Pulsed, gridded electron reversal ionizer

Abstract

The design and performance of a new-type source of negative ions is presented. This device retains the idea of the so-called reversal electron attachment detector [READ, M. T. Bernius and A. Chutjian, J. Appl. Phys. 66, 2783 (1989)] to produce low-energy electrons by reversing electron trajectories at an electrode. Electrons are produced in the present device by several ribbon filaments that surround a cylindrical wire grid. They are accelerated through the grid, then decelerated to zero, or near-zero velocity at the surface of a tube. The tube (anode) is perforated with small holes through which effuse the target molecules. Electrons attach to form either parent and/or fragment negative ions. Attachment takes place at a location above the anode corresponding to a match between the electron energy and the attachment resonance energy. Negative ions are pulsed out of the attachment region and focused onto the entrance plane of a quadrupole mass selector. Electron and ion trajectory calculations are presented, and the performance illustrated with six molecules having attachment resonances in the range 0.0–2.2 eV. The detection sensitivity as measured using the method of standard additions is below 1 part-per-trillion, with a calculated conversion rate (ions/electrons) of 1.3%. Signal nonlinearity is discussed in terms of ion space-charge effects. The device also produces positive ions, and an analogous treatment may be carried through to characterize its performance in this mode.