selectively labelled

An isotopically labelled compound is designated as selectively labelled when a mixture of isotopically substituted compounds is formally added to the analogous isotopically unmodified compound in such a way that the position(s) but not necessarily the number of each labellingnuclide is defined. A selectively labelled compound may be considered as a mixture of specifically labelled compounds.
Source:
Blue Book, p. 515
PAC, 1981, 53, 1887 (Nomenclature of inorganic chemistry: II.1 - Isotopically modified compounds) on page 1893