Establishing Accuracy of Position‐Specific Carbon Isotope Analysis of Propane by GC‐Pyrolysis‐GC‐IRMS

Rationale

Position-Specific (PS) δ¹³C values of propane has proven its ability to provide valuable information on evolution history of natural gases. Two major approaches to measure PS δ¹³C values of propane are isotopic ¹³C NMR and GC-Py (rolysis)-GC-IRMS. Measurement accuracy of the isotopic ¹³C NMR has been verified, however, the requirements of large sample size and long experimental time limit its applications. The GC-Py-GC-IRMS is a more versatile method with a small sample size, but its accuracy has not been demonstrated.

Methods

We measured PS δ¹³C values of propane from nine natural gases using both methods and evaluated the accuracy of 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

Therefore, 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 of PS isotope standards for propane and other molecules (e.g., butane and acetic acid).