A molecular or fragment ion which is made up of the most abundant
isotopes of each of its atomic constituents. In the case of compounds that have been artifically
isotopically enriched in one or more positions such as
CH313CH3 or
CH2D2
the principal ion may be defined by treating the heavy
isotopes as new atomic species. Thus, in the above two examples, the principal ions would
be of masses 31 and 18, respectively.
Source:
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 1550