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 lateral confinement of both ion polarities. Traveling wave ion mobility (IM) separations experienced by both polarities 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, where the traveling wave speed is less than the ion velocity, the two polarities are transported losslessly and non-reactively in their respective potential minima (higher absolute voltage regions confines negative polarities and lower absolute potential regions are populated by positive polarities). In separation mode, where the traveling wave speed is greater than the ion velocity, the two polarities can interact during rollovers over the traveling wave. 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.

Dual Polarity Ion Confinement and Mobility Separations

“…The electric potentials and fields created for ion confinement and manipulation in SLIM have been previously reported in conjunction with the development of SLIM-based technology. [46][47][48][49][50]52,53 Figure 2a shows a schematic diagram of the present SLIM configuration, illustrating 2 sets of the 3 arrays of 8 TW electrodes sequence sets interspaced by the 4 extended electrodes (i.e. a 4,3 arrangement 58 ) on which the RF potentials are applied (with opposite polarity to adjacent RF electrodes).…”

“…As a result, most of the time cations and anions will not occupy the same physical space, spending most of their time in their different wave troughs. 48 However, in the IM separation mode, ions do not stay in the minima as they roll over a passing wave, and lower mobility ions of opposite polarity will have a higher probability of interaction. An approach for the control of such potential ion-ion interactions in the separation mode will be discussed later.…”

“…neutralization, charge transfer). 48,[60][61][62][63] As noted earlier, there is an axial displacement of different ion polarities due to the TW, such that ions of different polarity and high mobility that are moving with the wave (i.e. surfing) will not interact; ions of increasingly lower mobility (that are being separated) will experience increasing interactions.…”

“…Under conditions where lower mobility species will undergo separation, a cation and an anion in the SLIM device could potentially interact every time they roll over a passing wave. The critical distance, the minimum distance between the two ions at which point they have the highest probability to form an orbit bound ion complex, has been estimated to be 162 µm at a relative velocity of 2 m/s, 48 and to avoid possible interactions the separation between these two species should ideally be significantly greater. A bias voltage applied to one SLIM surface can move ions of opposite polarity into regions closer to the other surface.…”

“…In this context, our laboratory has been developing a new approach for IM-based separations based upon Structures for Lossless Ion Manipulations (SLIM) that has already demonstrated large gains in IM separation resolution and other aspects of performance. [42][43][44][45][46][47][48][49][50][51][52][53] SLIM-based design implementations using traveling waves (TW) have provided the ability to achieve exceptionally long IM path lengths and thus increased resolution without the need for the higher voltages that would be required e.g. for extending the length of IM drift tube designs.…”