reagent ion

https://doi.org/10.1351/goldbook.12547
Ion that reacts with a neutral to produce an ionized form of such species through an ion/molecule reaction, for example, as in chemical ionization.
Source:
PAC, 2013, 85, 1515. (Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013)) on page 1583 [Terms] [Paper]