Biomarker isoprenoid alkane

Advances in petroleum characterization at the molecular structure level by GC-MS methods renewed interest in OSC. Within the past few years, at least one-thousand new and novel OSC that previously were not known to be present in petroleum and bitumens have been reported. Tentative molecular structures inferred from GC-MS and other techniques have been confirmed in many cases by synthesis of authentic reference-compounds. The difficult and time-consuming synthetic work has been crucial in validating many of the novel structures. Another key finding has been that immature bitumens and crude oils (samples that have not received significant thermal stress) differ markedly from the previously known OSC in that they have carbon-skeletons resembling ubiquitous biomarker hydrocarbons (e.g., n-alkanes, isoprenoid alkanes, steranes, and hopanes). This similarity, of course, suggests that the hydrocarbons and OSC have common biogenic precursors. 

Stranger C. (1993) The structure of a new C25 isoprenoid alkane biomarker from diatomaceous microbial communities. Aust. J. Chem. 46, 907-915. 

Other than rt-alkanes, pristane, phytane, and other isoprenoid alkanes that are present in abundance in unaltered oils and easily recognizable by GC and GC-MS analyses, most of the biomarker components are present in trace amounts. Among these, naphthenic hydrocarbons, including tricyclic terpanes, pentacyclic triterpanes, and steranes, have been studied most extensively because they carry more specific information on source and depositional environment than normal and isoprenoid alkanes [21]. 

Fig. 12. Salient features of the GC-MS data for a hydrothermal petroleum (Guaymas Basin, Gulf of California, Mexico) (a) TIC trace of total oil, (b) mjz 191 key ion for hopanes, and ctmjz 217 key ion for steranes. Numbers refer to carbon chain (/ -alkanes) or skeleton, UCM = unresolved complex mixture, Pr = pristane, Ph = phytane, asterisks = other isoprenoids, ot, ot, R, S = configurations of biomarkers.

Recent advances in isotope analysis include the ability to analyse isotopically smaller proportions of individual compounds by the use of more sensitive online isotope ratio mass spectrometers (cf. Merritt Hayes 1994 Merritt et al. 1994) and stable carbon isotope determination of individual n-alkanes, isoprenoids and biomarkers in petroleums is now a standard tool. Despite early work on collection of chromato-graphically separated individual n-alkanes followed by combustion and isotope determination (Welte 1969) it was for a number of years impossible to analyse isotopically long chained alkanes with the same comfort and ease as the... 

Figure 4 Selected example structures (carbon skeletons and functional groups) for biomarkers shown in Table 1 (A) n-Cag alkane (B) n-Ci6 0 alkanoic acid (C) n-C24 alkanol (D) C30 alkanediol (E) C40 2cy isoprenoid (F) C27 -sterol (cholesterol) (G) C32 hopanol.

Compound-specific A C values for 31 different lipid biomarker molecules are shown in Figure 8 for sedimentary horizons corresponding to pre-bomb (before ad 1950) and post-bomb (1950-1996) eras. These organic compounds represent phytoplank-tonic, zooplanktonic, bacterial, archaeal, terrestrial higher plant, and fossil carbon sources. The lipid classes include long-chain n-alkanes, alkanoic (fatty) acids, -alcohols, C30 mid-chain ketols and diols, sterols, hopanols, and C40 isoprenoid side chains of the ether-linked glycerols of the Archaea. 

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