Corona discharge ionization source

An ion mobility spectrometer comprising a sample introduction chamber into which sample material is introduced, a reaction chamber, a corona discharge ionization source provided in the reaction chamber for ionizing the sample material, a drift tube into which ionized sample material is passed, a collector electrode, a gating grid and a series of electrodes provided in the drift tube to draw ionized sample material toward the collector electrode, and a detecting unit for detecting ionized sample material reaching the collector electrode wherein the improvement comprises said corona discharge ionization source comprising a corona discharge electrode, said corona discharge electrode includes the distal end of a point electrode, a target electrode and means for applying a potential difference between the corona discharge electrode and the target electrode in order to establish a corona discharge between the two electrodes, said potential difference further comprises a substantially constant component, not of itself sufficient to cause corona discharge, plus a pulsed component, thereby ionizing material introduced into the region of the discharge.