Ions provided from an ion source are separated ions into a plurality of different ion groups according to at least one ion property. At least some of the different ion groups are stored in an ion storage array, which comprises a plurality of independently operable storage cells, each storage cell being arranged to receive and store a different ion group. A controller is programmed to cause selective switching of each of the storage cells between an ion receiving mode and an ion storage mode, and between the ion storage mode and an ion release mode. In particular, the switching of each storage cell is controllable independently of the switching of any of the other storage cells. Upon release from a respective storage cell of the array, ions are provided to one or more mass analyzers for subsequent analysis.