Many an undergraduate biologist will remember the excitement/fear of using radio-labelled substrates (usually 14C) during carefully supervised laboratory practicals. They will also remember the precautions taken to avoid contact with the radioactive source and to account for all the materials used. The use of radiolabelled compounds (mainly 3H, 14C, 32P or 35S) is commonplace though carefully controlled, and the utilisation of such compounds for field experiments is severely restricted. A further limitation in ecological research is that the radioactive isotope of nitrogen (13N) has a half life of only a few minutes.