Measurement of compound‐specific carbon isotope ratios (δ¹³C values) via direct injection of whole crude oil samples

Abstract

RATIONALE: Stable isotope analysis is a powerful tool in understanding the generation, history and correlation of hydrocarbons. Compound-specific δ 13 C measurements of oils allow detailed comparison of individual compound groupings; however, most studies of these sample materials separate and isolate individual fractions based on the chemistries of particular compound groups, potentially losing considerable valuable isotopic data. Even if all fractions are analyzed, this represents a large increase in the data-processing burden, effectively multiplying data evaluation time and effort by the number of fractions produced. Gas chromatography/isotope ratio mass spectrometry (GC/IRMS) of untreated, whole crude oils allows the immediate collection of a larger suite of valuable isotopic data for these studies. METHODS: Untreated ('neat', undiluted), whole crude oils were directly injected and measured on a GC/IRMS system, using split (40:1) injections and a 50 m HP-PONA column. The GC method, 97 min in duration, was designed to maximize baseline separation of target analyte peaks, while an additional oxygen flow was admitted into the combustion reactor to maximize the lifetime of the combustion chemicals. RESULTS: The method and setup utilized allow the measurement of a much greater range of the n-alkanes (n-C 4 to n-C 25+ ) than traditional methods, while also retaining important cycloalkane, aromatic and isoprenoid peaks within the same analysis. Carbon isotope (δ 13 C) evaluation of these additional compound classes reveals trends in maturity and origins which are not identifiable when exclusively assessing the traditional n-alkane package (>n-C 12 ). CONCLUSIONS: The described setup and method open up new possibilities for assessing the origins and histories of crude oil samples. The data generated for the whole oil n-alkanes by this method is equivalent to that reported for isolated n-alkane studies, while also providing valuable additional data on many other important compounds. The end result of this method is a more complete assessment of the carbon isotopic composition of crude oils.

Environmental impact and recovery of the Bohai Sea following the 2011 oil spill

“…Crude oil is mainly composed of carbon and hydrogen, with very little nitrogen, which would have resulted in much higher TOC/TN ratios, compared to marine and terrigenous organic matter, if a significant fraction remained. Previous studies showed that d 13 C of crude oil normally range from À32 to À24‰ (Wang et al, 2013b;Barrie et al, 2016;Muhammad et al, 2017). The ranges and mean values of TOC/TN and d 13 C in our study indicated the presence of impacts from oil.…”

Simultaneous determination of compound‐specific isotopic compositions of acyclic isoprenoids and n‐alkanes in light crude oil

“…The δ 13 C values of Pr and Ph determined directly from the whole oil and/or the saturated fraction were consistent with those from the branched/cyclic alkane fraction, with an RSD of less than 0.43%. The accuracy and precision were acceptable [10, 12].…”

“…Compound‐specific carbon and hydrogen isotopic ratios of individual organic compounds can be determined by compound‐specific isotope analysis (CSIA) using GC with isotope ratio mass spectrometry (IRMS). Reviews and multiple research papers have documented detailed information on analytical methods and the use of compound‐specific carbon and hydrogen isotopic data [9–12].…”

“…Barrie et al. [12] designed a method to simultaneously obtain the compound‐specific carbon (δ 13 C) signatures of n ‐alkanes ( n ‐C 4 to n ‐C 25+ ) and acyclic isoprenoids via the direct injection of whole crude oil using a 50 m HP‐PONA column. However, a method for the simultaneous determination of the compound‐specific hydrogen (δD) isotopes of n ‐alkanes, Pr, and Ph in crude oil has not been reported.…”

“…Article Related Abbreviations: CSIA, compound-specific isotope analysis; IRMS, isotope ratio mass spectrometry; Ph, phytane; Pr, pristane Reviews and multiple research papers have documented detailed information on analytical methods and the use of compound-specific carbon and hydrogen isotopic data [9][10][11][12].…”