A method and apparatus for tandem mass spectrometry is disclosed. Precursor ions are fragmented and the fragments are accumulated in parallel, by converting an incoming stream of ions from an ion source (10) into a time separated sequence of multiple precursor ions which are then assigned to their own particular channel of a multi compartment collision cell (40). In this manner, precursor ion species, being allocated to their own dedicated fragmentation cell chambers (41, 42 . . . 43) within the fragmentation cell (40), can then be captured and fragmented by that dedicated fragmentation chamber at optimum energy and/or fragmentation conditions.