Analysis of Organically Bound Tritium in biological samples Tritium, a radioactive isotope of hydrogen with atomic mass 3, is a clear indicator of numerous nuclear activities. Some released tritium is durably trapped in vegetation in the form of non-exchangeable Organically Bound Tritium (OBT), and can be determined several years after the waste has been produced. In the struggle against nuclear proliferation, the CEA has developed an OBT trace analysis methodology based on: prevention of cross contamination when handling samples, exchangeable tritium removal, OBT extraction by catalytic oxidation, and recovery in the form of water, measurement of tritium in collected water samples by low noise liquid scintillation. Organic molecules contain both exchangeable hydrogen atoms (bound to oxygen or nitrogen atoms) and non-exchangeable hydrogen atoms (bound to carbon atoms). Applications achieved OBT mapping (tree leaves) in areas near nuclear facilities, measurement of OBT concentration in tree rings, analysis of OBT in various biological samples (milk, corn, algae, sediment, etc.).   Tree section with annual growth rings detected using the X-ray microdensitometry technique.   Example of correlation between atmospheric concentration of tritium and the concentration measured in tree rings. Oven dedicated to Organically Bound Tritium extraction by catalytic oxidation in biological samples.