Ion funnels for the masses: experiments and simulations with a simplified ion funnel

Abstract

A modified ion funnel is described. Counterintuitively, increased spacing between electrodes results in enhanced "focusing" of the ions through the funnel. Consequently, the internal diameter (i.d.) of the funnel need not decrease to the conductance limit (as in previous designs). A simple dc-only lens, which also serves as the conductance limit, combined with the natural flow of gas is used to extract the ions from the funnel. Ions with mass to charge ratios varying between 75 and 3000 m/z are passed through the funnel with no apparent discrimination. The funnel can be operated under mild conditions that preserve weakly bound noncovalent complexes. After testing several designs, a thin closely spaced dc lens was found to be the best solution for extracting ions. A simple method for simulating ion trajectories at nonzero pressures based on ion mobility and explicit diffusion is described. This theoretical approach was used to design and calculate ion trajectories for the modified funnel presented here. Finally, the increased spacing between electrodes in the current funnel significantly relaxes machining constraints, reduces cost, and enhances ease of use versus previous funnel designs.