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 labelling nuclide is defined. A selectively labelled compound may be considered as a mixture of specifically labelled compounds.
Blue Book, p. 515 (
PAC, 1981, 53, 1887. 'Nomenclature of inorganic chemistry: II.1 - Isotopically modified compounds' on page 1893 (