A mass spectrometer is disclosed wherein ions from a pulsed ion source are dispersed in a drift region so that the ions become separated according to their mass to charge ratios. The ions are then received by an ion guide in which multiple trapping regions are created and wherein the multiple trapping regions are translated along the length of the ion guide. The ion guide receives the ions so that all the ions trapped in a particular trapping region have substantially the same or similar mass to charge ratios. The ions are released from the exit of the ion guide and the pusher/puller electrode of an orthogonal acceleration Time of Flight mass analyzer is arranged to be energised in synchronization with the ions emerging from the ion guide. The trapping regions may be translated along the ion guide with a velocity which becomes progressively slower and the delay time of the pusher/puller electrode may be progressively increased.