QUANTITATIVE AND STEREOISOMERIC DETERMINATION OF LIGHT BIOMARKERS IN CRUDE-OIL AND COAL SAMPLES

Indans and tetralins are considered biological markers (biomarkers). T hese C9-C11 hydrocarbons are present in small amounts in organic geolo gical samples. Methyl substituted indans or tetralins may possess a st ereogenic center (carbon). Thus they can exist as enantiomers and, in the case of disubstituted entities, also as diastereoisomers. The conc entrations of 1-methylindan, 1,3-dimethylindan, l-methyltetralin, and 2-methyltetralin were determined in sixteen crude oil samples of diffe rent sources and in fourteen coal samples of different sources and ran ks. Deuterated homologues were synthesized as standards to spike the s amples and to assure accurate quantitative analysis. A procedure using HPLC fractionation followed by GC/MS analysis allowed the determinati on of mu g/g (ppm) amounts of these compounds in oils. The concentrati on of substituted indans and tetralins was 3-4 orders of magnitude les s in coal than in crude oil. The select ion mass spectrometry (SIM) mo de in GC/MS and the deuterated standards allowed detection of the much lower amounts (ng/g, ppb down to pg/g, ppt) of these compounds in coa l samples. The stereochemistry of the biomarkers was determined, and t he relationship between their relative concentrations and the location and type of the deposits was examined. Racemic mixtures of the indans and tetralins studied were found in all samples of oil and coal. It i s postulated that there is an inverse relationship between the retenti on of stereochemical configuration and the molecular weight of hydroca rbons in crude oil. The chiral retention of configuration cut-off is t hought to be between molecular weights of 146 and 208. An excess of ci s-l,2-dimethylindan was found in all oil samples (average cis/trans ra tio: 3/2). The 2-methyltetralin concentration was found to be about tw ice that of 1-methyltetralin in all oil and coal samples. Similar conc entration correlations were found for the indan derivatives in oils an d coals. Copyright (C) 1998 Elsevier Science Ltd.