Greatly Increasing Trapped Ion Populations for Mobility Separations Using Traveling Waves in Structures for Lossless Ion Manipulations

The initial use of traveling waves (TW) for ion mobility (IM) separations using structures for lossless ion manipulations (SLIM) employed an ion funnel trap (IFT) to accumulate ions from a continuous electrospray ionization source and was limited to injected ion populations of ∼10⁶ charges due to the onset of space charge effects in the trapping region. Additional limitations arise due to the loss of resolution for the injection of ions over longer periods, such as in extended pulses. In this work a new SLIM “flat funnel” (FF) module has been developed and demonstrated to enable the accumulation of much larger ion populations and their injection for IM separations. Ion current measurements indicate a capacity of ∼3.2 × 10⁸ charges for the extended trapping volume, over an order of magnitude greater than that of the IFT. The orthogonal ion injection into a funnel shaped separation region can greatly reduce space charge effects during the initial IM separation stage, and the gradually reduced width of the path allows the ion packet to be increasingly compressed in the lateral dimension as the separation progresses, allowing efficient transmission through conductance limits or compatibility with subsequent ion manipulations. This work examined the TW, rf, and dc confining field SLIM parameters involved in ion accumulation, injection, transmission, and IM separation in the FF module using both direct ion current and MS measurements. Wide m/z range ion transmission is demonstrated, along with significant increases in the signal-to-noise ratios (S/N) due to the larger ion populations injected. Additionally, we observed a reduction in the chemical background, which was attributed to more efficient desolvation of solvent related clusters over the extended ion accumulation periods. The TW SLIM FF IM module is anticipated to be especially effective as a front end for long path SLIM IM separation modules.