Resonance shifts in the excitation of the n = 0, K = 1 to 6 quadrupolar resonances for ions confined in a linear ion trap.

Experiments examining the excitation of the quadrupolar n = 0, K = 1 to 6 resonances for the ion reserpine in a linear ion trap have been shown to produce resonance shifts that were dependent upon either or both of the excitation amplitude and trap pressure (Collings, B. A.; Douglas, D. J. J. Am,. Soc. Mass Spectrom., 2000, 11, 1016-1022). The extent of this dependency was determined by examining the effects of each parameter using an ion trajectory simulator. The simulations indicated that it is the change in excitation amplitude that is mostly responsible for the resonance shifts with a minor dependency upon the trapping pressure. It was found that the higher excitation amplitudes required to observe the higher order resonances resulted in greater shifts relative to the theoretical resonances predicted for an excitation amplitude of zero volts. The nature of these shifts can be understood by examining the equations of motion for an ion trapped in a quadrupolar potential during the excitation process. Rearrangement of the equations of motion lead to a Mathieu stability diagram in which the coordinate and ordinate variables are dependent upon the excitation frequency and amplitude. In such a diagram the resonances occur in the regions of instability. The calculated resonance shifts showed good correlation with the experimental and simulation results.