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.

			and	Br+
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.