Three-dimensional motional stabilization in the trapping field of an open-ended trapped-ion cell: application to the remeasurement experiment in Fourier transform ion cyclotron resonance mass spectrometry.

Three-dimensional motional stabilization of radial trajectories of low-mass organic ions in an open cell using only a dc trapping field is applied to the FT-ICR remeasurement experiment. More than 300 remeasurement cycles are observed with 99.59% remeasurement efficiency for benzene (m/z 78) using a high-pressure helium buffer gas. The enhancement in remeasurement efficiency is due to collisional stabilization of the guiding center of ion motion by dynamic motional averaging in the axial position-dependent radial electric field. Such dc-induced radial stabilization is in contrast to the stability produced by application of radio frequency fields characteristic of quadrupolar axialization or rf-only mode operation. The same effect is produced because ions experience a radial "pseudopotential" during axial oscillation as in time-varying fields. Trajectory simulations for ions oscillating in an open cell trapping well above a z-amplitude critical threshold energy of 0.60 eV (in a potential well of 0.84 V) indicate that radially stabilized trapping motion is achieved because the outward-directed radial electric field existing near the cell center line is compensated by an opposing inward-directed radial electric field at extended z-amplitude. Sufficient axial kinetic energy permits ion penetration into the inward-directed radial electric field regions, enabling > 50% residence time of each trapping oscillation period in regions inducing radial stability, thereby inhibiting magnetron radius growth. A high-pressure, low-mass buffer gas such as helium provides the requisite increase in the axial amplitude of the ion cloud, similar to the mechanism observed for axial excitation of low-mass ions observed in collision-induced dissociation. The result is radial stability at high pressure, even after multiple remeasurement cycles. An optimized excitation radius of 12.5% of the cell radius yields maximum remeasurement efficiency with a 500 ms relaxation delay between excitation events. Summed signal intensity decreases with increased trap potential due to the greater radial electric field and reduced axial expansion of the ion cloud and also decreases with buffer gas mass in response to greater radial scattering.