mixed labelled

A specifically labelled compound is referred to as mixed labelled when the isotopically substituted compound has more than one kind of modified atom e.g. CH3–CH2–[18O][2H].
Source:
Blue Book, p. 515
PAC, 1981, 53, 1887 (Nomenclature of inorganic chemistry: II.1 - Isotopically modified compounds) on page 1893