Increased ion throughput using tristate ion-gate multiplexing
For time dispersive ion mobility experiments detail control over the mechanism of ion beam modulation is necessary to establish optimum performance as this parameter greatly influences the temporal width of the ion beam arriving at the detector. When sampling continuous ion sources the temporal sampling or the incoming ion beam is often achieved by the electronic modulation of a grid or electric field. Not surprisingly, the rate at which a given ion population traverses this gating region is directly proportional to an ion's population and the applied electric field. This scenario establishes conditions where discrimination of the incoming ion beam may occur when the ion gate modulation rate is minimized. Recent developments in the mechanical construction of ion gates and their subsequent operation suggest that the mobility discrimination during ion gating may be minimized, however, it is remains unclear how this behavior will translate to ion beam multiplexing approaches. In this present work, we compare the performance levels of the tri-state ion shutter (3S-IS) to the two-state ion shutter (2S-IS) using a series of Fourier transform ion mobility mass spectrometry (FT-IMMS) experiments. The performance of the two different shutter operating principles were evaluated using ion multiplexing using tetraalkylammonium salts (TXA ions; T5–T8, T10, T12) bradykinin, and a set of reversed sequence isomeric pentapeptides using a variety of different ion gate frequency sweeps. Noticeable increases in ion throughput were observed for the 3S-IS with 95% and 45% increases in ion counts for the T5 and T12 ions respectively compared to the 2S-IS. Similarly, a 27% and 55% increase in ion counts was observed for the [M + 2H]2+ and [M + H]+ ions of bradykinin, respectively. In addition, a 10% increase in resolving power was also observed for the 3S-IS compared to the 2S-IS. Overall, utilization of the 3S-IS effectively minimizes both discrimination of slower ions and the impact of gate depletion effect common to traditional ion gating techniques.