Development of new method of δ13C measurement for trace hydrocarbons in natural gas using solid phase micro-extraction coupled to gas chromatography isotope ratio mass spectrometry

Abstract

Compound specific isotope analysis (CSIA) of normal-level hydrocarbons (C1–C4) in natural gas is often successfully used in natural gas origin identification and classification, but little progress so far has been made for trace level hydrocarbons (C5–C14) in natural gas. In this study, we developed a method for rapid analysis of carbon isotopic ratios for trace hydrocarbons in natural gas samples. This method can be described as a combined approach characterized by solid phase micro-extraction (SPME) technique coupled to gas chromatography isotope ratio mass spectrometry (GC/IRMS). In this study, the CAR–PDMS fiber was chosen as the SPME adsorptive material after comparative experiments with other four fibers, and the parameters, including equilibration time, extraction temperature and desorption time, for efficient extraction of trace hydrocarbons were systematically optimized. The results showed the carbon isotopic fractionation was not observed as a function of equilibration time and extraction temperature. And the δ13C signatures determined by SPME-GC/IRMS were in good agreement with the known δ13C values of C5–C14 measured by GC–IRMS, and the accuracy is generally within ±0.5‰. Five natural gas samples were analyzed using this method, and the δ13C values for C5–C14 components were obtained with satisfied repeatability. The SPME-GC/IRMS approach fitted with CAR–PDMS fiber is well suited for the preconcentration of trace hydrocarbons and provides so far the most reliable carbon isotopic analysis for trace compounds in natural gas.