multistage/mass spectrometry An analytical procedure which recognises ions as reactive entities that can be interrogated. The prototype MS/MS instrument consists of two independently operated mass analysers linked by a reaction region in which the ion can be induced to react. Induction often occurs by collision (collision-induced dissociation) in which a selected higher-mass ion dissociates to a smaller fragment ion. In this simple conceptual instrument, three experiments are possible: precursor ion scan, product ion scan, and constant neutral loss scan. MS/MS recurring over multiple steps is known as MSn.