Perturbation of ion trajectories by resonant excitation leads to occurrence of ghost peaks

We have shown in a recent paper (Kocher F, Favre A, Gonnet F, Tabet JC J. Mass Spectrom . 1998; 33: 921) that under particular ion trapping conditions, i.e. space charge, artefacts called ghost peaks were displayed in mass spectra and isolation spectra. All experimental factors which modified the space‐charge strength, influenced the intensities of the ghost peaks. It was demonstrated that only the injected ions having q z ≤0.25 (β z  ≤ 2/11) at the beginning of the analytical scan step gave rise to the formation of such artefact peaks. In fact, the ion cloud defocused by space charge was subjected to high‐order multipole fields (e.g. 22‐pole, Wang Y, Schubert M, Franzen J, Proc. 44th ASMS Conf. Mass Spectrom. and Allied Topics , Portland OR, 1996; p. 131), so that the trajectories of the analysed ions were destabilized thus affecting the ion ejection which then occurred at q z  = 0.908 (β z  = 1, natural boundary). However, ions having 0.25 (β z  ≤ 2/11) ≤q z ≤0.45 (β z  ≤ 1/3), at the beginning of the analytical scan step, were not destabilized, and were correctly ejected where the axial modulation was applied (β z  ≤ 1/3). Thus, under extreme space‐charge conditions, the ion excursion increased, the influence of the non‐linear resonance at β z  = 2/11 was enhanced, ion excursion was more enhanced, and the ions were not ejected at β z  = 1/3 but at β z  = 1. We shown here that, again under the influence of the non‐linear resonance at β z  = 2/11, under space‐charge conditions the ion excursion increased, and the application of a resonant excitation dramatically enhanced this ion excursion, compared to only the isolation step as was performed in the previous work. This led to these ions not being ejected at β z  = 1/3 but rather at β z  = 1, and thus being displayed as ghost peaks in the collisionally induced dissociation (CID) spectra. This behavior could take place for other working ejection points, which occur at non‐linear resonances (e.g. β z  = 2/11 as well as β z  = 2/3). Copyright © 2001 John Wiley & Sons, Ltd.