Alkanes, biomarker molecules

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. 

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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