Illustrated Glossary of Organic Chemistry

Fragment ion: In mass spectrometry, an ion formed by fragmentation of a molecular ion or other species in the mass spectrometer.

m/z = 170
Molecular ion (M)

m/z = 91
Fragment ion

m/z = 79
Fragment ion

Mass spectrum of benzyl bromide (PhCH2Br) showing peaks due to molecular ions and peaks due to fragment ions. m/z = 91 is the base peak.