Evaluation of on-line pyrolysis coupled to isotope ratio mass spectrometry for the determination of position-specific 13C isotope composition of short chain n-alkanes (C6–C12)

•Determination of intramolecular isotopic composition of short-chain n-alkanes.•High δ13C variation between terminal and sub-terminal C-atom positions.•Results from Py-GC-C-IRMS consistent with those obtained using isotopic NMR.

We measured 13C intramolecular isotopic composition of commercially available short-chain hydrocarbons (n-C6–n-C12) using 13C-NMR. Results show that the main variation is between the terminal and the sub-terminal C-atom positions. Site-preference (difference in δ13C values between terminal and sub-terminal C-atom positions) among all the samples varies between −12.2‰ and +8.4‰.Comparison of these results with those obtained using on-line pyrolysis coupled with GC-C-IRMS show that the thermal cracking of hydrocarbons occurs with a good isotopic fidelity between terminal and sub-terminal C-atom positions of the starting material and the related pyrolysis products (methane and ethylene). On-line pyrolysis coupled with GC-C-IRMS can thus be used for tracing hydrocarbons biogeochemical processes.

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