Ion selecting device for identification of ions in gaseous media

A differential mobility analyser (DMA) comprises ion separation chamber 2 and focusing chamber 16. Sample gas containing ions of different ion mobilities enters the focusing chamber from inlet 15 at a predetermined flow rate. A non-uniform charge density on surfaces 17 and 18 creates a non-uniform electric field in focusing zone 14, causing trajectories 19-21 of ions in the sample gas to converge. Ion stream 22 passes through sample gas inlet 30 into the ion separation chamber, in which sheath gas flows from inlet 5 to outlet 11 at a predetermined flow rate. The focusing chamber is at an angle of 30-90° to the sheath gas flow direction. Electrodes 6 and 7 provide an ion-separating electric field across the ion separation chamber, the field strength chosen such that ions of predetermined mobility follow trajectory 22 and are collected at ion outlet 9.