Exploration of intramolecular 13C isotope distribution in long chain n-alkanes (C11–C31) using isotopic 13C NMR

Abstract

n-Alkanes are ubiquitous and useful biomarkers in the biogeochemistry field. Their carbon isotope composition in sedimentary organic matter is therefore of particular importance for inferring their origin. The commonly used technique for δ13C determination, isotope ratio mass spectrometry (IRMS), gives access to the isotope composition of n-alkanes at the molecular level, but does not provide information on their intramolecular isotope distribution. Here, we evaluate the potential of isotopic 13C nuclear magnetic resonance (NMR) spectrometry for the determination of the intramolecular isotope composition of long chain n-alkanes (C11–C31). The relative isotope composition of the three terminal carbon positions can be determined with a precision of 1.2‰ or better. The results from commercially available samples show that (i) the intramolecular 13C isotope distribution is opposite between odd and even numbered n-alkanes in the C16–C31 range and (ii) those in the C11–C15 range show a 13C depletion of ca. 12‰ in the methyl position and no difference between odd and even numbered compounds. The results are consistent with a biological origin of heavy n-alkanes whereas lighter ones are proposed to originate from abiogenic degradation such as thermal cracking. Overall, although only partial intramolecular 13C patterns are obtained, the approach appears as promising tool in petroleum exploration and in the biogeochemistry field.