A current controlled miniaturized non-radioactive electron emitter for atmospheric pressure chemical ionization based on thermionic emission

In this paper, we introduce a simple to manufacture, emission current controlled, miniaturized non-radioactive electron source to provide free electrons at atmospheric pressure. In atmospheric pressure chemical ionization as usually used in ion mobility spectrometry radioactive beta minus sources provide free electrons with high kinetic energy to initiate a chemical gas phase ionization of the analytes. Here, classical thermionic emission from a heat controlled tungsten filament in vacuum is used to generate free electrons. These are then accelerated towards an electron transparent 300 nm thin Si ₃N₄-membrane through which the electrons are transferred from vacuum to atmospheric pressure. Both, the emitted electron current and the average kinetic electron energy can be controlled. For similar ionization processes as compared to radioactive beta minus sources we operate the electron source with an electron emission current at atmospheric pressure of 50 pA - corresponding to an activity of 300 MBq - and an average kinetic electron energy of 8.6 keV.