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The influence of drift gas composition on the separation mechanism in traveling wave ion mobility spectrometry: insight from electrodynamic simulations


The influence of three different drift gases (helium, nitrogen, and argon) on the separation mechanism in traveling wave ion mobility spectrometry is explored through ion trajectory simulations which include considerations for ion diffusion based on kinetic theory and the electrodynamic traveling wave potential. The model developed for this work is an accurate depiction of a second-generation commercial traveling wave instrument. Three ion systems (cocaine, MDMA, and amphetamine) whose reduced mobility values have previously been measured in different drift gases are represented in the simulation model. The simulation results presented here provide a fundamental understanding of the separation mechanism in traveling wave, which is characterized by three regions of ion motion: (1) ions surfing on a single wave, (2) ions exhibiting intermittent roll-over onto subsequent waves, and (3) ions experiencing a steady state roll-over which repeats every few wave cycles. These regions of ion motion are accessed through changes in the gas pressure, wave amplitude, and wave velocity. Resolving power values extracted from simulated arrival times suggest that momentum transfer in helium gas is generally insufficient to access regions (2) and (3) where ion mobility separations occur. Ion mobility separations by traveling wave are predicted to be effectual for both nitrogen and argon, with slightly lower resolving power values observed for argon as a result of band-broadening due to collisional scattering. For the simulation conditions studied here, the resolving power in traveling wave plateaus between regions (2) and (3), with further increases in wave velocity contributing only minor improvements in separations.

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JCM acknowledges David Manura (Scientific Instrument Services, Inc.) for insightful direction in developing the simulation model. JAM acknowledges initial discussions with Alex Shvartsburg (Pacific Northwest National Laboratory) on the separation mechanisms in traveling wave. Financial support for this work was provided by the National Institutes of Health (NIDA RC2DA028981), the Defense Threat Reduction Agency (HDTRAI-09-1-001), the National Science Foundation (MRI CHE-1229341), the Vanderbilt Institute of Chemical Biology, the Vanderbilt Institute for Integrative Biosystems Research and Education, and the Vanderbilt University College of Arts and Sciences.

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Correspondence to John A. McLean.

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May, J.C., McLean, J.A. The influence of drift gas composition on the separation mechanism in traveling wave ion mobility spectrometry: insight from electrodynamic simulations. Int. J. Ion Mobil. Spec. 16, 85–94 (2013).

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  • Traveling wave ion mobility spectrometry
  • TWIMS resolving power
  • Electrodynamic ion simulations
  • Alternate drift gases
  • Helium
  • Nitrogen
  • Argon