STUDY OF GHOST PEAKS RESULTING FROM SPACE-CHARGE AND NONLINEAR FIELDSIN AN ION-TRAP MASS-SPECTROMETER

Ion trap improvements require some geometrical modifications which can alter significantly the relative linear and non-linear field contribu tions. In this work, where the ion trap was interfaced with an electro spray source, it was shown that under particular ion trapping conditio ns 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 mit ratio of the expected peaks (for positive and negative ions) when the analyzed ions are ejected at beta(z) = 1/ 3 (extended mass range mode). They were unambiguously identified by us ing isolation sequences. The various experimental factors governing th e observation of ghost peaks were explored in order to rationalize the ir origin. Especially the capillary exit voltage, ion accumulation tim e and low mit cut-off of the analytical scan, which modify the space c harge strength, influence the ghost peak intensities. Furthermore, it is evidenced that only the injected ions which are located at q(z) les s than or equal to 0.25 (beta(z) less than or equal to 2/11) at the be ginning of the analytical scan step give rise to the formation of such artefact peaks. Under these experimental conditions, the ion cloud de focused by space charge is subjected to high-order multipole fields (e .g. 22-pole). Then the motion of the analyzed ions is destabilized, af fecting the ion ejection. It seems that two ion populations can co-exi st: (i) one submitted to a normal ion ejection at q(z) = 0.45, where t he axial modulation is applied, i.e. normal peaks, and (ii) a second e jected at the natural q(z) = 0.908 boundary, leading to ghost peaks at twice the mit value of normal signals. (C) 1998 John Wiley & Sons, Lt d.