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 delta and internal electric field distribution was investigated, and the impact of delta 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 delta = 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.