Theoretical Study On Nonlinear Ion Trap Mass Spectrometers

Ion trap is a general tool used for ion storage and mass analysis. The ion trap isdivided into three dimensional (3D) and two dimensional (2D) ion trap, such asquadrupole ion trap (Paul trap) and quadrupole mass filter (QMF). In general, higherresolution can be achieved in higher stability regions, and the nonlinear fields willinfluence the performance of the device.Therefore, we analyzed the performance of the ion trap in the second stabilityregion. We calculated the trap’s acceptance, investigated ion sources with differentspatial distribution, and found that a proper initial RF phase can greatly improve theperformance of the device operated in this region.The Poincare-Lighthill-Kuo (PLK) method was used to derive an analyticalexpression on the stability boundary and ion trajectory. The octopole component isadopted to represent the inhomogeneities of the field. It was found that the frequencyshift exists between the ideal and nonlinear conditions, which is not only related toMathieu parameters, a and q, but also dependent on the percentage of the nonlinearfield and ion initial amplitude. We discussed the influences of positive and negativeoctopole fields on the mass resolution, respectively.We obtained the potential function in the curved quadrupole under the minimalrotation frame (MRF) coordinates, and confirmed the results by SIMION method.Based on the PLK method, the nonlinear effects of ion motion were investigated. Itwas found that the frequency shift of ion motion can be eliminated by coupling thehexapole with a positive octopole. Moreover, the ion transmission efficiencydecreases dramatically with the increment of the ionic kinetic energy. A modifieddirect current (dc) voltage supply pattern was hence proposed to compensate for thiseffect.The surface-electrode ion trap, compared with the quadrupole ion trap, has easiergeometrical structure and fabrication, and readily realized miniatured trap and arrayedstructure. Because of the asymmetric geometrical structure, the field and ion motionof the surface-electrode ion trap are asymmetric in the x and y directions. Weoptimized the trap by maximizing the trapping well depth.