  The isotopic composition, for example, 14C/12C, 13C/12C, 2H/1H, 15N/14N and
18O/16O, of the elements of matter is heterogeneous. It is ruled by physical,
chemical and biological mechanisms. Isotopes can be employed 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 in 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 pioneering studies
revealing isotopic breakthroughs at the molecular and intramolecular levels,
this paper reviews selected applications of compound-specific isotope analysis
in various scientific fields.
