Cyclotron Motion - Excitation and Detection

Upon excitation, the circular micromotion is superinposed by the macroscopic cyclotron motion of the whole ion cloud, i.e., the excitation field preserves the coherence of ion packages composed of ions of the same m/z value. As the initial kinetic energy of the ions is small as compared to the energy uptake from the RF field, it is of minor importance for the experimait [189]. Nonetheless, the complexity of the overall motion affects frequency-to-mass calibration if accurate results are required [190]. 

In practice, mass-selective excitation, so-called resonant excitation, is achieved by applying a transverse electric field alternating at the cyclotron frequency f (cOc = 2nfc) to accelerate the ions. Such a field can be applied by a pair of RF electrodes placed on opposite sides of the orbit. As the ions accelerate, the radius of their orbit increases, and the resulting overall motion is a spiral (Fig. 4.52a) 

The first-geaieration ICR instrumHits essentially performed an energy sean the m/z value was obtained from the number of half cycles, until the ions struck an electrometer plate at r = r [171,172], The disadvantages of this concept are clear i) mass accuracy and resolution are limited to N (Nc = number of half cycles) ii) the electric signal for ion detection is solely due to neutralization of the ions, and there is no anplification as obtained with multiplier-type detectors used with all other analyzers and Hi) the ions are removed from the cell upon detection precluding its use for MS/MS. 

Note In ICR cells, the ions circulate like separate swarms of birds rather than like matter in the rings of Saturn. If ions of the same m/z noncoherently circulated at the same frequency and radius, but occupied the total orbit rather than a small sector of it, there would be no image current induced upon their passage at the detector plates, i.e., coherence of the circulating ion motion is crucial. 

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