Principles of Ion Selection, Alignment, and Focusing in Tandem Ion Mobility Implemented Using Structures for Lossless Ion Manipulations (SLIM)

  • Rachel M. Eaton
  • Samuel J. Allen
  • Matthew F. BushEmail author
Focus: Ion Mobility Spectrometry (IMS): Research Article


Tandem ion mobility (IM) enables the characterization of subpopulations of ions from larger ensembles, including differences that cannot be resolved in a single dimension of IM. Tandem IM consists of at least two IM regions that are each separated by an ion selection region. In many implementations of tandem IM, ions eluting from a dimension of separation are filtered and immediately transferred to the subsequent dimension of separation (selection-only experiments). We recently reported a mode of operation in which ions eluting from a dimension are trapped prior to the subsequent dimension (selection-trapping experiments), which was implemented on an instrument constructed using the structures for lossless ion manipulations (SLIM) architecture. Here, we use a combination of experiments and trajectory simulations to characterize aspects of the selection, trapping, and separation processes underlying these modes of operation. For example, the actual temporal profile of filtered ions can be very similar to the width of the waveforms used for selection, but depending on experimental parameters, can differ by up to ± 500 μs. Experiments and simulations indicate that ions in selection-trapping experiments can be spatially focused between dimensions, which removes the broadening that occurred during the preceding dimension. During focusing, individual ions are thermalized, which aligns and establishes common initial conditions for the subsequent dimension. Therefore, selection-trapping experiments appear to offer significant advantages relative to selection-only experiments, which we anticipate will become more pronounced in future experiments that make use of longer IM separations, additional dimensions of analysis, and the outcomes of this study.

Graphical Abstract


Ion mobility Tandem ion mobility Protein structure 



This material is based upon work supported by the National Science Foundation under CHE-1807382 (M.F.B.) and DGE-1256082 (R.M.E.) and by the ACS Division of Analytical Chemistry (fellowship to R.M.E.). We thank Prof. Robert Synovec (University of Washington) for useful discussions. We thank Benjamin Zercher (University of Washington) for critically reviewing the manuscript. We thank the SLIM Consortium at Pacific Northwest National Laboratory, including Dr. Richard Smith, Dr. Yehia Ibrahim, Dr. Randy Norheim, Dr. Tsung-Chi Chen, Spencer Prost, and Dr. Ian Webb for sharing designs, software, and technical expertise. We thank Gordon Anderson (GAA Custom Engineering) for assistance with electronics.

Supplementary material

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Copyright information

© American Society for Mass Spectrometry 2019

Authors and Affiliations

  1. 1.Department of ChemistryUniversity of WashingtonSeattleUSA

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