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

Abstract

We measured (13)C intramolecular isotopic composition of commercially available short-chain hydrocarbons (n-C6-n-C12) using (13)C-NMR. Results show that the main variation is between the terminal and the sub-terminal C-atom positions. Site-preference (difference in δ(13)C 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.

Keywords: Hydrocarbons; Intramolecular isotope distribution; Isotopic (13)C-NMR; On-line pyrolysis.