U.S. Department of Energy

Pacific Northwest National Laboratory

Fundamentals of Ion Dynamics in Structures for Lossless Ion Manipulations

Introduction:

While much effort has gone into developing improved separation strategies for use with MS analysis, the extensive demands for more effective characterization of complex biological mixtures drives further efforts to meet these needs. Gas phase separations based upon ion mobility (IM) are fast, amenable to high-throughput application, and provide high reproducibility. New platforms that allow complex ion manipulations, e.g. mobility based ion selections, CID, ion/ion reactions, in addition to higher resolution separations, are of interest. Here we characterize the fundamentals of ion dynamics and consider novel ion processing approaches in Structures for Lossless Ion Manipulations (SLIM). Ion confinement, ion dynamics, heating effects and separation performance and other insights from simulations and theory will be discussed.

Methods:

Theoretical and simulation methods are used to study ion trajectories in SLIM at a pressure of ~4 Torr. Ion energies, heating due to the applied fields for confinement and IM separation in SLIM are explored. Computational models and SIMION simulations were used to optimize the design of SLIM modules for comparison with experimental performance. Optimized designs implemented and interfaced with an Agilent QTOF mass spectrometer to e.g. evaluate separations and their utility in conjunction with MS. The details of ion motion in SLIM ion traps based upon traveling waves (TW) have been explored using theoretical and simulation methods. Using appropriate combination of static, TW and RF fields, novel SLIM modules for ion separations were developed and characterized.

Preliminary Data:

The fundamentals of ion confinement and motion in SLIM modules were studied. The RF trapping potentials and pseudo-potentials created at short distances from the electrode surfaces (i.e. the SLIM PCBs) indicated that lossless ion performance could be achieved, which was verified experimentally. Applying TW fields to sets of electrodes on the SLIM surfaces was found to provide higher IM peak resolutions compared to constant field IMS for a given path length. The TW-SLIM mobility resolution was characterized for different TW speeds and amplitudes. Theoretical characterization of the fields experienced by ions, showed that ions are separated in TW-SLIM based on their relative motion with respect to the TWs. While faster ions that “surfed” on the edge of the traveling wave were not separated, ions slower than a threshold were separated to extents that depended upon the number of times they ‘rolled over’ (or were passed over by) the TWs. Under optimum resolution conditions, ions being separated were found to experience a broad range of fields (2 to 60 V/cm). Ions resided in intermediate field regions for the majority of the time (the extent of which depended on TW speed and amplitude). The arrival time of ions and peak widths calculated from simulations and experiments were in agreement, and were greater than peak widths due to purely diffusion. The factors that contribute to the additional peak width in TW IMS will be discussed. The useful range of mobilities and m/z of the TW-SLIM device were also evaluated. Methods to ‘turn’ ions around corners were feasible with no significant “race track” effect related degradation of IM resolution. With insights from simulations, the developments in SLIM modules for manipulating dual polarity populations of ions, and also development of a SLIM ion mobility filter will be reported.

Novel Aspect:

An improved understanding of the fundamentals of gas phase ion manipulations and dynamics in SLIM.

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