Characterisation of Unresolved Complex Mixtures of Hydrocarbons

Mark A. Gough

PhD – June 1989

Department of Environmental Sciences, Polytechnic South West

The hydrocarbons of Recent polluted sediments, in-reservoir and laboratory biodegraded crude oils, and certain petroleum products (e.g. lubricating oils) often display "humps" or Unresolved Complex Mixtures (UCMs) when analysed by gas chromatography (GC). Although widespread and often abundant, to date little is known of their detailed molecular composition.

Standard chromatographic methods of isolation of modal aliphatic and aromatic hydrocarbon UCMs from lubricating oils followed by conventional methods of analysis provided little compositional detail. Thus GC and CC-electron impact mass spectrometry (GC-EIMS) was limited to an estimate of carbon number ranges and to the identification of certain series of "biological marker" compounds. However, these were well resolved and were estimated to account for <10% of the total detector response. Further analyses were performed by chemical ionisation-MS (CI-HS), probe distillation EI-MS, field ionisation-MS (FIMS), and elemental analysis; yet the information provided by each was limited to a few "average" molecular types.

In view of the limitations of conventional methods of analysis, alternative methods were adopted. These utilised novel chemical and pyrolytic degradations of the UCM hydrocarbons. Chemical oxidation with Cr03 in glacial acetic acid produced reasonable yields of total revoverable material (40-80%). Furthermore, a high proportion were functionalised (>90%), and many resolved, which allowed their identification by El and CI GC-MS.

Surprisingly, the most abundant products of oxidation of hydrocarbon UGMs were straight chain monocarboxylic acids. This appeared to contradict literature consensus on UCM composition, namely a predominance of highly branched and/or cyclic hydrocarbons. However, from literature reported Cr03 oxidations of hydrocarbons, potential precursor compounds were proposed. These were monoalkyl substituted "T"-branched acyclic and monocyclic alkanes for the aliphatic UCM and alkyl "T"-branched monoaromatic hydrocarbons for the aromatic UCM.

Proposed precursor UCM hydrocarbons were confirmed by synthesis and chemical oxidation under the same conditions. Thus each of the synthetic candidate UCM hydrocarbons [7-n-hexylnonadecane, 9-(2-phenylethyl)-heptadecane, and 9-(2-cyclohexylethyl)-heptadecane] produced n-acids on oxidation with Cr03. Further correlations were found for products of other synthetic alkanes and less abundant UCM oxidation products. For example, n-alkan-2-ones, iso alkan-2-ones, and γ-methyl-7-lactones could all be correlated with methyl substituted acyclic alkyl linkages on UCM hydrocarbons.

The application of chemical oxidation to aliphatic UCMs of varied origin showed the technique has great potential for "fingerprinting" such samples. GC-MS analysis of a selected series of resolved product compounds (alkyl ketones, γ-methyl-γ-lactones) showed good correlations for samples of the same origin, yet distinct differences for UCMs from different sources.

Biodegradation of the three candidate UCM hydrocarbons alongside acyclic isoprenoid alkanes and normal and monomethyl alkanes showed the UCM hydrocarbons were at least as resistant to microbial degradation as the isoprenoid alkanes. In this context it is therefore concluded that the candidate UCM compounds serve as good molecular models for hydrocarbon UCMs.

Copyright 1989 M.A. Gough. All rights reserved.

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