A simulation study of coupled secular oscillations in nonlinear Paul trap mass spectrometers
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AbstractThis paper presents the results of a simulation study to understand coupling of axial and radial secular motion in nonlinear Paul trap mass spectrometers having hexapole and octopole superposition in the presence of an external excitation. The equations of motion has the form of a inhomogeneous coupled Duffing oscillators with quadratic and cubic nonlinearities with an periodic forcing function in the axial direction with a frequency corresponding to the ideal axial secular frequency. The study relies on using both numerical as well as analytical techniques to study the role of field inhomogeniety and external excitation in the appearance of coupled frequencies in axial and radial directions. It has been seen that the strength of coupling can be characterized by the magnitude of the coefficient of the cross terms in the equations of motion and the appearance of coupled frequencies in each direction is a function of this coupling strength. Hexapole superposition plays a role only in the axial direction and its influence in coupled oscillations is minimal. Octopole superposition plays the predominant role in determining the appearance of coupled frequencies in both directions. It was further seen that in the presence of an external excitation the axial secular motion was amplitude and phase modulated and resulted in a beat frequency corresponding to the difference between the perturbed axial frequency and the applied excitation frequency. Increasing the amplitude of external excitation results in appearance of an increased number of sidebands in the frequency spectrum in both directions.
Sevugarajan, S and Menon, AG (2001) A simulation study of coupled secular oscillations in nonlinear Paul trap mass spectrometers. In: International Journal of Mass Spectrometry, 209 (2-3). pp. 209-226.