https://doi.org/10.1351/goldbook.S05556
An @[email protected] compound is designated as selectively labelled when a mixture of @[email protected] compounds is formally added to the analogous @[email protected] compound in such a way that the position(s) but not necessarily the number of each @[email protected] @[email protected] is defined. A selectively labelled compound may be considered as a mixture of @[email protected] compounds.