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.