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Determination of Biomarkers in Petroleum by Multidimensional Gas Chromatography: Fundamentals, Applications, and Future Perspectives
Noroska Gabriela Salazar Mogollón, Paloma Santana Prata, Fabio Augusto*. Institute of Chemistry, University of Campinas (IQ/UNICAMP). Instituto Nacional de Ciência e Tecnologia em Bioanalítica (INCTBio).
*Corresponding author: Dr Fabio Augusto,
Instituto de Química, Universidade Estadual de Campinas, Caixa Postal 6154, 13084-971 Campinas, SP, Brazil Phone: +55 19 3521-3057 FAX: +55 19 3521-3023 E-mail address:
augusto@iqm.unicamp.br
Several improvements were observed in the analytical methods employed for chemical characterisation and determination of biomarkers in petroleum. Emphasis has been placed on sample preparation and instrumental analysis. Comprehensive two-dimensional gas chromatography (GC×GC) has allowed better characterisation of potential biomarkers by dampening co-elution and increasing the signal-to-noise ratio during chromatographic analyses, which has lead to the acquisition of more accurate and reliable mass spectra. As a consequence, reliable biomarkers are now available to ascertain the thermal maturity, extent of biodegradation, evaluation of the oil’s migration, and age of the source rock surrounding the petroleum. This review addresses some fundamentals, advances, and future perspectives of gas chromatographic techniques in petroleomics.
1. Introduction
Petroleum or crude oil is primarily a fossil fuel and a non-renewable source of energy. The economic importance of crude oil alongside with the need to perform technological improvements related to the exploration and production of petroleum have stimulated the assessment of the origin of petroleum, which controls its physical properties and chemical composition and, therefore, determines its quality. Crude oils are naturally occurring complex mixtures comprised of more than 20,000 compounds with distinct elemental compositions that exhibit a broad structural and chemical diversity. These oils are predominantly comprised of aliphatic hydrocarbons, aromatics, and many other constituents that contain heteroatoms (N, O, S) with different volatilities [1].
Biological markers (i.e, biomarkers) or chemical fossils are used to ascertain the maturation of crude oils and the extent of their biodegradation [1,2]. The most important biomarkers known today are, but not exclusively, hydrocarbons [1,3]. For instance, biodegraded oils possess little to no aliphatic hydrocarbons in its composition [1]. These compounds are originally found in the organic matter that yields petroleum and are able to withstand
the process of biodegradation over the course of its production [1-4]. The resistance to biodegradation of these biomarkers depends largely on its chemical structure. Hence, determination of the biomarkers is used to extract information regarding the organic matter in the source rock that originated the crude oil and the conditions during its deposition [1,4-6]. So, chemical fingerprinting of petroleum is commonly used to ascertain the thermal maturity, extent of biodegradation, evaluation of the oil’s migration, and age of the source rock surrounding the petroleum [1-6].
The biomarkers, namely, isoprenoids, terpanes, and steranes are used for the determination of the geochemical parameters mentioned above [1,4-6]. However, chemical characterisation of petroleum remains an extremely challenging task to analytical scientists due to the exceedingly complex nature of the sample: (i) very large number of constituents, (ii) broad structural diversity, (iii) many compounds are isobaric, and (iv) often times important markers are found in trace concentrations. In order to address these shortcomings, chromatographic techniques coupled to mass spectrometry are frequently used to generate accurate analytical profiles for chemical fingerprinting of
petroleum. However, the use of conventional chromatographic techniques does not exhibit the required peak capacity (or the number of theoretical plates required) necessary to fully resolve all analytes found in crude oil samples [1,7-8]. Also, even if a highly efficiency GC column was employed for such analyses, it would not display the necessary selectivity/solvation capabilities required to resolve all of the analytes. The latter is crucial in petrochemical studies, where many compounds may have the same boiling point, but have other different physiochemical properties; being an alternative, the introduction of new parameters (selectivities, or polarity index) for the separation of these compounds that suffer from coelution in one-dimensional GC.
2. Conventional Gas Chromatography
The analysis of biomarkers is commonly performed by gas chromatography coupled with mass spectrometry (GC-MS) employing highly efficient and thermally stable open tubular capillary columns (e.g., the ASTM method) [1,9]; however, under practical experimental conditions, GC-MS offers a resolving power far smaller than the required to adequately resolve the analytes that can co-eluting, such as, tri- and pentacyclic terpanes, or that eluete very close, as the
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