A new method, combining headspace solid phase microextraction (HS-SPME) with an online pyrolysis system coupled with isotope ratio mass spectrometry (IRMS), is developed for the determination of the intramolecular 13C isotope composition of ethanol in aqueous solutions. The δ13C values of the pyrolytic fragments (CO, CH4, C2H4) are shown to be highly reproducible (sd <0.4‰). Furthermore, using 14 ethanol samples of known intramolecular isotope distribution, the CO and CH4 fragments are shown to arise solely from the methylene (CH2OH) and methyl (CH3) carbon atom positions of the original ethanol, respectively. Although the different steps (extraction and pyrolysis) fractionate between 12C and 13C, the isotopic fractionation is reproducible (sd <0.4‰), allowing correcting factors to be applied in order to back-calculate the original δ13CCH2OH and δ13CCH3 values of ethanol. The method thus allows the determination of the isotope composition of ethanol at the intramolecular and molecular levels, within a single run and a short experimental time (30 min), and with a very easy sample preparation. The method is then applied to alcoholic beverages to show its potential for authentication purposes.

•Intramolecular 13C isotopic distribution in n-alkanes determined via isotopic 13C NMR.•Relative isotopomer concentration of the three terminal carbon atom positions available.•Alternation of intramolecular 13C pattern between odd and even heavy n-alkanes.•Strong 13C depletion on the methyl position of light n-alkanes.•Promising tool for the study of the sources and sinksof n-alkanes.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.

The stable carbon isotope 13C is used as a universal tracer in plant eco-physiology and studies of carbon exchange between vegetation and atmosphere. Photosynthesis fractionates against 13CO2 so that source sugars (photosynthates) are on average 13C depleted by 20‰ compared with atmospheric CO2. The carbon isotope distribution within sugars has been shown to be heterogeneous, with relatively 13C-enriched and 13C-depleted C-atom positions. The 13C pattern within sugars is the cornerstone of 13C distribution in plants, because all metabolites inherit the 13C abundance in their specific precursor C-atom positions. However, the intramolecular isotope pattern in source leaf glucose and the isotope fractionation associated with key enzymes involved in sugar interconversions are currently unknown. To gain insight into these, we have analyzed the intramolecular isotope composition in source leaf transient starch, grain storage starch, and root storage sucrose and measured the site-specific isotope fractionation associated with the invertase (EC 3.2.1.26) and glucose isomerase (EC 5.3.1.5) reactions. When these data are integrated into a simple steady-state model of plant isotopic fluxes, the enzyme-dependent fractionations satisfactorily predict the observed intramolecular patterns. These results demonstrate that glucose and sucrose metabolism is the primary determinant of the 13C abundance in source and sink tissue and is, therefore, of fundamental importance to the interpretation of plant isotopic signals.