Hydrocarbons, biomarker molecule

The biomarker molecules are particularly resistant to microbial attack, and thus the ratio of other hydrocarbon components to the biomarker will decrease as the crude oil is biodegraded (Wang et al., 1994). In the case of an ongoing oil discharge into the soil, this ratio will be highest nearest the source and will decrease with increasing distance from the source. Thus, the ratio may be used to locate the source of the contaminant (Whittaker et al., 1995). 

Fig. 8. The distribution of fraction-specific maturity for the oils and condensates in the data set. Mean maturities for the data set based on representative biomarker, aromatic hydrocarbon and light hydrocarbon parameters are indicated, based on parameters proposed by Radke (1988) and Schaefer Littke (1988). Biomarker maturities are based on proprietary source rock correlations. The cyclic biomarkers are generated earlier than the aromatic hydrocarbons with the quantitatively most abundant light hydrocarbons being generated, on average at higher maturity than the more structurally exotic molecules.

Biological marker (biomarker) molecules are compounds that have distinctive biotic sources and that retain their identity after burial in sediments, even after partial alteration. Most biomarkers are lipids, which are hydrocarbons and hydrocarbon-like molecules. Because of their low susceptibility to microbial degradation compared to other types of organic matter, saturated hydrocarbons — those having only carbon-carbon single bonds — can record many aspects of the depositional history of organic matter sources in lake sediments. A number of detailed reviews of the biomarker contents of lake sediments exist (e.g., Barnes Barnes, 1978 Cranwell, 1982 Johns, 1986 Muller, 1987 Meyers Ishiwatari, 1993). 

Mineral Oil Hydraulic Fluid. Limited studies were located that suggest biomarkers of exposure to mineral oil hydraulic fluids. No data that indicate quantitative or qualitative biomarkers of exposure to mineral oil hydraulic fluid were located. Mineral oil (hydrocarbons containing 15-30 carbon atoms per molecule) is a major component that is common to all mineral oil hydraulic fluids. Following exposure to food-grade mineral oil, most of the administered radioactivity was excreted in the feces as mineral oil (Ebert et al. 1966). Although the presence of mineral oil is a biomarker of exposure to mineral oil hydraulic fluids, it is also a biomarker of exposure to other readily available products that contain mineral oils. 

The separated hydrocarbon fraction is then analyzed by capillary column gas chromatography, which simultaneously separates the mixture of hydrocarbons into its components and measures how much of each compound is present. Individual n-alkanes can usually be identified by their relative retention times in gas chromatograms, although sometimes branched hydrocarbon molecules will share the same retention time as an n-alkane. For this reason, positive identification of biomarker hydrocarbons require the combination of gas chromatography and mass spectrometry using verified mass spectra. 

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