Enabling Multidimensional Ion Mobility Separations Using Structures for Lossless Ion Manipulations

Abstract

This dissertation describes the development and application of multidimensional ion mobility (IM) techniques using the modular Structures for Lossless Ion Manipulations (SLIM) architecture. IM is an analytical technique in which charged ions are separated in a neutral background gas under the influence of an applied electric field. IM separates ions based on their size, shape, and charge. IM was paired to mass spectrometry (MS), an analytical technique that determines an ion’s mass to charge (m/z) ratio. To provide context for multidimensional techniques described in this dissertation, Chapter 1 provides an overview of next-generation IM instrumentation that enables both multidimensional and ultrahigh-resolution IM separations. The following chapters describe experimental efforts to realize multidimensional experiments on a modular IM instrument that utilizes the Structures for Lossless Ion Manipulations (SLIM) architecture. Chapter 2 investigates the structural stability of native-like protein ions, which are ions generated from non-denaturing solution conditions that exhibit compact structures that can retain memory of solution-phase structure. Using time-dependent, multidimensional IM experiments, individual charge states of native-like protein ions were mobility-selected and trapped for varying periods of time (up to 14 s). Structural dynamics during ion trapping were probed in a second dimension of IM. While some native-like ions exhibited structural dynamics, the unfolding that was observed was less extensive than complete unfolding, demonstrating that ions can retain memory of the solution-phase structure for remarkably long times in the gas phase. These results inform applications of next-generation IM instrumentation that could potentially separate ions using much longer timescales.In Chapter 3, a new strategy for multidimensional IM separations on instruments that use electrostatic gradients is presented. In this strategy, the potentials of trapped ions are “reset” between dimensions of IM, which allows for increased flexibility in the application of electrostatic gradients to different regions of the instrument. Potential-resetting experiments were compared to standard tandem IM experiments, which demonstrated that ions are neither lost nor activated in this strategy. We propose to use this strategy on the final implementation of the modular instrument described here, in which the installation of four additional SLIM modules will create a “cyclic” path to enable flexible experiments. As the experiments described herein are performed using electrostatic gradients, it is possible to determine an ion’s mobility directly from experimental observables without calibration. The conversion of collision cross section values from nitrogen to estimated helium values is then described in Chapter 4, further increasing the utility in cross-platform comparison of IM results.