Trapped-ion mass analysers

Quadrupole ion traps ions are dynamically stored in a three-dimensional quadrupole ion storage device (Fig. 10.6) [37]. The RF and DC potentials can be scanned to eject successive mass-to-charge ratios from the trap into the detector (mass-selective ejection). Ions are formed within the ion trap or injected into an ion trap from an external source. The ions are dynamically trapped by the applied RF potentials (a common trap design also makes use of a bath gas to help contain the ions in the trap). The trapped ions can be manipulated by RF events to perform ion ejection, ion excitation, and mass-selective ejection. This provides MS/MS and MS experiments, which are eminently suited for structure determinations of biopolymers [38] (see Section 10.4). 

Space-charge effects (ion—ion repulsion) limit the inherent dynamic range of the ion trap. This is usually handled by auto-ranging a pre-scan is performed to determine the ion current and the ionising electron current is then adjusted to reduce the number of ions formed to within the working range. This can be done wherever the ion formation event can be manipulated to control the number of ions formed. 

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