To Control Ionization

Ionization Voltage Typically, 70eV electrons from the filament are used for EI. This energy is determined by voltage deference between the filament and the ion source. This voltage is changeable in some commercial GC/MS instruments
Emission Current The number of ions produced in unit time is proportional to the current of emitted electrons, i.e., emission current. To stabilize formation of ions, the monitored emission current is always compared to the setting value and the result is feed-backed to the filament current source.
Ion Source Temperature The ion source is usually used at about 200 oC.