Establishing the Accuracy of Position-Specific Carbon Isotope Analysis of Propane by GC-pyrolysis-GC-IRMS.

RationalePosition-specific (PS) delta C-13 values of propane have proven their ability to provide valuable information on the evolution history of natural gases. Two major approaches to measure PS delta C-13 values of propane are isotopic C-13 nuclear magnetic resonance (NMR) and gas chromatography-pyrolysis-gas chromatography-isotope ratio mass spectrometry (GC-Py-GC-IRMS). Measurement accuracy of the isotopic C-13 NMR has been verified, but the requirements of large sample size and long experimental time limit its applications. GC-Py-GC-IRMS is a more versatile method with a small sample size, but its accuracy has not been demonstrated. MethodsWe measured the PS delta C-13 values of propane from nine natural gases using both C-13 NMR and GC-Py-GC-IRMS, then evaluated the accuracy of the GC-Py-GC-IRMS method. ResultsThe results show that large carbon isotope fractionations occurred for both terminal and central carbons within propane during pyrolysis. The isotope fractionations during the pyrolysis are reproducible at optimum conditions, but vary between the two GC-Py-GC-IRMS systems tested, affected by experimental conditions (e.g., pyrolysis temperature, flow rate, and reactor conditions). ConclusionsIt is necessary to evaluate and calibrate each GC-Py-GC-IRMS system using propane gases with accurately determined PS delta C-13 values. This study also highlights a need for PS isotope standards for propane and other molecules (e.g., butane and acetic acid).