Accurate Collisional Cross Section Measurement by Multi-pass Cyclic Ion Mobility Spectrometry
Authors: Xia Chaoshuang , Mernie Elias , Zaia Joseph , Costello Catherine , Lin Cheng 
Published: February 2024

Ion mobility-mass spectrometry (IM-MS) is a powerful analytical tool for structural characterization. IM measurement provides collision cross section (CCS) values that facilitate analyte identification. While CCS values can be directly calculated from mobility measurements obtained using drift tube ion mobility spectrometry (DT-IMS), this method has limited mobility resolution due to the practical constraints on the length of the ion drift path. Consequently, DT-IMS cannot differentiate analytes with similar mobilities or resolve fine mobility features of individual ions. Cyclic IMS (cIMS) instruments leverage a cyclic path enabled by traveling wave ion mobility (TWIM) technology and offer increased mobility solution to address this challenge. While TWIM devices must first be calibrated to enable CCS measurements, current calibration strategies are primarily tailored for single-pass analyses. This preference is partly attributed to the challenges associated with multi-pass calibration methods, which require both calibrants and analytes to experience the same number of passes. Achieving this consistency can be complicated due to factors like peak splitting and diffusion, and may not be feasible for on-line IM-MS analyses. A recent report employed average ion velocities obtained from multiple measurements under different separation pathlengths as a pathlength-independent metric for CCS calibration. However, the ability to exploit this averaging approach is limited by observed variation in ion drift time/velocity in these measurements. In this study, we introduce a novel calibration strategy designed for multi-pass cIMS analyses, directly targeting the root cause for the pathlength- and mobility-dependent variations in ion drift time. With this method, we demonstrate that CCS values derived from multi-pass measurements closely align with those obtained from single-pass analyses, with an average deviation of 0.1%. We apply this method to characterize four isomeric trisaccharides. Our approach not only results in excellent agreement between our measured cIMSCCS values and the reported DTCCS values, with an average difference of only 0.5%, but also allows us to effectively identify subtle mobility characteristics of each compound and determine their respective CCS values. This level of detail and accuracy was previously unattainable using DT-IMS or single-pass cIMS measurements. We developed an algorithm for reconstructing arrival time distribution in cases where wrap-around has resulted in peak splitting. Collectively, the new calibration strategy and the reconstruction procedure maintain reproducibility and precision in CCS measurements while largely eliminating the need for meticulous selection of separation times. We expect that our method will empower researchers to harness the high mobility resolution offered by multi-pass cIMS analyses without compromising the accuracy of CCS measurement, making it appropriate for straightforward use across a wide range of applications.