A technique for detecting low concentrations of specific compounds, based on the rate at which their ions migrate through an electric field. The instrument operates in the gas phase at atmospheric pressure. The sample vapour enters an ionizing region, where ions can be produced by a variety of methods. In compact instruments the source is usually a small amount of radioactive material. The ions are allowed in pulses into a drift tube, where they move to a detector under the influence of a homogeneous electric field. The rate of movement depends on the way the ions interact with neutral molecules in the tube and this depends on the ion’s size and shape. The spectrum is a plot of detector signal against time, and is characteristic of the sample being ionized. Ion-mobility spectrometers are compact, sensitive, and fast-acting. They are widely used in screening for drugs and explosives at airports, border crossings, etc. Often the technique is to wipe a swab over luggage and place it in the instrument. More sophisticated instruments combine IMS with gas chromatography or mass spectrometry. The technique is sometimes referred to as gas-phase electrophoresis.