フーリエ変換イオンサイクロトロン共鳴（ＦＴ‐ＩＣＲ）セル内の３次元イオン軌道解析：励起電圧波形がイオンの磁場方向の運動に及ぼす影響

Abstract

Effects of the three cyclotron resonance excitation waveforms on the trapping oscillation (z-motion) of ions confined in a cylindrical Fourier transform ion cyclotron resonance (FT-ICR) cell are examined by computer simulation of ion trajectories.As to the fast frequency sweep waveform, ion signal intensity varies with the direction of the frequency sweep: by upward sweeping the amplitude of z-motion is increased at frequencies slightly higher than the cyclotron resonance frequency and by downward sweeping the z-motion is damped at slightly lower frequencies. Thus the down-sweep is suitable for ICR signal detection and for ion ejection the up-sweep is preferable.In regard to the stored waveform inverse Fourier transform (SWIFT) waveform, the z-motion varies with the modulation of phase with frequency. In the case of the quadratic phase modulation, where ions are successively excited from low to high masses, the motion of ions in the z-direction is similar to that observed with the down-sweep excitation waveform.Because of the extremely short duration of the impulse waveform it might be thought that the waveform hardly has time to excite the z-motion. However, the displacement of ion position by the strong electric field activates the z-motion after the impulse, and the ions are eventually ejected towards the trapping electrodes of the cell.Influence of the DC potential applied to the trapping electrodes on z-motion is studied with the down-sweep excitation experiments. The threshold amplitude of the excitation waveform at which ions are ejected to the trapping electrodes increases with the DC trapping voltage. At a trapping voltage above a certain amplitude the ions are no longer ejected and the radii of the cyclotron orbits continually increase with excitation amplitude so that they collide with the inner surface of the cylindrical electrodes and are neutralized and lost.A method is proposed for deterring the z-motion by applying an AC potential at a frequency twice the frequency of trapping oscillation of ions to the trapping electrodes. Effectiveness of the method is confirmed by trajectory calculation.