An
isotopically labelled compound may be designated as isotopically deficient when the isotopic content of
one or more elements has been depleted, i.e. when one or more
nuclide(s) is (are) present in less than a natural ratio.
Source:
PAC, 1981, 53, 1887
(Nomenclature of inorganic chemistry: II.1 - Isotopically modified compounds)
on page 1896