Operation of the Quadrupole Ion Trap

Scans based on resonant ejection may either be carried out in a forward, i.e., from low to high mass, or a reverse manner. This allows for the selective storage of ions of a certain m/z value by elimination of ions below and above that m/z value from the trap. Thus, it can serve for precursor ion selection in tandem MS experminents. [156,158] Axial excitation can also be used to cause collision-induced dissociation (CID) of the ions as a result of numerous low-energy collisions with the helium buffer gas that is present in the trap in order to dampen the ion motion. [150,156] A substantial increase of the mass range is realized by reduction of both the RF frequency of the modulation voltage and the physical size of theQIT. [154,159,160] 

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 QTT is about 10 -10, but it reduces to about 10 -10 if unit mass resolution in an RF scan is desired. Axial modulation, a sub-type of resonant ejection, allows to increase the number of ions stored in the QIT by one order of magnitude while maintaining unit mass resolution. [160,161] 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., = 0.98, we have z = (0 + 0.98/2) = 0.49 x 

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 resolution is not important, scanning of QITs can be very fast, a property that can be employed to make a pre-scan. Then, the result of the pre-scan is used to adjust the number of ions inside the QIT close to the optimum for the subsequent analytical scan. Such an automatic gain control (AGC) [138,162] gives increased sensitivity at low sample flow and avoids overload of the QIT at high sample flow. 

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. 

---