Simulation of unidirectional ion ejection in linear ion traps with asymmetric radiofrequency voltage.

RATIONALE: Linear ion traps with simplified structure have been widely used in miniaturized mass spectrometers. However, linear ion traps usually have low ion detection efficiency when used in a miniaturized mass spectrometer, in which only one ion detector can be installed. To solve this problem, an asymmetric radiofrequency (RF) voltage was applied to introduce asymmetric electric fields in the trapping volume of linear ion traps, which would lead to unidirectional ion ejection. METHODS: An asymmetric RF voltage was applied on both half-round-rod electrode and triangular electrode linear ion traps, and their performances including unidirectional ion ejection efficiency and mass resolution were evaluated using computer simulation. The relationship between asymmetric RF voltage difference δ and internal electric field distribution was investigated, and the impact of δ and resonance excitation signal frequency on the unidirectional ion ejection efficiency and mass resolution was also explored. RESULTS: A unidirectional ion ejection efficiency of around 83% and a mass resolution of over 2700 were achieved with δ = 7% in both half-round-rod electrode and triangular electrode linear ion traps. CONCLUSIONS: By applying an asymmetric RF voltage, the ion detection efficiency could be significantly improved without any change to the structures of existing linear ion traps. This method provides a simple and general solution for improving the ion detection efficiency and sensitivity of miniaturized linear ion trap mass spectrometers.

Research of Unidirectional Ion Ejection in Printed-Circuit-Board Ion Trap / 分析化学

Printed-circuit-board ion trap (PCBIT) is a novel ion trap mass analyzer,which is capable of optimizing its internal electric field distributions by adjusting the radio frequency (RF) voltage-divided ratio to improve its analytical performance.This work introduced odd electric field components into the trapping volume to achieve unidirectional ion ejection by applying asymmetric RF voltages to x electrode pairs of PCBIT.In this case,the center of ion vibration was displaced away from the geometrical center of PCBIT and ions were ejected predominantly through one of x electrode pairs.The relationship between asymmetric voltage-divided ratio (AV) and internal electric field distributions was investigated by simulation software SIMION and AXSIM.At the same time,the ion trajectories and simulated mass spectrum peaks were calculated.The results showed that,for ions with m/z 609 Th,a mass resolution over 2500 and an ion unidirectional ejection efficiency of over 90％ were achieved in PCBIT with AV=20％ at an appropriate frequency of AC.Using this method,ion unidirectional ejection efficiency of PCBIT can be significantly improved while maintaining a high mass resolution,which makes the PCBIT more suitable for developing miniaturized mass spectrometer.