电压不稳定性对离子阱中离子运动特征的影响

在离子阱共振激发过程中，电压的微小变化可能导致离子的运动特征发生显著改变。本工作基于赝势理论，建立了电压不稳定时3D离子阱中离子运动的理论模型，并以辅助交流电压AC的毛刺和端盖电极施加的直流电压为例，揭示端盖电极上辅助交流电压AC的稳定性对离子阱性能的影响。结果表明，当辅助交流电压存在毛刺时，离子的运动速度和振幅将发生突变，扰乱离子阱内离子的冷却状态，致使部分离子在激发阶段打在阱壁上而无法被检测，从而降低了离子阱质谱仪的灵敏度。同时，该模型表明，对于处于冷却状态的离子，可以通过测量其在脉冲激励作用下的响应来测量离子运动的久期频率。若在端盖电极上施加直流电压，离子运动的平衡位置将发生明显的偏移，通过将离子检测器置于偏移的一侧，可以明显地提高离子的检测效率，从而提高离子阱质谱仪的灵敏度。

Effect of Voltage Instability on Motion Characteristics of Ions in Ion Trap

Resonant excitation is a widely used ion ejection technique in ion trap mass spectrum. Combined with mass selective instability scan mode, it can remarkably improve mass resolution and scanning range. The performances of ion trap mass spectrometers, such as mass resolution, sensitivity etc., depend on the motion characteristics of ions, which is vulnerable to both initial conditions and external field parameters. Small changes in voltage can lead to observably oscillation in the motion of ions confined in ion traps. To explore this effect, a theoretical model was established based on the pseudo potential theory. Taking the burrs of the additional auxiliary AC voltage and the DC voltage applied on end cap electrodes as examples, the influence of the stability of voltage on the performance of ion trap was revealed using this model. The results showed that if the auxiliary AC voltage existed burrs, sudden changes in ion velocity and motion amplitude would occur, leading to disruption of the cooling state of ions in ion trap, some of which would hit the trap wall in the excitation phase and could not be detected, thereby reducing the sensitivity of the ion trap mass spectrometer. In addition, the model showed that the secular frequency could be determined through the measurement of the response to the action of voltage burr. By applying a pulse voltage on the ion trap electrode, measuring the ion response to the excitation, and implementing Fourier transformation to the time-zone response, the frequency corresponding to the peak value of the frequency-zone curve was the secular frequency. If a DC voltage was applied on the end cap electrodes, the equilibrium position of the ion motion would be significantly shifted, the amplitude of which was considerable compared with the small size of the ion trap. It could be expected that more than 50% of the ions would eject from the hatch of the end cap electrode on the offset side. Therefore, placing the ion detector outside this electrode can greatly improve the detection efficiency, and hence improve the sensitivity of the ion trap mass spectrometer. The theoretical model established in this work can reasonably explain experimental phenomena, proving its effectiveness. This work can provide theoretical reference for the optimal design of ion trap mass spectrometer. During the actual design and implementation of the ion trap mass spectrometer, the RF signal is more prone to burr than the additional auxiliary AC voltage. In the future, it will have important practical significance to study the influence of the stability of RF signal on the ion motion characteristics in ion trap.