An ion guide generates a radio frequency (RF) field to radially confine ions to an ion beam along a guide axis as the ions are transmitted through the ion guide. The effective potential of the RF field includes an alternating series of barriers and wells. Ions may be trapped in individual wells in mass-dependent order, with larger masses trapped closer to a guide exit than smaller masses. The RF field may be scanned so as to release the ions from the ion guide in mass-dependent order, with larger masses released before smaller masses. The operating conditions may be set such that ions over the entire mass range arrive at a desired downstream focal point simultaneously, for example at an accelerator of a time-of-flight analyzer.