Abstract: A method of ion mobility spectrometry and an ion mobility spectrometer. The method comprises introducing a packet of sample ions into a chamber, the sample ions including an ion for analysis and the chamber housing a drift region and a deflection region. The sample ions are passed on a drift trajectory through the drift region towards the deflection region, wherein the sample ions separate according to their ion mobility as they pass through the drift region. The sample ions received from the drift region are then passed on a deflection trajectory through the deflection region whilst changing the direction of the sample ions on the deflection trajectory to travel towards the same drift region or a further drift region.