precursor ion

in mass spectrometry
https://doi.org/10.1351/goldbook.P04807
Synonymous with parent ion.
Sources:
Orange Book, 2nd ed., p. 205 (http://media.iupac.org/publications/analytical_compendium/)
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 (https://doi.org/10.1351/pac199163101541)