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Chromatographic techniques in molecular organic geochemistry analysis

Molecular organic geochemistry investigation involves modern analytical techniques needed for classification of bitumen and crude oil samples. The basic compounds in this investigation are geochemical biomarkers, compounds different from other biomarkers used in biomedicine and environmental protection. They are molecular fossils derived from once living organisms, through a number of complex chemical reactions during diagenesis . Biomarkers are complex organic compounds composed of carbon, hydrogen, and other elements and they are found in rocks and sediments as well as in crude oils. They show little or no change in structure from their parent organic molecules in living organisms [1]. There is a huge number of geochemical biomarkers starting from simple acyclics ( pristane, phytane ), multi-cyclics ( sterane, hopane ) to very complex aromatics ( steroids ). The more complex is the structure of biomarkers, more geochemical information could be revealed about their origins. Biomarkers are determined in bitumen part of source rocks ( organic matter extracted from fine-grained rocks using common organic solvents like methylene chloride ). In crude oil samples they are derived from maltene components ( part soluble in normal alkanes like pentane, hexane or heptane ). Bimarker parameters can be use to: - correlate oils and bitumens -define different depositional environments -indicate the degree of oxicity under which marine sediments were deposited -effectively rank the relative maturity of oils -determine source and maturity, even for biodegraded oil -provide information on the lithology of the source rock -offer information on the age of the source for the oil. Large number of oils and/or rock samples can be „ screened „ using simple geochemical tools supported by common capillary GC, for non-biomarkers parameters. For biomarker parameters determination a hyphenated GC/MS, GC/MSMS are obligatory. But GC / MS determination followed after a very complex procedure of sample preparation and concentration. The preparation of sample rocks starts with some physical methods ( washing, drying, cuttings, grinding ) and afterwards different extraction techniques follow ; SPE, LLE, soxhlet and, soxtec extraction, column adsorbtion chromatography [2]. The analysis of crude oil samples starts with the removing of asphaltenes ( part insoluble in light hydrocarbons ). From prepared bitumen and maltene fraction common procedure involves cleanup with adsorption column chromatography, giving cuts of aromatics and parrafinics on one side and polars and resins on another side. As level of biomarkers is very low before GC/MS measurements, additional separation and concentration are required. In this stage NPHPLC or different liquid extraction techniques are used giving several groups of compounds ( parrafinics, aromatics, polyaromatics, polars ). In parrafinic group satured based biomarkers could be determined and in aromatic group steroids could be determined by using GC/MS. Besides a cleanup and group type separation, HPLC are used as analytical tool for estimation of parrafinic or aromatic character of samples. But the same technique could be used for the collection of needed fraction after repeating the analysis a few times. To overcome this time-consuming procedure a semi-prep or prep. HPLC, as well as open column adsorption chromatography could be used. Separation on Pasteur pipette filled with alumina or silica has the same meaning. It is possible to obtain in very short time a fraction in mg or higher level, amount sufficient for further GC/MS analysis. By introducing back flush HPLC, it is possible to collect from two to four fractions in the same analysis ( saturates, mono-aromatics, di-aromatics, and poly-aromatics together with polars ) . The gas chromatogram of an oil sample obtained by routine GC/FID is dominated by the homologous series of parrafinics. Mass chromatogram of the sterane and hopane are determined by GS/MS, by monitoring principal fragment ions at m/z 217 and 191, respectively. Many biomarkers, including steranes and hopanes elute from GC column between n-parrafins with 24 and 36 carbon atoms. The common method used for sterane and hopane analysis involve single quadropole analyzer and selective ion recording ionization for known fragments ( 217 and 191 ), but for more specific analysis, multiple reaction monitoring and triple quadrupole are needed. [1] K.E. Peters, J.M. Moldowan, The Biomarker Guide. Interpreting Molecular Fossils in Petroleum and Ancient Sediments, Prentice Hall, XVI Englewood Cliffs, New Jersey, 1993., str.363. [2] Y. Li, R. Michels, L. Manusy, S. Fleck, P. Faure, Fuel 81 (2002) 747.

molecular organic geochemistry; biomarkers; biomarkers analysis by HPLC and GC/MSmolecular organic geochemistry; biomarkers; biomarkers analysis by HPLC and GC/MS

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