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Hyperbolic electrode linear ion trap is a good choice as the mass analyzer of miniaturized ion trap mass spectrometer for the reason that it has a compact size and excellent performance. However, ions are ejected from both opposite directions along the x-axis for existing hyperbolic electrode linear ion trap, which means that the ion detection efficiency could not be higher than 50% if only one ion detector is employed in the miniaturized ion trap mass spectrometer. This symmetrical structure significantly influences the analysis performance of hyperbolic electrode linear ion trap and limits the application at miniaturized ion trap mass spectrometer. Therefore, in order to further improve the ion detection efficiency of the ion trap, an asymmetric hyperboloid linear ion trap structure was proposed. By optimizing the unidirectional stretching distance Δra of the x-axis electrode in the direction of ion ejection after the optimization to the stretching distances Δrx of both x axis electrodes, a reasonable asymmetric radio frequency electric field was introduced for improving ion unidirectional ejection efficiency. The samples used in the simulation experiments were ions with m/z 609, 610 and 611. The internal electric field distribution, ion motion trajectory and simulated mass spectra peak of the asymmetric hyperboloid linear ion trap for different electrode structures were analyzed by simulation software SIMION and AXSIM. Then the ion unidirectional ejection efficiency and mass resolution were calculated, and the data were drawn as line charts. The simulation results showed that on the basis of the optimized symmetrical hyperboloid ion trap structure, whose x-axis electrodes were both stretched by Δrx=0.9 mm, an ion unidirectional ejection efficiency of over 90% and a mass resolution over 5 100 for m/z 610 were achieved by optimizing the parameters such as AC frequency and scanning rate, when one of the x-axis electrodes was unidirectionally stretched by Δra=0.8 mm. In addition, under the condition of ions with large m/z 1 890, a mass resolution up to 10 590 and an ion unidirectional ejection efficiency of 93.3% were achieved when Δra=0.7 mm. It proved that the asymmetric hyperboloid linear ion trap with the optimized geometric structure could greatly improve the ion detection efficiency with only one ion detector under the guarantee of high mass resolution, which had a significant advantage for developing miniaturized ion trap mass spectrometer in the future. On the whole, the research could provide a theoretical foundation for further investigations, but there were still some problems to be solved, such as the actual processing, assembly and experiment. |
