Development of new method for D/H ratio measurements for volatile hydrocarbons of crude oils using solid phase micro-extraction (SPME) coupled to gas chromatography isotope ratio mass spectrometry (GC-IRMS)

Abstract

In compound-specific isotope analysis (CSIA), one major challenge is the effective sample treatment and the isolation of sufficient amount of analytes for accurate measurements for isotope ratios. The hydrogen isotope ratio (D/H) of normal level hydrocarbons in crude oils has been successfully used as an isotopic tracer in oil-oil and oil-gas correlations, but little progress so far has been made for small size hydrocarbons (C6C15). In this paper, we presented a novel method for sample treatment and achieved the D/H ratio analysis for these particular compounds. The method can be characterized as an efficient approach featured by SPME (solid phase micro-extraction) coupled to GC-IRMS (gas chromatography-isotope ratio mass spectrometry). In this investigation, the CAR/DVB/PDMS fiber was evaluated and used as the absorptive material, and the operating conditions, including the extraction time (0.1 min–150 min) and the extraction temperature (−30 °C–140 °C), which may affect the adsorption amount, were carefully evaluated, and it was found that the adsorption yields varied considerably not only with the operating conditions but also with the molecular size. In addition, the possible hydrogen isotopic fractionation, which may exert strong influence on D/H ratios, were also discussed, it was found that the hydrogen isotopic fractionation were not observed in the extraction process, and the D/H ratios obtained via SPME-GC/IRMS were in good agreement with the EA-IRMS. Notably, the difference between the two approaches was generally within the expected uncertainties from 0.8‰ to 4.2‰, indicating the method is high efficient for volatile hydrocarbon enrichments and the D/H measurements. To assess the validity of the established method, one typical crude oil sample was analyzed, and the D/H ratio results displayed considerable large variation, enough to distinguished between different compounds, showing the established method is well suited for trace hydrocarbon enrichment, and provides so far the most efficient and reliable means for D/H analysis.