tracer substance

https://doi.org/10.1351/goldbook.16163
Substance that can be tracked through one or more reactions or systems, often by detecting an incorporated isotope.
Source:
PAC, 2007, 79, 1153. (Glossary of terms used in toxicology, 2nd edition (IUPAC Recommendations 2007)) on page 1328 [Terms] [Paper]