The chemical characterisation of the unresolved complex mixture (UCM) observed in the gas chromatograms of biodegraded petroleums.

Abstract

This research was undertaken to elucidate the chemical composition of the unresolved complex mixture (UCM) found in heavily biodegraded crude oils. The isolation of total hydrocarbons fraction by alumina column chromatography of the deasphalted, biodegraded crude oil (West Sak, from the Shublich formation, Alaska), was subjected to a variety of techniques, including chromatographic methods such as column chromatography and TLC to obtain aliphatic and aromatic sub-fractions. Molecular inclusion techniques such as urea, thiourea adduction and 13X molecular sieving were applied to gain some general indication on the UCM component sizes and the distribution of cyclic components, which was also aided by elemental analysis. All fractions obtained by the above-mentioned methods were analysed by gas chromatography (GC) and some by combined gas chromatography-mass spectrometry (GC-MS). Chemical oxidative degradation using chromic acid was employed to derive more specific structural data on the various units present in the UCM. The chemical oxidation products obtained were analysed by GC and GC-MS as full methyl esters. Spectroscopic/spectrometrie techniques such as FT-IR, FT-NMR (1H, 13C), UV, EI-MS and CI-MS were also employed to elucidate the general chemical nature of the UCM components. It is apparent from NMR and IR studies that the UCM is mainly aliphatic (ca. 90%) and comprises a significant proportion ofcyclic systems with probably up to four and five fused rings. In contrast, TLC resulted in a ca. 50:50 mixture between aliphatic and aromatic components in the UCM. An average empirical formula of C4H7 was obtained from the elemental analysis implying that mixture of di- and tri-cyclic components are present in the UCM. Quantification data of the adducted UCM by urea (ca. 50 + 25%) reflected the presence of a significant proportion of branched/cyclic systems with n-alkyl chains attached at one end. With thiourea, it was found that ca. 65% of the UCM was confined to the thiourea adduct, suggesting that branched and/or cyclic structures with sizes up to four fused rings are important contributors to the UCM structure. Aliphatic/aromatic class fractionation with 13X molecular sieve indicated that ca. 70% of the UCM components are aliphatic and ca. 30% are aromatic. Oxidation product data suggested that cyclic systems present are substituted with alkyl chains. The maximum n-alkyl chains would appear not to exceed C20 in length and their frequency distribution seems to maximise around C18. Molecular ion information was not obtained from EI-MS and CI-MS, although evidence was obtained to support a predominantly aliphatic mixed cyclic/acyclic composition.