collisional focussing

In the quadrupole ion trap, ions are focussed towards the centre of the trap by collisional cooling with helium ions, where focussing forces  are balanced by space charge. Ions with the lowest mass-to charge ratios are nearest the centre, with  ions of higher mass arranged further out in layers, like the rings of an onion. These ions can be stored for long periods, providing improvements in mass selectivity and axial ejection, and enhancing process such as laser dissociation, since the ion cloud is more easily irradiated. Collisional  focussing can also be carried out in other traps and focussing devices such as wave guides and hexapoles.