U.S. Department of Energy

Pacific Northwest National Laboratory

Simultaneous and co-located dual polarity ion confinement and mobility separation in traveling wave-based structures for lossless ion manipulations (SLIM)

Introduction 

Ion mobility (IM) coupled with mass spectrometry has gained prominence as a powerful analytical tool. To advance IM technology performance to higher levels SLIM technology has recently been developed in our laboratory, and has provided the basis for large gains in IM resolution as well as sensitivity. In many applications both positive and negative ion separations provide complementary information. In this work we explore the use of traveling waves in SLIM to simultaneously confine and separate by IM co-located cations and anions.

Method 

SIMION ion trajectory software was used to simulate ion confinement in SLIM, as well as ion transport. Ion-neutral collisions during ion transport simulations were modelled using the SDS collision model, which employs statistical methods to account for ion collision with buffer gas. The simulations were used to optimize the SLIM design process as well as predict possible experimental performance. MATLAB software package was also used to obtain and analyze the ion confinement potentials.

 

Preliminary Data 

Static voltages applied to guard electrodes in traditional SLIM configurations provide good lateral confinement for single ion polarity experiments, but such conditions lead to the loss of opposite polarity ions. It is well recognized that rf ion traps can simultaneously confine ions of both polarities.  In this work we have explored the potential for developing instrumentation allowing the simultaneous introduction, and manipulation (including IM separation) of both positive and negative ions in a new SLIM design. Preliminary data obtained from ion trajectory simulations have shown the possibility to simultaneously confine and transport both positively and negatively charged ions. Simultaneous confinement for ions of both polarities was achieved by replacing the guard electrodes in the traditional SLIM configuration which employed static voltages (typically 5V above the travelling wave (TW) voltage) for lateral confinement of ions between the SLIM boards with RF “guards” which use dynamic voltages for the lateral confinement of the ions. Concurrent ion transport is also achieved due to the nature of the dynamic voltage profile of the TW which presents a potential minima at opposite ends of the voltage wave for each ion polarity as the wave transverses the segmented TW electrodes, and thus subsequently provide efficient ion transport. We have also shown using ion trajectory simulations, the capability to manipulate the spatial separation of ion populations in SLIM based on their polarities, by biasing the RF guards on each side of the ion conduit so as to limit the interactions between the two ion polarity populations if ion-ion interactions could lead to ion loss during transmission.  This presentation will also describe our progress in experimental implementation.

Novel Aspect 

First exploration and demonstration of simultaneous positive and negative collocated confinement and IM separation

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