ABSTRACT
Whereas Fourier Transform (FT) techniques are common in Ion Cyclotron Resonance (ICR) mass spectrometry [1], and more recently in the Orbitrap™ instrument,* only a few works refer to non-destructive FT techniques applied to an RF quadrupole ion trap. In most of the studies, ion image currents induced between the two end-cap electrodes are employed as the detection principle. At the beginning of connement, a fast DC pulse or a dipolar AC frequency matching the frequencies of ion motion moves the ions out of the trap center and makes the motion coherent, as in FT-ICR instruments. Syka and Fies gave an account of the experimental use of this detection principle for mass analysis [2,3]. The trapping (or axial) motion has been detected in a Penning trap [4]. Two different schemes of excitation and detection were proposed and applied between the two end-caps of the trap. Wang and Wanczek proposed a new electrode conguration for an ion trap with cone-shaped boundaries [5]. An homogeneous eld was then superimposed along the axial direction to the connement eld within the same region. As a result the excitation of the transient axial ion motion, which renders the motion coherent may attain a greater accuracy of mass selection. Consequently, enhanced resolution may be achieved with the image current detection method. Lammert et al. measured the frequencies of the radial and axial secular ion motion experimentally [6]. For this purpose, a fast DC pulse displaced a kinetically cooled ion cloud from the center of the trap. The ion cloud displacement was observed during connement by means of a laser probe. Subsequently, Cooks et al. examined by simulation the operating methods of an ion trap involving various ion cloud excitations with non-destructive FT detection [7,8]. Other work followed concerning the inuence of DC pulses on the coherence of motion and axial kinetic energy of the ions during connement (DC pulse tomography) [9,10].
