U.S. Department of Energy

Pacific Northwest National Laboratory

Very Long Path Length Ion Mobility Separations using Structures Lossless Ion Manipulations

Introduction

Ion mobility-based separations are of increasing importance in conjunction with analytical applications of mass spectrometry, not only providing additional structure-related information, but potentially more complete analysis of complex samples, detection of lower level constituents, and greater measurement throughput than feasible with on-line liquid phase separations.  The benefits of mobility-based separations generally increase as separation power increases, but high resolution mobility separations have only been achieved in conjunction with significant ion losses, substantially limiting their practicality and range of applications.  This presentation will describe initial efforts at implementing approaches using Structures for Lossless Ion Manipulations (SLIM) for enabling very long path length high resolution ion mobility separations.

 

Methods 

Extended mobility separations in SLIM have been initially explored using modeling and ion simulations, and then implemented using printed circuit board (PCB) technologies.  The SLIM include arrays of electrodes to which RF potentials are applied so as to create pseudopotentials that prevent ion losses. DC potential gradients can be concurrently applied to drive ion motion.   The two SLIM PCB surfaces were closely spaced to create the desired ion pathways and situated in a chamber maintained at a pressure where RF ion confinement is effective (~4 torr in this work).  An external nanoESI source and a dual ion funnel interface was utilized prior to transfer to the SLIM, and exiting ions were detected using a TOF MS.

 

Preliminary data 

We have previously demonstrated the capabilities of PCB-based SLIM designs to enable simple ion manipulations, including low resolution ion mobility separations, extended and lossless ion trapping, and rapid switching of ions to orthogonal pathways and without significant loss of mobility resolution. The designs initially studied used extended arrays of electrodes for RF confinement that had dimensions of 0.75 mm x 5 mm with 0.75 mm spacing between electrodes, and ~5 mm gap between the two surfaces. Ions introduced from the external ESI source were injected into one of the ion pathways in which RF frequencies from ~1 MHz and 100 to 300 Vp-p were applied to the rung electrode arrays on the opposing PCB surfaces.  Based upon these initial developments we have developed a SLIM cyclic ion mobility separation module that switches ions to move in a rectangular ~90 cm path that includes four 90-degree turns, designed to minimize loss of resolution due to the ‘race track’ effect. A linear DC field gradient is maintained across three of the four linear segments of the cyclic path, and the field gradient is switched to effectively rotate the electric field in the four segments at a rate determined by the mobility of the ions being targeted. SLIM switches both initiate cyclic motion and terminate ion motion after the selected number of cycles by directing ions to the TOF MS.  The detailed design of both the SLIM module and the control system will be described, and initial results presented.  Additionally, alternative concepts for achieving long path length ion mobility separations and their initial evaluation will also be presented.

 

Novel aspect

The development of SLIM modules enabling very long path length ion mobility separations provides both high resolution and high sensitivity.

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