5


of flight mass analyser (GC×GC-TOFMS). Also, the use of MS offers an additional dimension for separation and ascertains qualitative analysis [14,24,28].


3.1. Biomarker Determination by Comprehensive Two-Dimensional Gas Chromatography


In conventional GC-MS analyses of crude oils, the chromatograms exhibit a pronounced “hump” in the detector’s baseline that is comprised of an overwhelming number of co-eluting analytes; hence, samples that exhibit this chromatographic profile are known as the unresolved complex mixture (UCM), [12]. The co-elution observed in the UCM is so severe because of the high degree of isomerisation of these compounds that a characterisation by ion monitoring becomes impossible, in this manner, the amount of the information related to geochemical evolution, migration of oils, and extent of biodegradation severely decreases; all of which can provide critical information to understand the factors that regulate the presence of petroleum in surface environments. From the technological standpoint, chemical characterisation of crude oil to determine its composition is used, among other objectives, to ascertain the possibility of generating new cuts in extra heavy gas oil to generate commodities with high revenue (i.e., diesel). Analysis of extra heavy gas oil by GC×GC-TOFMS allowed detection and identification of hopanes, moretanes, paraffins, steranes, and tricyclic terpanes [30]. Frysinger and co- workers examined the chemical composition of crude oil developed in marine environment by combining the qualitative information obtained from both GC×GC- FID and GC-MS analyses [30]. Alkylated aromatics, sulphur-containing aromatics, isoprenoids, steranes, triaromatic steranes, and triterpanes were identified by analysis of the mass spectra and confirmed by their characteristic elution patterns observed in GC×GC, where an overview of oil facilitates the identification of groups of families into the sample, this allowed the calculation of important geochemical parameters, such as biodegradation, thermal maturation and oil migration. As the co-elution between important biomarkers as tri- and pentacyclic terpanes was deleted. [30]. The elution patterns of known biomarkers from crude oil by GC×GC-FID analysis is shown in Figure 2, [31]. Also, the chromatographic resolution of important critical pairs, namely, tricyclic terpanes/pentacyclic terpanes, C30/C30R demethylated homohopane, and hopanes/sterenes from Brazilian crude oils were resolved by GC×GC-TOFMS, which co-eluted by conventional GC-MS analysis [10]. This was also the first report


Figure 2. GC×GC-FID chromatogram from Exxon Valdez cargo oil, developed in marine environment. 1 column: 10 m × 0.10 mm Rtx-1 (polydimethylsiloxane) (df = 0.40 µm). 2


(polydimethyldiphenylsiloxane with 50% diphenylsiloxane monomer incorporation) (df from Gaines et al. [31]


of demethylated tri- and tetracyclic in Brazilian oils which allowed the study of a new geochemical parameter, these compounds were not developed during diagenesis, indicating that a medium level of biodegradation in the oil is present [10].


Oils of marine origin were analysed by GC×GC-TOFMS and four types of complex mixtures were observed, namely, UCM, UCM I, UCM II, and UCM III. The UCM comprised of C36


-C40 mono- to tricyclic


archaeal isoprenoid diastereoisomers, UCM type I exhibited predominantly mono- to hexacycloalkanes of unknown origin, and UCM type II consisted of C35


-C40


D


D column: 100 cm × 0.10 mm DB-17 = 0.10 µm), adapted


[11]. Also, the identification of biomarkers in oils from the upper Magdalene valley basin (Colombia) by GC×GC-TOFMS was jeopardised by the oil’s chemical complexity due to its severe thermal maturity. A chemometric technique, namely multi-way principal component analysis (MPCA), was employed for unsupervised data mining of petroleomic data generated by GC×GC- TOFMS to examine the differences in the oils sampled from several reservoirs [34].


hydrocarbons that originated from


archaeal lipids. Interestingly, the chemical composition of UCM type III was similar to UCM I and II [12].


Several chromatographic techniques, namely, GC-MS, GC×GC-FID, and GC×GC- TOFMS were examined to determine of key petroleum biomarkers to ascertain the cretaceous age and terrestrial organic matter input [32]. Biomarkers of angiosperm origin, namely, lupanoides, olenoides, 18α-oleonane, and 18β-oleonane were identified. GC×GC-TOFMS was used to evaluate the chemical composition of oils from different basins to extract geochemical information from new Brazilian reserves [11]. Several markers including hopanes, steranes, tricyclic terpanes, 8,14-seco-hopanes, onocerane, 3β- and 2α-methylhopanes, and mono- and triaromatics were identified [11]. The 3β-methylhopanes and a series of onoceranes were detected exclusively in samples from depositional lacustrine environments; while 2α-methylhopanes were determined in marine oils [11,33]. It was determined that a ratio between the concentration of C31


3β-methylhopane and C30 hopane 100 (3βMHC31/H30) can be used


to differentiate lacustrine (3βMHC31/H30 > 1) and marine samples (3βMHC31/H30 < 1)


The extent of biodegradation increases the complexity of UCM, which in turn reduces the availability of potential geochemical information, namely, maturity, and origin through peak overlap by GC/MS. Marriott and co-workers were able to resolve several C1


-C7 alkyl decahydrohopanes from


marine, terrestrial, and hybrid oil samples by GC×GC [35]. However, many resolved analytes were not identified possibly due to limited availability of mass spectra in commercial databases.


4. Conclusions and Future Perspectives


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. GC×GC has allowed better characterisation of potential biomarkers by reducing the number of compounds co-eluting 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. It is hoped that improvements in column technology may help expand


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