Resonance excitation ejection

Splendore, M. Lausevic, M. Lausevic, Z. March, R.E. Resonant Excitation and/or Ejection of Ions Subjected to DC and RF Fields in a Commercial Quadrupole Ion Trap. Rapid Commun. Mass Spectrom. 1997,11, 228-233. 

This instrument can be operated as a normal triple quadrupole with all its scan modes or as a trap in various combinations with the use of the other quadrupoles. If a slow scan rate is used to expel the ions a resolution up to 6000 FWHM can be reached by scanning at 5 Th s 1 using q2 and at 100 Th s 1 using Q3, which is at a lower pressure. As fringe field effects are used, only ions close to the exit lens are expelled. In consequence, mass selective ejection in the axial direction based on this technique is characterized by low ejection efficiency. For instance, an ejection efficiency of less than 20 % is achieved at 1 Th ms-1 scan rate. Different techniques have been proposed to improve the axial ejection efficiency [21], but the most promising technique for mass selective axial ejection is the technique named axial resonant excitation (AREX) [22]. Lenses are introduced between each rod of the quadrupole... 

Hashimoto, Y., Hasegawa, H., Baba, T. and Waki, I. (2006) Mass selective ejection by axial resonant excitation from a linear ion trap. J. Am. Soc. Mass Spectrom., 17, 685-90. 

The first obstacle for FT-ICR-based CID, that the ions must be excited transla-tionally, may be surmounted by resonant excitation of the ions (as in pre-detection excitation, or radial ejection, described above). However, the extent of excitation is limited by the magnetic field and the size of the trap. Excess excitation wonld result in ejection of the ions from the ICR cell. A second problem is that the products are formed off-axis because the precursor ions are increasing their cyclotron radius this effect results in reduced resolving power and prevents any further fragmentation (MS ). 

SORI-CID, introduced by Gauthier and co-workers [28], is not beset by these problems. As the name suggests, ions are excited slightly off-resonance (500-2000 Hz). Such excitation results in acceleration and deceleration of the ions with a period equal to the difference between the excitation frequency and the ion cyclotron frequency. The periodic decrease in cyclotron radius means that ions are not ejected from the ICR cell. Prior to off-resonance excitation of the precursor ions, inert gas is leaked into the ICR cell. As the ions are excited, collisions with the gas resnlt in conversion of translational energy to internal energy. Again, as a resnlt of the periodic decrease in cyclotron radius, the product ions are formed close to the center of the cell, eliminating resolution issues. It is possible that the product ions have a cyclotron frequency equal to that of the applied excitation waveform. If this were the case, those product ions would be ejected from the ICR cell (resonant ejection). To avoid this occurrence, off-resonance excitation is performed in both directions, for example, 500 Hz. 

When an auxiliary ac field is applied to induce radial resonant excitation, coupling of radial and axial motions effected axial ion ejection when the ion radial secular frequency matched that of the ac field. 

Resonant excitation is used to remove ions from the trap in order to isolate a range of mJz values by increasing the kinetic energy of the unwanted ions so that they escape from the trapping potential and are ejected. This mode can be used to eject unwanted ions and isolate in the trap only ions with a selected mJz value (e.g., for subsequent MS/MS experiments, see later) or a range of mJz values... 

In addition, use of axial modulation scanning was also found to increase resolving power this effect is believed to arise as follows (March 1998). Immediately prior to ion ejection, as Vg is scanned upwards the axial secular motions of ions of a particular m/z value become resonant with the supplementary potential applied between the end caps, so that the axial excursions of the resonantly excited ions (and only these ions) increase in magnitude. As a result these ions escape from the space charge effects of the cloud of ions of higher m/z while the latter are stiU coUisionally cooled at the center of the trap. In this manner the ions that are resonantly excited become tightly... 

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