Non-radioactive electron source with nanosecond pulse modulation for atmospheric pressure chemical ionization.

Ion mobility spectrometers (IMSs) are well-known instruments for fast and ultrasensitive trace gas detection. In recent years, we introduced a compact nonradioactive electron source providing a defined current of free electrons with high kinetic energy at atmospheric pressure for initiating a chemical gas phase ionization of the analytes identical to radioactive sources. Besides its nonradioactivity, one major advantage of this electron source is its controlled electron emission current even in pulsed mode. By optimizing the geometric parameters and developing faster control electronics, we now achieve electron pulses with extremely short pulse widths down to 23 ns. This allows us to kinetically control the formation of reactants and analyte ions by chemical gas phase ionization (e.g., reducing discrimination processes caused by competing ionization), enhancing the analytical performance of the IMS. However, this paper concentrates on the pulsed electron source. For its characterization, we developed a measurement setup, which allows the detection of nanosecond electron pulses with amplitudes of only a few nanoamperes. Furthermore, we investigated the spatial ion distribution in the ionization region depending on several operating parameters, such as the kinetic electron energy or the ionization time.