Determination of position-specific carbon isotope ratios of propane from natural gas

On-line gas chromatography-pyrolysis coupled to gas chromatography-isotope ratio mass spectrometry was used here for the position-specific isotope analysis (PSIA) of propane. First, based on the conversion rate of propane and its products, 800-840 degrees C was considered optimal for propane pyrolysis. The major pyrolytic fragments of propane included CH4, C2H4, C3H6, and C2H6. Subsequent isotope labeling experiments showed that CH4 and C2H6 were derived entirely from the terminal carbons, whereas C2H4 and C3H6 were derived from both terminal and central positions of propane. Therefore, the C-13 enrichment factor associated with the major reactions during the pyrolysis process and position-specific delta C-13 values of propane can be estimated from the amount and delta C-13 values of the pyrolytic fragments using isotope mass balance. The obtained enrichment factors depended on the pyrolysis temperature, which can be used to calculate position-specific delta C-13 values for propane measured with this system. The results suggest that a relatively accurate site-preference value for propane can be obtained by this method. Therefore, the combination of compound-specific isotope analysis and PSIA of propane will be a powerful tool to discriminate the different origins of gases. (C) 2018 Elsevier Ltd. All rights reserved