Exploration of intramolecular 13 C isotope distribution in long chain n-alkanes (C 11 –C 31 ) using isotopic 13 C NMR Alexis Gilbert a,⇑ , Keita Yamada a , Naohiro Yoshida a,b a Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan b Earth-Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan article info Article history: Received 16 October 2012 Received in revised form 5 July 2013 Accepted 8 July 2013 Available online 16 July 2013 abstract n-Alkanes are ubiquitous and useful biomarkers in the biogeochemistry field. Their carbon isotope com- position in sedimentary organic matter is therefore of particular importance for inferring their origin. The commonly used technique for d 13 C 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 13 C nuclear magnetic resonance (NMR) spectrometry for the determination of the intramolecular isotope composition of long chain n-alkanes (C 11 –C 31 ). 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 13 C isotope distribution is opposite between odd and even numbered n- alkanes in the C 16 –C 31 range and (ii) those in the C 11 –C 15 range show a 13 C depletion of ca. 12‰ in the methyl position and no difference between odd and even numbered compounds. The results are consis- tent 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 13 C patterns are obtained, the approach appears as promising tool in petroleum exploration and in the bio- geochemistry field. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction n-Alkanes are ubiquitous compounds biosynthesized by several organisms such as terrestrial and aquatic plants, algae and insects. The stable carbon isotopic composition (noted as d 13 C and ex- pressed in ‰ relative to the international standard Vienna Peedee Belemnite, V-PDB) of n-alkanes biosynthesized by such organisms is therefore a useful indicator for inferring the origin of n-alkanes in sedimentary organic matter (OM; Hayes, 1993; Chikaraishi and Naraoka, 2003; Chikaraishi et al., 2012). The isotopic composition of n-alkanes is commonly determined using gas chromatography coupled with isotope ratio mass spec- trometry via a combustion module (GC-C-IRMS; Hayes et al., 1990). The technique has the advantage of allowing determination of the d 13 C values of various compounds present in the same ma- trix with a precision as low as 0.2‰ and with a sample amount of the order of nmol. The main limitation is that it provides d 13 C values at the molecular level, i.e. the average isotopic composition of each carbon position of the analyte. Indeed, since chemical and biological reactions break or form specific C–C bonds, the isotope fractionation (variation in d 13 C values) expected in natural compounds must occur at specific carbon atom positions. As stated by Monson and Hayes (1982), ‘‘isotopic differences between whole molecules [...] must be the attenuated and superficial manifesta- tions of isotopic differences within molecules’’. In other words, determining the d 13 C for each carbon position of a compound, namely the intramolecular isotope distribution, might lead to re- fined information about the chemical, physical and/or biological history of the compound. Little is known about the intramolecular isotopic distribution in natural compounds. The pioneering work of De Niro and Epstein (1977) and later Monson and Hayes (1982), allowed a comprehen- sive view of the metabolic steps that could lead to a heterogeneous 13 C isotopic distribution in fatty acids (FAs). Given that n-alkanes are formed from the FAs of living organisms (Cheesbrough and Kolattukudy, 1984; Zhou et al., 2010), the 13 C intramolecular het- erogeneity in FAs is likely to be preserved to a certain extent in n-alkanes and could thus be a valuable indicator of the sources and fate of organic compounds in sedimentary OM. Yet, to our knowledge, the intramolecular isotope distribution in n-alkanes has never been determined. The most conventional way to obtain intramolecular isotope distribution at natural abundance levels consists in the degradation of the analyte into fragments, the d 13 C values of which are determined separately using IRMS. Nevertheless, the high stability of n-alkanes, that makes them valuable biomarkers in 0146-6380/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.orggeochem.2013.07.004 ⇑ Corresponding author. Tel.: +81 45 924 5555; fax: +81 45 924 5143. E-mail address: gilbert.a.aa@m.titech.ac.jp (A. Gilbert). Organic Geochemistry 62 (2013) 56–61 Contents lists available at SciVerse ScienceDirect Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem