The gaseous neutral sample molecules enter into the ion chamber through the molecular leak. A tight helical beam of highly energetic accelerated electron beam (70ev) from a glowing tungsten or Rhenium filament pass perpendicular to the incoming gas molecule.
These electrons are drawn off by a positive charged slit called as electron trap, which is on the opposite side of the filament. Thus electrons travel across the ion chamber. Ions are generated by the exchange of energy during the collision of the electron beam and sample molecules. As the electron beam is highly energetic (70 ev), it provides sufficient energy to gas molecules to have ionization and to cause the characteristic fragmentations of sample molecules either by loss of radicals or by loss of neutral molecules.