Establishing the accuracy of position-specific carbon isotope analysis of propane by GC-pyrolysis-GC-IRMS

Position-specific (PS) δ¹³ C values of propane have proven their ability to provide valuable information on the evolution history of natural gases. Two major approaches to measure PS δ¹³ C values of propane are isotopic ¹³ C nuclear magnetic resonance (NMR) and gas chromatography-pyrolysis-gas chromatography-isotope ratio mass spectrometry (GC-Py-GC-IRMS). Measurement accuracy of the isotopic ¹³ C 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.

We measured the PS δ¹³ C values of propane from nine natural gases using both ¹³ C NMR and GC-Py-GC-IRMS, then evaluated the accuracy of the GC-Py-GC-IRMS method.

The 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).

It is necessary to evaluate and calibrate each GC-Py-GC-IRMS system using propane gases with accurately determined PS δ¹³ C values. This study also highlights a need for PS isotope standards for propane and other molecules (e.g., butane and acetic acid).