A molecular or fragment ion which is made up of the most abundant @I03331@ 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 @I03331@ as new atomic species. Thus, in the above two examples, the principal ions would be of masses 31 and 18, respectively.