Radioactive or stable isotope of a specified element distinguishable by the observer but not by the system used to identify an isotopic tracer.
Notes: - 'Isotopic labelling' is the incorporation of an isotopic label in a substance. It may be qualified by the manner of introduction of the label, e.g. exchange labelling, conjugation labelling, recoil labelling.
Examples: Deuterium ('isotopic label') that is substituted for protium in the illegal drug methylamphetamine, for use in analysis by mass spectrometry. Methylamphetamine incorporating deuterium is the 'isotopic tracer'.Example: - In general usage, 'label' and 'tracer' are used synonymously.
Source:
PAC, 2021, 93, 69. 'Vocabulary of radioanalytical methods (IUPAC Recommendations 2020)' on page 85 (https://doi.org/10.1515/pac-2019-0302)