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

Rationale

Position-specific (PS) δ¹³C values of propane have proven their ability to provide valuable information on the evolution history of natural gases. Two major approaches to measure PS δ¹³C values of propane are isotopic ¹³C nuclear magnetic resonance (NMR) and gas chromatography-pyrolysis-gas chromatography-isotope ratio mass spectrometry (GC-Py-GC-IRMS). Measurement accuracy of the isotopic ¹³C 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.

Methods

We measured the PS δ¹³C values of propane from nine natural gases using both ¹³C NMR and GC-Py-GC-IRMS, then evaluated the accuracy of the GC-Py-GC-IRMS method.

Results

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).

Conclusions

It is necessary to evaluate and calibrate each GC-Py-GC-IRMS system using propane gases with accurately determined PS δ¹³C values. This study also highlights a need for PS isotope standards for propane and other molecules (e.g., butane and acetic acid).