In a mass spectrometer a target volume is filled with ions of different mass but substantially the same energy from a distant storage device by forming a plurality of spatially-limited ion swarms consisting of ions having the same mass. The ion swarms are ordered either by a mass-sequential extraction from the storage device or by rearranging the order of flight as the ions are in flight, so that swarms of different mass ions simultaneously enter the target volume despite having different flight velocities. A mass-sequential extraction in the order of decreasing mass can be achieved in one embodiment by decreasing a pseudopotential barrier at the storage device which causes the heavy ions to emerge first. In another embodiment, the ions can be rearranged in flight by applying a bunching potential. A second reverse bunching potential then restores the energy of the ions to their original values.