Miniaturized Drift Tube Ion Mobility Spectrometer with Ultra-Fast Polarity Switching

Ion mobility spectrometers (IMS) are well suited for detecting trace gases down to levels at ppb_v and even ppt_v within 1 s of analysis time when using chemical ionization. The measuring principle is based on the separation and detection of the ionized constituents of a sample. Depending on the sample composition, certain ionization sources create both positive and negative analyte ions, but the simultaneous detection of both ion polarities usually requires two drift tubes. Contained within this effort, we present an alternative approach for detecting both ion polarities using one single drift tube that can switch the polarity of the drift tube within 12 ms. This technique allows for generating one positive and one negative ion mobility spectrum, each with a drift time range of 13 ms (minimum reduced ion mobility of K₀ = 0.72 cm² V^-1 s^-1), within a total experiment time of 50 ms. Additionally, ions are continuously generated in the ionization region during both the polarity switching and the analysis of one of the polarities, which allows for an effective ionization/reaction time of 25 ms. Comparable to the performance of similar instrument designs we reported previously, the presented device has a high resolving power of R_P = 70 with a drift length of 51 mm. The limits of detection are for the monomers between 70 and 370 ppt_v and for the dimers between 450 and 800 ppt_v for 1 s of averaging for various ketones, methyl salicylate, and chlorinated hydrocarbons. Although this work focuses on applying ultra-fast polarity switching to an existing IMS, the techniques shown here may be applied to other IMS implementations for different applications.