Impact of Multipole Fields on the Performance and Dynamics of Quadrupole Linear Ion Traps

Additional multipole fields are unavoidable in real quadrupole linear ion traps (QLITs) and play a crucial role in influencing their performance. In this study, the impact of these multipole fields on ion ejection and dynamics in QLITs is exhaustively analyzed using a vectorized Runge-Kutta method and a comprehensive theoretical model of ion vibration involving all the common multipole fields. The comparison of nonlinear resonance under different added multipole fields reveals obvious ion ejection from hexapole and octopole resonances as well as multiple resonance points in most multipole fields. Ion ejection with dipole excitation under these fields demonstrates distinct variations at different excitation working values, influenced by the inherent power distribution of ion motion in a linear quadrupole and the energy dispersion caused by the added multipole fields at varying stability parameters. Furthermore, theoretical and numerical analyses of ion dynamics mutually validate each other, offering the first comprehensive demonstration of ion excitation responses under various multipole fields across a wide stability range. The results show that for positive even-order multipole fields, forward scans lead to lower and more stable maximum amplitude responses compared to reverse scans, while the opposite is true for negative fields. In hexapole fields, the forward scan responses are lower than the reverse scan responses, and both increase sharply near nonlinear resonance points, regardless of field polarity. This work provides a thorough theoretical foundation for optimizing multipole field applications in QLITs.