Experiments in driving ion axial motion by application of an AC dipolar electric field will be described. The application of AC can cause de-excitation of ion axial motion if applied out of phase with respect to ion motion and excitation (and eventual ejection from the orbitrap) if applied in phase with ion motion. Both de-excitation and excitation can be applied mass-selectively. Ion axial motion can be sufficiently de-excited that ion signal cannot be observed above baseline noise. After de-excitation, ion motion can be re-excited by application of another AC waveform. Broadband and narrowband waveforms have been used to de-excite and re-excite ion motion. Simulations, using ITSIM 6.0, were performed for these experiments. Generally good agreement with the experimental data is observed. The effects of AC amplitude, frequency, phase relative to ion motion, and bandwidth of applied waveform were simulated.
Also described is our recently-implemented method of phase-enhanced selective ion ejection. Normally, the mass resolution achieved in selective ion isolation using resonance excitation is limited by the frequency resolution of the ac waveform and by unintended off-resonance excitation. However, the new method is demonstrated to allow an isolation resolution of 28,400 and is estimated to provide a mass resolution for ion ejection of up to 100,000.
The effects of de-excitation/re-excitation upon resolution and peak shape of the re-excited ion signal have also been examined. This process improves peak shapes by re-exciting ions to produce a more narrow and symmetric distribution. Resolution is increased 10% to 200% and mass accuracy is improved up to seven-fold. These results suggest that re-excitation may be useful with sub-optimal injection conditions. (We note here that the ion injection method used on our orbitrap differs from the commercial instrument.) Simulations of this process suggest that space charge effects are necessary to bring about the peak narrowing. These results will be discussed in conjunction with preliminary observations of space-charge effects (peak coalescence) in other contexts and supporting simulations.
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