Application of combined gas chromatography-mass spectrometry to the analysis of organic products of biogeochemical significance

A large number of organic products of diverse origins have been studied by gas chromatography and by combined gas chromatography-mass spectrometry. This combination technique allows the rigorous structural characterization of the individual compounds present in multicomponent mixtures. Its application to the determination of bioorganic lipids provides data on the biological order at the molecular level. Thus important correlations, relevant to the distributions, origins and biosynthetic pathways of the compounds, can be established. Among the lipids, the hydrocarbons and fatty acids have been the most extensively studied but in general the interest has been centered around the isoprenoid hydrocarbons because of their structural specificity and diagenetic stability. Also the characteristic bimodal distribution of the isoprenoids in natural products was found to be in line with their presumed biological origin. The correlations and distributional criteria established as a result of this investigation are based on the following observations: Microorganisms contain significant amounts of aliphatic hydrocarbons and fatty acids. Branched paraffins belonging to the iso and anteiso series have been identified in a few cases. In general, isoprenoids are absent. All of the microscopic algae analyzed contain high amounts of n-C[lowered 17] and in two instances, high molecular weight olefins (C[lowered 17]-C[lowered 33]) were found to be present in relatively high proportions. Several isomeric unsaturated isoprenoids were found in higher plants although their saturated counterparts are absent. High amounts of isoprenoids (pristane and squalene) were found in shark liver oil. The saponifiable fraction contains mainly normal unsaturated fatty acids. In general the hydrocarbons and fatty acids of all these products are unrelated in their distributions. Relatively large amounts of isoprenoids from the C[lowered 14] to C[lowered 21] as well as of n-alkanes were identified in a crude oil. The sample contains too, iso and anteiso alkanes and cycloalkanes. Paraffins, olefins, and methyl alkanes ranging from C[lowered 10] to C[lowered 33] were identified in a concentrate of tobacco smoke. The isoprenoids in this sample are mainly unsaturated. Terrestrial graphites contain n-alkanes, small amounts of methyl alkanes and isoprenoids from C[lowered 14] to C[lowered 22]. The outside parts of the graphite contain more hydrocarbons than the inner parts. The possibility of abiotic synthesis of isoprenoids has been investigated using the Fischer-Tropsch reaction between hydrogen and carbon monoxide. Although n-alkanes and methyl branched isomers have been obtained in the products of more than 50 synthesis experiments, no traces of isoprenoids are present. A detailed study of a number of hydrocarbons observed in gas chromatographic analyses of six carbonaceous chondrites has resulted in the identification of nine homologous series of isomeric alkanes in addition to the n-alkane series. Two of them show isoprenoid structures corresponding to 2,6,10-trimethyl and 2,6,10,14-tetramethyl alkanes, with members in the C[lowered 14]-C[lowered 19] and C[lowered 19]-C[lowered 21] ranges, respectively. The rest is made up of five series of monomethyl alkanes (6-,5-,4-,3-,2-methyl alkanes) and two of monocycloalkanes (cyclohexyl and cyclopentyl). An identical study of the graphite-troilite nodules of three iron meteorites yielded results qualitatively very similar to the carbonaceous chondrites. Nine homologous series have been identified mass spectrometrically: the n-alkane series (C[lowered 13] to C[lowered 26]); the 2-,3-,4-,6-methyl alkane series; the two isoprenoid series, 2,6,10-trimethyl and 2,6,10,14-tetramethyl alkanes (C[lowered 16] to C[lowered 19] and C[lowered 19] to C[lowered 21] respectively), and two series of monocycloalkanes (cyclohexyl and cyclopentyl). The internal parts of the nodules show (1) smaller amounts of hydrocarbons (from three to seven times less than the surface), and (2) an increase in the ratio of isoprenoids to normal alkanes. Isoprenoids are present in all meteoritic samples analyzed. Usually they show a major maximum at C[lowered 19] (pristane) and a secondary maximum at C[lowered 16] (methyl farnesane) with a minimum at C[lowered 17] Comparative studies of isoprenoid distributions typical of meteorites show a much greater degree of correlation with their distributions in sediments and petroleum crudes than with those produced by continuous flow Fischer-Tropsch processes.