specifically labelled

An isotopically labelled compound is designated as specifically labelled when a unique isotopically substituted compound is formally added to the analogous isotopically unmodified compound. In such a case, both position(s) and number of each labellingnuclide are defined.
Source:
Blue Book, p. 514
PAC, 1981, 53, 1887 (Nomenclature of inorganic chemistry: II.1 - Isotopically modified compounds) on page 1891