Generation of Primary Electrons

The beam of ionizing electrons is produced by thermionic emission from a resis-tively heated metal wire ox filament typically made of rhenium or tungsten. The filament reaches up to 2000 °C during operation. Some reduction of the working temperature without loss of electron emission (1-10 mA mm ) can be achieved by use of thoriated iridium or thoriated rhenium filaments. [22] There is a wide variety of filaments available from different manufactures working almost equally well, e.g., the filament can be a straight wire, a ribbon, or a small coil (Fig. 5.9). 

To achieve a more stable mode of operation, in modem instruments the heating current for the filament is emission-controlled, i.e., the current of the electron trap is used to keep emission comparatively independent from actual ion source conditions. Typical emission currents are in the range of 50-400 pA. 

Note The lifetime of a filament is several weeks under proper operating conditions. However, harsh conditions such as aggressive analytes or reagent gases in chemical ionization, too high emission current, and in particular sudden breakdown of the high vacuum have devastating consequences. 

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