Electron source

A gas-source mass spectrometer comprising an electron source, the electron source comprising:

• an electron emitter cathode presenting a thermionic electron emitter surface in communication with a gas-source chamber of the gas-source mass spectrometer for providing electrons there to;
• a heater element electrically isolated from the electron emitter cathode and arranged to be heated by an electrical current therein and to radiate heat to the electron emitter cathode sufficient to liberate electrons thermionically from said electron emitter surface, therewith to provide a source of electrons for use in ionising a gas the gas-source chamber;
• an electron grid permeable to electrons and positioned to receive electrons emitted from the thermionic electron emitter surface such that electrons are permitted to pass through the electron grid from a side thereof facing the thermionic electron emitter surface to a side facing the gas-source chamber, the electron grid responsive to a voltage applied thereto;
• and
• an anode positioned to receive electrons that have passed through the electron grid and transmit the electrons received thereby to the gas-source chamber, the anode responsive to a voltage applied thereto;
• and
• an electron trap operable to receive electrons which have traversed the gas-source chamber as a current between 0.5 mA and 10 mA in response to the electron emitter cathode being heated by the heater element to a temperature not exceeding 2000° C., the electron trap having a trap current associated therewith,
• wherein the electron source is responsive to a measured trap current to stabilise operating conditions in the electron source with the electron grid and/or the anode configured to control the energy of thermionic electrons input to the gas-source chamber by controlling the voltages applied to the electron grid and/or the anode.