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

✍ Scribed by Kocher, F.; Favre, A.; Gonnet, F.; Tabet, J.-C.

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 406 KB
• Volume: 33
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/(sici)1096-9888(1998100)33:10<921::aid-jms700>3.0.co;2-s

✦ Synopsis

Ion trap improvements require some geometrical modiÐcations which can alter signiÐcantly the relative linear and non-linear Ðeld 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 (extended mass b z = 1/3 range mode). They were unambiguously identiÐed 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-o † of the analytical scan, which modify the space charge strength, inÑuence the ghost peak intensities. Furthermore, it is evidenced that only the injected ions which are located at at the beginning of the analytical scan step give rise to the formation of such q z O 0.25 (b z O 2/11) artefact peaks. Under these experimental conditions, the ion cloud defocused by space charge is subjected to high-order multipole Ðelds (e.g. 22-pole). Then the motion of the analyzed ions is destabilized, a †ecting the ion ejection. It seems that two ion populations can co-exist : (i) one submitted to a normal ion ejection at q z = 0.45, where the axial modulation is applied, i.e. normal peaks, and (ii) a second ejected at the natural q z = 0.908 boundary, leading to ghost peaks at twice the m/z value of normal signals.