Compound-specific isotope analysis

The isotopic composition, e.g. 14C/12C, 13C/12C, 2H/1H, 15N/14N and 18O/16O, of matter elements is heterogeneous. It is ruled by physical, chemical and biological mechanisms. Isotopes thus be enable to follow the fate of mineral and organic compounds during biogeochemical transformations. The determination of the isotopic composition of organic substances occurring at trace level into very complex mixtures such as sediments, soils and blood, has been made possible during the last 20 years due to the rapid development of molecular level isotopic techniques. After a brief glance at pioneer studies revealing isotopic breakthroughs the molecular and intramolecular levels, this paper reviews selected applications of compound-specific isotope analysis in various scientific fields. INTRODUCTION Most stable isotopic research relies on two concepts. First, natural and artificial chemical reactions fractionate isotopes, thus leading to the occurrence of various organic and inorganic materials having different isotopic compositions. Isotopes can thus record biogeochemical changes. Second, isotopes can be used as natural or artificial tracers to follow the behaviour of organic molecules in complex media such as living organisms and ecosystems. The determination of 13C/12C ratios1 of natural organic matter has been restricted for a long time to the analysis of bulk samples2. Then, the development of gas chromatography coupled to a combustion furnace then to an isotope ratio mass spectrometer3-5 (GC-C-IRMS, Figure) has allowed the analysis of individual substances occurring at trace levels in very complex mixtures. It thus opened new research fields in various scientific areas6-8 such as organic geochemistry9,10, food science11, medecine12, nutrition13, pharmacy14, sport15, phytochemistry16,17, archaeology18, soil science19,20, environment21-25, humic substances26-28, extraterrestrial science29 and forensic science30,31. 15N/14N and 2H/1H ratios of individual molecules have also been measured using a GC coupled to an IRMS32-35. Besides, since GC is limited to the study of volatile substances, another hyphenated technique has been developed, coupling liquid chromatography to an isotopic ratio mass spectrometer via a combustion furnace (LC-C-IRMS), thus enabling high-molecular, polar, or thermally sensible molecules to be analysed36,37. Further, determination of 2H/1H and 13C/12C ratios of each atomic