Axial Modulation and Automatic Gain Control

Ion trapping devices are sensitive to overload because of the detrimental effects of coulombic repulsion on ion trajectories. The maximum number of ions that can be stored in a QIT is about 10 -10 , but it reduces to about 10 -10 if unit mass resolution in an RF scan is desired. Axial modulation, a subtype of resonant ejection, allows to increase the number of ions stored in the QIT by one order of magnitude while maintaining unit mass resolution [152,153]. During the RF scan, the modulation voltage with a fixed amplitude and frequency is applied between the end caps. Its frequency is chosen slightly below V2 of the fundamental RF frequency, because for Pz 1, e.g., Pz = 0.98, we have Qz = (0 + 0.98/2) x Q = 0.49 x O.. At the stability boundary, ion motion is in resonance with this modulation voltage, and thus ion ejection is facilitated. Axial modulation basically improves the mass-selective instability mode of operation. 

If resolving power is not a major concern, scanning of QITs can be very fast, a property that can be employed to make a pre-scan. The actual ion current into the trap is then determined from the pre-scan and the result is used to adjust the number of ions admitted to the QIT by a timed ion gate for the subsequent analytical scan. Thus, the number of ions, and hence, charge density inside the QIT are con-tinously held close to the optimum. This is known as automatic gain control (AGC) [125,154]. AGC gives increased sensitivity at low sample flow and avoids overload of the QIT at high sample flow. 

Note AGC is not only relevant for QITs but also for all otha- types of ion traps. AGC is well implemented in LITs that are not only employed as standalone devices but also in conjunction with FT-ICR analyzers to serve for ion selection and dosing into the ICR cell (Chap. 4.9.1). 

Provided sufficiently high scan rates are also available whilst resolution is preserved, the pre-scan can be omitted. Instead, a trend analysis based on a set of two or three preceding analytical scans can be performed. This procedure avoids wasting of ions and results in further optimization of the filling level of the QIT. The exploitation of the phenomenon of nonlinear resonances turned out to be of key importance for the realization of this method. 

Example Tandem mass spectrometric experiments in quadrupole ion traps (more in Chap. 9.8) are performed by combining the techniques of resonant ejection, and forward and reverse scanning to achieve an optimum in precursor ion selection, ion activation, and fragment ion scanning (Fig. 4.48) [147]. 

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