Abstract

A new method to eject ions from an electrodynamic ion trap is discussed. An asymmetric trapping field is electronically generated with electrodes, comprised of a ring electrode and two end-cap electrodes having symmetrical hyperbolic surfaces. The asymmetric trapping field is generated by adding an alternating voltage out of phase to each end cap at the same frequency as the ring electrode. The resulting multipole field has a dipole component in the axial direction, as well as a significant hexapole component. All ions confined in the combined trapping field are uniformly displaced away from the geometrical center of the ion trap, defined to be the midpoint along the axis of symmetry between the end-cap electrodes. The displacement is independent of the mass-to-charge ratio of the ions. During mass analysis ions are ejected predominantly through one end-cap electrode and the detected ion signal intensity is doubled. Mass analysis is performed by the application of phase-locked supplementary dipole and quadrupole excitation fields at an operating point corresponding to β z = 2/3. Ions at this operating point are simultaneously in resonance with three fields: (1) the nonlinear hexapole trapping field resonance, (2) the parametric resonance with the supplementary quadrupole field, and (3) the sideband resonance with the supplementary dipole field. The simultaneous triple resonance causes extremely fast power absorption by the ions and improved mass resolution.

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