radioactive tracer
a chemical compound labeled with a radioactive isotope, such as 45Ca (calcium) or 14C (carbon), so that its metabolic pathway can be traced through body tissues. Radioactive tracers may be used for diagnostic, therapeutic, or research purposes. For example, in tobacco research, nicotine molecules can be tagged with radioactive tracers, and the path of the alkaloid can be traced through the lungs and bloodstream.