riva del Garda, Italy, 27 May-1 June 2012.

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Brief Summary

Characterisation of the ‘supercomplex’ mixtures of polar organic compounds occurring in crude oils and in degraded oil sands is both economically and environmentally important. Particular interest in such mixtures is focussed on the characterisation of 'Naphthenic Acids', which are considered problematic due to: (1) their association with corrosion and deposition problems in oil pipelines and (2) their reported toxicity to some organisms. Indeed, concern has recently been raised over the potential hormonal activity associated with acid-extractable organic matter of process waters derived from oil extraction activities. Consequently, chemical characterisation of the structures of the individual acids which may be responsible for such deleterious effects is vital in order to design effective remediation strategies. Since they remain unresolved by conventional chromatographic methods, the identification of the overwhelming majority of individual carboxylic acids in such complex mixtures has eluded scientists for over a century. Recently however, we successfully used GC×GC/ToF-MS to separate and identify numerous individual acids (as methyl esters) in both commercial petroleum-derived mixtures and oil sands process water (OSPW) samples. A variety of column combinations were investigated, including so called 'reverse-phase' GC×GC using polar 1st dimension columns, each presenting different advantages, to facilitate chromatographic separations. The results demonstrate that refined petroleum and OSPW contain very different distributions of acids, with some structural classes detectable in both. Identification was based on interpretation of the electron ionisation mass spectra of individual esters and where possible, comparison with the 1st and 2nd dimension retention times and mass spectra of synthesised compounds. Interestingly, numerous individual diamondoid and aromatic acids were identified in OSPW, which suggests an unprecedented degree of biodegradation of some of the oil in the oil sands. Our most recent results, presented herein, extend these reports to more detailed studies of the aromatic acids.

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