Ion mobility spectrometer

An ion mobility spectrometer has a non-radioactive electron source 7 to generate ions inside a reaction chamber 1. The reaction chamber 1 consists of two partial chambers 3,4 one of which 3 is evacuated and comprises the electron source 7, and the other one 4 is connected to the drift chamber 2 of the IMS via a shutter grid 17. The partition wall 5 between both partial chambers is transparent to electrons but impermeable for gas molecules. It may be made of mica for example. The electron source may comprise a thermoemitter,as shown, or a photocathode, which is illuminated from outside through a window. In this way, any contact between analyte and electron source is avoided, thereby improving the measuring stability and enabling operation in positive and negative mode.