Iterative SLIM (itSLIM) for Ultrahigh Resolution Targeted Ion Mobility Analysis

We present a novel approach for achieving ultrahigh-resolution ion mobility (UHRIM) separations using a structures for lossless ion manipulation (SLIM) ion mobility-mass spectrometry (IM-MS) system. By incorporating a rounded-turn ion path design, ions can be transmitted and separated bidirectionally, enabling an iterative workflow in which mobility-separated ions are returned to the entrance of the serpentine path while preserving their separation order and position. This "iterative SLIM" (itSLIM) process can be repeated multiple times with little to no target ion loss to increase the effective path length of the mobility separation. As IM resolution scales with the square root of the separation path length, this method enhances IM resolution without increasing the form factor of the mobility device. It is particularly beneficial for targeted mobility analysis and confident identification where high specificity is required. UHRIM separation was achieved with a two-peak resolution of 2.48 for 18:1 <i>Δ9-cis</i>- and <i>Δ9-trans</i>-phosphatidylethanolamine (PE) lipid isomers at a path length of 120 m. By combining itSLIM separation over a 90-m path with MS/MS fragmentation analysis, a liquid chromatography (LC)-free workflow capable of definitive identification of the isomeric small molecule drugs norcodeine and norhydrocodone was achieved. The UHRIM separation simplifies the resulting MS/MS spectra and improves identification accuracy. Such capabilities are anticipated to enhance the throughput by reducing or eliminating LC run times and robustness of screening workflows. Iterative SLIM provides a powerful strategy for enhancing IM resolution, offering broad utility for high-performance analytical separations.