Toward Compact High-Performance Ion Mobility Spectrometers: Ion Gating in Ion Mobility Spectrometry

Printed circuit board (PCB) based drift tube ion mobility spectrometers enable the use of state-of-the-art production techniques to manufacture compact devices with excellent performance at minimum cost. The new PCB ion mobility spectrometer (PCB-IMS) presented here is equipped with either a 140 MBq tritium or a 95 MBq nickel-63 ionization source and consists of a combination of horizontally arranged 6-layer PCBs for the drift and reaction regions and vertically arranged PCBs for interfacing the ionization source, ion shutter, and detector. The design allows the reproducible manufacturing and thus comparison of different IMS topologies. Here, we investigate different ion shutters, field-switching, Bradbury-Nielsen, and tristate and their effects on resolving power and limits of detection considering two different ionization region geometries and ionization sources, tritium and nickel-63. It is shown that the high resolving power of R_P > 80 at low drift voltage of 3 kV and short drift length of 50 mm can be achieved independent of the used ion shutter mechanism and reaction region geometry. While the resolving power of all ion shutters is excellent, the Bradbury-Nielsen shutter shows a pronounced discrimination of slow ion species when using short shutter opening times for small initial ion cloud widths, as required for high resolving power. Thus, the intensity of the proton-bound dimer of 2-pentanone is reduced by 30% compared to the signal intensity obtained with both the field-switching and tristate shutter. The detection limits employing the Bradbury-Nielsen shutter and a 50 mm reaction region as required for nickel-63 are 58 ppt_v for the protonated monomer and 3.4 ppb_v for the proton-bound dimer of 2-pentanone. The detection limits achieved with the tristate shutter utilizing the same reaction region are slightly higher for the protonated monomer at 68 ppt_v, but lower for the proton-bound dimer at 2 ppb_v due to the advanced ion shutter principle not discriminating slow ions. However, the lowest detection limits of 13 ppt_v and 301 ppt_v can be achieved with the field-switching shutter and a 2 mm reaction region, sufficient for a tritium ionization source.