Tristate Bradbury–Nielsen Gate for Enhancing the Sensitivity of High-Resolution Ion Mobility Spectrometry

Resolution and sensitivity are crucial for standalone ion mobility spectrometry (IMS) used in the field detection of hazardous chemicals. However, it is challenging to achieve a high resolution and high sensitivity simultaneously. For commonly used Bradbury-Nielsen gate (BNG)-based IMS, increasing the gating voltage difference (GVD) between the two sets of BNG wires enhances the temporal compression effect and improves the resolving power; however, this also significantly diminishes IMS sensitivity due to the accompanying enhanced ion depletion effect. In this work, an ion focusing effect induced by increasing the GVD of BNG was first illustrated, and a nearly 200% increase in ion number density before BNG injection was achieved. To fully utilize the temporal compression effect and the ion focusing effect, a new tristate BNG working mode was proposed by inserting a compressing state between the normal BNG opening and closing states. The extra compressing state was able to reduce the mobility discrimination between ions with large mobility difference of ∼0.90 cm²/V·s to 1/19 of the original. Using the tristate BNG mode to analyze hazardous chemical mixtures, the ion current of less mobile di-2-ethylhexyl phthalate ions (DEHP)H⁺ and methyl salicylate ions (MS·O₂)^- increased 22 and 26 times, respectively, compared to normal bistate BNG mode, and their resolving powers still remained around 100. The corresponding estimated limits of detection for DEHP and MS were lowered to 228 and 143 ppt, respectively, demonstrating a promising potential for developing high-performance IMS systems that function over wide mobility ranges.