Abstract: A compact ion mobility spectrometer (IMS) with increased sensitivity and resolution for detecting the composition of an unknown substance is disclosed. The IMS comprises a unitary cylindrical housing, inlet means for introducing sample gas, outlet means for withdrawing sample and drift gas, two high voltage electrodes, and a detector. The first electrode is pulsed to inject sample ions through an opening located in a second electrode which defines a drift region. The second electrode is cup-shaped having an open end and is electrically biased to create a non-uniform electric field gradient across said drift region. The electric field in said drift region is axial and radial resulting in an increase in ion drift velocity at the detector and a corresponding increase in detected peak signal amplitudes thereby increasing resolution. A shorter drift region and the absence of an aperture grid near the detector which cause ion collisional losses, helps to increase IMS sensitivity.