Improving Ion Mobility Spectrometer Sensitivity through the Extended Field Switching Ion Shutter

Abstract

Field switching ion shutters allow generating short ion packets with high ion densities by first ionizing for several milliseconds in a field-free ionization region and then quickly pushing the entire ion population out into the drift region. Thus, they are an excellent choice for compact ion mobility spectrometers with both high resolving power and low limits of detection. Here, we present an improved setup, named the extended field switching ion shutter. By generating a second field-free region between the ionization region and the drift region, shielding of the ionization region is significantly improved, even when using grids with higher optical transparency to improve ion transmission into the drift region. Furthermore, it is shown that under certain conditions, ion transmission through multiple grids in series can even surpass transmission through a single grid of the same transparency. For the studied ions, the signal intensity at low concentrations increases by approximately a factor of 7 to 9 for protonated monomers, 10 to 14 for proton-bound dimers, and 25 for the proton-bound 1-octanol trimer compared to the classical field switching shutter. However, due to the nonlinear response for ions containing multiple analyte molecules, the limits of detection improve only by a factor of 3 to 4 for proton-bound dimers and 3 for the proton-bound 1-octanol trimer. Nevertheless, this still leads to single-digit pptv limits of detection for protonated monomers and hundred pptv limits of detection for proton-bound dimers measured for a series of ketones. However, for the most intense peaks such as the reactant ion peak, a significant loss of resolving power by a factor of up to 1.4 was observed due to Coulomb repulsion.

Compact and Sensitive Dual Drift Tube Ion Mobility Spectrometer with a New Dual Field Switching Ion Shutter for Simultaneous Detection of Both Ion Polarities

“…For example, increasing the potential of the pusher electrode above the potential of the injection grid transfers positive ions into the drift tube, while negative ions are discharged at the pusher electrode. For maximum ion accumulation leading to maximum IMS sensitivity, the field free ionization region during ionization is important . However, it is not trivial to obtain such a field free ionization region as the constant drift field always penetrates the injection grid reaching into the ionization region.…”

“…The electric potentials of both injection grids are chosen in such a way that during injection a potential gradient with the desired injection field strength is achieved. For a compact construction of a dual drift tube IMS with high resolving power this requires a field strength that is significantly higher than the drift field strength. , The electric potentials of the pusher grids are then linearly approximated from the distances between the grids. During ionization, the electric potentials of both pusher grids are set to the mean potential between both injection grids leading to a large potential difference between each pair of pusher grid and injection grid.…”

“…In this variant, an additional grid is used to apply the blocking voltage, leading to better shielding of the ionization region. It is important to mention that additional grids do not necessarily lower ion transmission . This idea can be adapted to the dual FS ion shutter for the dual drift tube IMS by adding two more grids, the so-called extension grids.…”

“…Ion mobility spectrometers (IMS) combining atmospheric pressure chemical ionization (APCI) with field switching ion shutters are extremely sensitive trace gas detectors, reaching limits of detection (LoD) in the single digit ppt v range within measurement times of a second. − However, depending on the target molecule properties, positive and/or negative ions may form during the ionization process. − Thus, analyzing of both ion polarities is required in many applications.…”

Novel ion drift tube for high-performance ion mobility spectrometers based on a composite material

“…Ion mobility spectrometers (IMS) are compact analytical instruments, mainly for the rapid detection of volatile and semivolatile substances at trace levels in the range of ppt V (partsper-trillion by volume) within a few seconds [1,2]. Equipped with electrospray ionization, IMS are also used for measuring larger molecules in liquids [3][4][5].…”