Study of ghost peaks resulting from space charge and non‐linear fields in an ion trap mass spectrometer

Abstract

Ion trap improvements require some geometrical modifications which can alter significantly the relative linear and non-linear field contributions. In this work, where the ion trap was interfaced with an electrospray source, it was shown that under particular ion trapping conditions due to space charge, artefacts called ghost peaks are displayed in the mass spectra. They are characterized by particular width and shape and appear at twice the m/z ratio of the expected peaks (for positive and negative ions) when the analyzed ions are ejected at βz=1/3 (extended mass range mode). They were unambiguously identified by using isolation sequences. The various experimental factors governing the observation of ghost peaks were explored in order to rationalize their origin. Especially the capillary exit voltage, ion accumulation time and low m/z cut-off of the analytical scan, which modify the space charge strength, influence the ghost peak intensities. Furthermore, it is evidenced that only the injected ions which are located at qz⩽0.25 (βz⩽2/11) at the beginning of the analytical scan step give rise to the formation of such artefact peaks. Under these experimental conditions, the ion cloud defocused by space charge is subjected to high-order multipole fields (e.g. 22-pole). Then the motion of the analyzed ions is destabilized, affecting the ion ejection. It seems that two ion populations can co-exist: (i) one submitted to a normal ion ejection at qz=0.45, where the axial modulation is applied, i.e. normal peaks, and (ii) a second ejected at the natural qz=0.908 boundary, leading to ghost peaks at twice the m/z value of normal signals. © 1998 John Wiley & Sons, Ltd.