An ion mobility spectrometer is disclosed wherein ions are passed through an ion mobility spectrometer (5) and become temporally separated. Ions having a relatively high ion mobility are transmitted by a non-destructive ion gate (6) but ions having a relatively low ion mobility are subsequently trapped within the ion mobility spectrometer (5) when the ion gate (6) is switched so as to prevent ions from being onwardly transmitted. Ions which are transmitted by the ion gate (6) are trapped in a downstream ion trap (7). The ions are then returned back upstream to a second upstream ion trap (4).