Any ion containing one or more of the less abundant naturally occurring @I03331@ of the elements that make up its structure, e.g. CH2D+.
Sources: Orange Book, 2nd ed., p. 205 [Terms] [Book] PAC, 1991, 63, 1541. (Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). (Recommendations 1991)) on page 1549 [Terms] [Paper]