U.S. Department of Energy

Pacific Northwest National Laboratory

Computational Evaluation of an Ion Peak Compression Concept for Ion Mobility Spectrometry

Separations using conventional drift-tube ion mobility (IMS) or travelling wave IMS (TW-IMS) when coupled to Mass Spectrometry (MS) have great potential e.g., for enabling high throughput pan-omics methodologies. High resolution separations need long drift times (through cyclic designs or using long drift paths), which leads to diffusional broadening and decreased signal-to-noise in most situations as well as ultimately limiting the use of cyclic devices. In the present work, using computational methods and SIMION simulations, we explore the feasibility and performance of an ‘ion peak compressor’ device that can periodically bunch diffused ions over a range of mobilities into sharper peaks. The aim is to increase peak intensity while minimizing loss of resolution for ion mobility separations.

SIMION was used to explore the effect of nonlinear voltage profiles on ion mobility separations. An SDS collision model was used to account for background gas collisions at a pressure of ~4 Torr. Ions were initially mobility separated using a linear drift field followed by switching to a nonlinear drift field. The non-linear field is applied such that ions that are lagging behind the packet centroid have greater drift velocity compared to ions that are leading. This results in decreased packet widths. The effect of ion packet compression on IMS-MS signal intensity and resolution was studied.

Preliminary Data
Ions were assumed to have drifted a distance of 87.2 cm before they enter the compressor region. After the ions of interest were in this region, nonlinear potential profiles were applied to the drift region for different time periods (i.e. a compression time tc). After the time tc, the fields were switched back to the linear profile. When ions reached the exit of the compressor device, the ion statistics were recorded. Space charge effects were also considered as they ultimately limit the extent of ion peak compressibility. An ion current of 100 pA was considered. Simulation results indicated that as tc was increased the FWHM of the arrival time distribution of the ion packet decreased up to ~50%.  Due to lower drift velocities for later portions of the nonlinear potential profile; the overall drift velocity through the compressor device decreases. When baseline-separated ion peaks are individually compressed there is a decrease in peak FWHM by ~1.5 times and a corresponding increase in signal intensity by ~1.5 times. For species having close mobilities which are partially separated before the compression field is applied, a small decrease of about ~1.4-fold in resolution accompanies the increase in signal intensity. Optimal compressor times and approaches that can allow for increase in peak intensities while further minimizing any loss in resolution are being explored. These methods will be discussed in light of the novel “Structures for Lossless Ion Manipulations” (SLIM) modules that provide very long drift lengths.

Novel Aspect: A new concept to overcome diffusional broadening and consequent decrease in ion mobility peak intensities using non-linear drift fields.


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