MOTIONAL AVERAGING OF IONS FOR CONTROL OF MAGNETRON MOTION IN FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE OPEN-GEOMETRY TRAPPED-ION CELLS

Open trapped-ion cell geometries have been utilized extensively in ext ernal source Fourier transform ion cyclotron resonance (FTICR) applica tions. A fundamental difference between open and closed cells is that in open cells the radial electric held undergoes a shift in orientatio n 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 amplitu de encounter an inward-directed stabilizing force. Radial destabilizat ion at the center of the cell is compensated by radial stabilization a t increased z amplitude through motional averaging along the z axis. T his suggests that conditions can be created in the open cell for which deleterious effects due to magnetron motion can be minimized. This co ntrasts with the closed cell, for which the radial electric field is o utward directed at every point along the z axis. This unique aspect of the open geometry trapped-ion cell is demonstrated to reduce radial d rift by control of the axial amplitude of the ion cloud. Suspended tra pping and resonance excitation of the trapping motion are used to incr ease the ion z amplitude. For ions with increased z amplitude, the eff ective cyclotron frequency (omega(+)) approaches the unperturbed cyclo tron frequency (omega(c)) with virtual elimination of the magnetron fr equency (omega(-)). This is observed in benzene for which omega(+) is observed only 16 Hz below omega(c). Additionally, resonant excitation at the trapping frequency (2 omega(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 r adial trajectory stabilizing characteristic of the open cell was maxim ized using shallow trapping wells in nested traps that yielded cyclotr on frequency shifts 300 Hz higher than the unperturbed cyclotron frequ ency. 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 cl oud due to ion Coulomb repulsion, and the shallower trapping well. The presence of large numbers of charges (electrons, negative ions) in th e outer trapping wells of polarity opposite the positive ions in the c entral trapping well may compensate for the depression of cyclotron fr equency due to space charge. (Int J Mass Spectrom 178 (1998) 173-186) (C) 1998 Elsevier Science B.V.