Characterization of applied fields for ion mobility separations in traveling wave based structures for lossless ion manipulations (SLIM)

Abstract

Ion mobility (IM) is rapidly gaining attention for the separation and analysis of biomolecules due to the ability to distinguish the shapes of ions. However, conventional constant electric field drift tube IM separations have limited resolving power, constrained by practical limitations on the path length and maximum applied voltage. The implementation of traveling waves (TW) in IM removes the latter limitation, allowing higher resolution to be achieved using extended path lengths. Both of these can be readily obtained in structures for lossless ion manipulations (SLIM), which are fabricated from arrays of electrodes patterned on two parallel surfaces where potentials are applied to generate appropriate electric fields between the surfaces. Here we have investigated the relationship between the primary SLIM variables, such as electrode dimensions, inter-surface gap, and the applied TW voltages, that directly impact the fields experienced by ions. Ion trajectory simulations and theoretical calculations have been utilized to understand the dependence of SLIM geometry and effective electric fields on IM resolution. The variables explored impact both ion confinement and the observed IM resolution using SLIM modules.

Keywords: Electric field; Ion mobility; Ion trajectory simulations; Resolution; Structures for lossless ion manipulations (SLIM); Traveling waves.

Fig. 1.

(A) Schematic diagram of the experimental setup. (B) Portion of a TW SLIM module surface using a 3,2 electrode arrangement of three RF electrodes interspersed by two segmented electrode arrays (used to create the TW) (C) Potential (black) and the field (red) due to TW only along x-axis in middle plane between the two boards for 2 mm electrode size and 2 mm inter-surface gap. Region between points #1 to #2 represents one quarter of the wave which is equivalent to 2 electrodes distance, hence the average over this range represents the average field. (D) The plot of the equipotential surface generated by the RF pseudopotential for 622 m/z for a 2 mm inter-surface gap. The two surfaces are on the left and right.

Fig. 2.

The average electric field for a TW SLIM with a 30 V_p-p amplitude calculated for different board spacing as a function of varying electrode size.

Fig. 3.

IM resolution observed for m/z 622 and 922 ions as a function of TW speed for a SLIM module of 3,2 configuration using a symmetric TW sequence (HHHHLLLL), TW amplitude of 30 V, guard bias of 15 V, and RF amplitude of 300 V_p-p for TW electrode size 0.5-mm (red circles) and 2-mm (black squares) for SLIM surface gaps of (A) 4.8-mm and (B) 2.7-mm. (C) IM resolution for m/z 622 and 922 as a function of TW speed for a SLIM module of 6,5 configuration using the same conditions for 1-mm (black squares), 2-mm (red circles) and 4-mm (blue triangles) long TW electrodes with an inter-surface gap of 2.7-mm. (D) Ion mobility spectra corresponding to the TW speed at which the highest resolution obtained for the 6,5 configuration with electrode sizes 1-mm (black), 2-mm (red) and 4-mm (blue) and an inter-surface gap of 2.7-mm.

Fig. 4.

The SIMION simulations showing the RF confinement for different geometry scaling. (A) RF voltage 150 V peak to peak, frequency 1 MHz, TW 30 V, TW speed of 45 m/s. (B) Scaled by a factor of 2, RF voltage 250 V peak to peak, frequency 1 MHz, TW 30 V, TW speed of 45 m/s. (C) Scaled by a factor of 4, RF voltage 420 V peak to peak, frequency 1 MHz, TW 30 V, TW speed of 45 m/s.