Design of a TW-SLIM Module for Dual Polarity Confinement, Transport, and Reactions

Abstract

Here we describe instrumental approaches for performing dual polarity ion confinement, transport, ion mobility separations, and reactions in structures for lossless ion manipulations (SLIM). Previous means of ion confinement in SLIM, based upon rf-generated pseudopotentials and DC fields for lateral confinement, cannot trap ions of opposite polarity simultaneously. Here we explore alternative approaches to provide simultaneous lateral confinement of both ion polarities. Traveling wave ion mobility (IM) separations experienced in such SLIM cause ions of both polarities to migrate in the same directions and exhibit similar separations. The ion motion (and relative motion of the two polarities) under both surfing and IM separation conditions are discussed. In surfing conditions the two polarities are transported losslessly and non-reactively in their respective potential minima (higher absolute voltage regions confine negative polarities, and lower absolute potential regions are populated by positive polarities). In separation mode, where ions roll over an overtaking traveling wave, the two polarities can interact during the rollovers. Strategies to minimize overlap of the two ion populations to prevent reactive losses during separations are presented. A theoretical treatment of the time scales over which two populations (injected into a DC field-free region of the dual polarity SLIM device) interact is considered, and SLIM designs for allowing ion/ion interactions and other manipulations with dual polarities at 4 Torr are presented. Graphical Abstract ᅟ.

Keywords: Dual polarity; Ion mobility; Ion/ion reaction; SLIM; Traveling wave.

Figure 1

(a) Top panel shows schematic of SLIM-TW board layout, with iso-potential surface (at 20 V) showing ion conduits created in SLIM for confinement of positive ions, and bottom panel shows the potential experienced by ions along a line equidistant between the two SLIM boards. (b) Top panel shows the ion trajectories for positive ions (red traces) and negative ions (black traces) under the same effective potential conditions. Bottom panel shows the ion trajectories when the guard potential is made negative (−5 V), in which case the positive ions are lost laterally while negative ions are confined.

Figure 2

(a) Left panel shows the schematic of board design to confine dual polarities. Guard electrodes as shown in purple in Figure 1a are replaced with rf electrodes and the boards are paired in such a way that counter-polarity rf electrodes are laid out in the opposing board. Right panel shows the view between the boards in the YZ plane with SIMION ion trajectories super-imposed showing quadrupole fields confining the ions (unlike in Figure 1a where Guard electrodes create a field penetration leading to skewed dumbbell shaped confining potential) (b) Top panel shows ion conduits created for confining dual polarities representing the iso-surface of potential at 30V, bottom panel shows the potential on an equidistant line between the SLIM boards (c) ion trajectory simulations in SIMION showing lossless transmission of both positive (red traces) and negative (black traces) ions. (d) ATDs for m/z 622 (K_o 1.17 cm2/Vs) and m/z 922 (K_o = 0.97 cm2/Vs) over a path length of 4 cm.

Figure 2

(a) Left panel shows the schematic of board design to confine dual polarities. Guard electrodes as shown in purple in Figure 1a are replaced with rf electrodes and the boards are paired in such a way that counter-polarity rf electrodes are laid out in the opposing board. Right panel shows the view between the boards in the YZ plane with SIMION ion trajectories super-imposed showing quadrupole fields confining the ions (unlike in Figure 1a where Guard electrodes create a field penetration leading to skewed dumbbell shaped confining potential) (b) Top panel shows ion conduits created for confining dual polarities representing the iso-surface of potential at 30V, bottom panel shows the potential on an equidistant line between the SLIM boards (c) ion trajectory simulations in SIMION showing lossless transmission of both positive (red traces) and negative (black traces) ions. (d) ATDs for m/z 622 (K_o 1.17 cm2/Vs) and m/z 922 (K_o = 0.97 cm2/Vs) over a path length of 4 cm.

Figure 3

(a) The evolution of distance between two hypothetical ions of same mass, mobility and opposite charge as they traverse through the dual polarity SLIM device in the surfing mode where ions are confined to bins of TW. Ions are started at same point in the simulation and they separate into respective potential minima as shown in the inset where positive ions (blue) populate low potential regions shown by (darker colored TW electrodes) and negative ions (red) populate the high voltage regions (b) Evolution of distance between polarities when they are in the separation mode. Ions of same mobility and opposite polarity are considered, showing that they pass each other during every cycle of the TW. The distance between polarities is lesser than critical distance for ion/ion reaction for a small fraction of time (discussion below in text). Inset shows the relative velocities between the ions (magnitude varying between 1 m/s and 150 m/s) over the course of their transport. (c) The two polarities are laterally separated to by applying +4 V and −4 V guard biases on each side. Inset shows separation between the plumes at 1 V (with ~25% overlap of the plumes) and 2 V biases. (d) The two polarities separated between the two boards by biasing one board at +7 V.

Figure 4

(a) SLIM design with the two polarities introduced from either side of a 16 mm long dc field free region. (b) The relative approach velocity between populations (of 100,000 charges each of protonated peptide neurotensin m/z 558.6473 and reduced mobility 0.9 cm²/Vs and deprotonated 1H,1H-Perfluoro-1-octanol m/z 399.07 and reduced mobility 1.2 cm²/Vs) as a function distance between polarities. (c) Distance between the two polarities as a function of time beginning with an initial separation of ~14 mm.