An ion mobility spectrometer has a pair of electrodes (13A) and (13B) midway along the drift chamber (7). A high field is applied between the electrodes (13A) and (13B) sufficient to modify (e.g. fragment) ions in the region of the electrodes such that they move at a different rate towards the collector plate (8) . This is used to modify the time of flight of selected ions or ion clusters and enable identification of ambiguous peaks on the IMS spectrum.