principal ion true OBSOLETE Most abundant ion of an isotope cluster, such as the 11B79Br2 81Br+ ion of m/z 250 of the cluster of isotopologue molecular ions of BBr3. The term principal ion has also been used to describe ions that have been artificially isotopically enriched in one or more positions such as CH3 13CH3+ or CH2D2 +, but those are best defined as isotopologue ions. MS:1000373 This child of the former purgatory term ion chemical type was made obsolete. MS