Motional averaging of ions for control of magnetron motion in fourier transform ion cyclotron resonance open-geometry trapped-ion cells

Abstract

Open trapped-ion cell geometries have been utilized extensively in external source Fourier transform ion cyclotron resonance (FTICR) applications. A fundamental difference between open and closed cells is that in open cells the radial electric field undergoes a shift in orientation as a function of z displacement. Ions confined near the cell radial centerline in the z center of the cell experience an outward-directed destabilizing field gradient force whereas ions at large axial amplitude encounter an inward-directed stabilizing force. Radial destabilization at the center of the cell is compensated by radial stabilization at increased z amplitude through motional averaging along the z axis. This suggests that conditions can be created in the open cell for which deleterious effects due to magnetron motion can be minimized. This contrasts with the closed cell, for which the radial electric field is outward directed at every point along the z axis. This unique aspect of the open geometry trapped-ion cell is demonstrated to reduce radial drift by control of the axial amplitude of the ion cloud. Suspended trapping and resonance excitation of the trapping motion are used to increase the ion z amplitude. For ions with increased z amplitude, the effective cyclotron frequency (ω +) approaches the unperturbed cyclotron frequency (ω c ) with virtual elimination of the magnetron frequency (ω −). This is observed in benzene for which ω + is observed only 16 Hz below ω c . Additionally, resonant excitation at the trapping frequency (2ω t ) was used to increase the axial extent of the ion cloud, raising cyclotron frequency up to 550 Hz. The cyclotron frequency shift also occurs without peak broadening. This radial trajectory stabilizing characteristic of the open cell was maximized using shallow trapping wells in nested traps that yielded cyclotron frequency shifts 300 Hz higher than the unperturbed cyclotron frequency. This is a result of longer ion residence time at the turnaround points in the trapping well (where the inward-directed radial electric field is greatest in magnitude), increased axial extent of the ion cloud due to ion Coulomb repulsion, and the shallower trapping well. The presence of large numbers of charges (electrons, negative ions) in the outer trapping wells of polarity opposite the positive ions in the central trapping well may compensate for the depression of cyclotron frequency due to space charge.