Mass-Selective Instability Mode

The mass-selective instability mode of operation permits the selection and trapping of all ions created over a specified period with subsequent ejection to the detector.26 Ions with different m/z values can be confined within the ion trap and scanned singly by application of voltages that destabilize the orbits of the ions and eject them to the detector. Ion trap instruments interface readily with liquid chromatography, ESI,15 and MALDI.27 The motions of the ions and the dampening gas (e.g., helium) concentrate around the middle of the ion trap, thereby diminishing ion loss through collisions with electrodes. 

Fig. 4.44. Timing sequence used for mass-selective instability mode (about 1.5 cycles shown). With an external ion source the ionization time is replaced by the ion injection pulse. Reproduced from Ref. [150] by permission. Elsevier Science, 1984.

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. 

Chemical ionization (Cl) mass spectra were first obtained by using the mass-selective instability mode of the QIT. [154,155,170] The reagent gas was admitted into the QIT, ionized and then allowed to react with the analyte. 

The Paul trap, popularly known as a quadrupole ion trap (QIT), was introduced in 1958 by Paul and colleagues [33]. This contribution was recognized by the award of the 1989 Nobel Prize for Physics to Wolfgang Paul. Because it is a three-dimensional analog of a quadrupole mass filter, it is also called a three-dimensional ion trap to distinguish it from the two-dimensional ion trap described in Section 3.7. The QIT became popular as a mass spectrometer after development of the mass-selective instability mode of mass analysis by Stafford and co-workers [34]. For further reading, several review articles [35-41] and books are cited at the end of the chapter. 

The secular frequency of ion motion in the aiy-plane is given by P r /2, where is a proportionality constant that can be calculated from and q. The term P is equal to for the mass-selective instability mode of operation. 

Figure 3.24. Stability diagram and scan line of an orbitrap in the mass-selective instability mode. (Reproduced from ref. 66 by permission of the American Chemical Society, Washington, DC, cop)night 2000.)...

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. 

Figure 7 Stability diagram in space for the region of simultaneous stability in both rand z directions near the origin for the three-dimensional quadrupole ion trap the iso-yS, and iso-/S lines are shown. The axis intersects the = 1 boundary at q = 0.908, which corresponds to in mass-selective instability mode.

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