Resonant ejection

Scans based on resonant ejection may either be carried out in a forward, i.e., from low to high mass, or a reverse direction. However, the scan direction has significant influence on the attainable resolving power, the reverse direction being clearly inferior in that respect. [149,150] The combination of forward and reverse scanning 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 experiments [147,149]. Axial excitation can also be used to cause collision-induced dissociation (CID) of the ions as a result of 

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As with the quadmpole ion trap, ions with a particular m/z ratio can be selected and stored in tlie FT-ICR cell by the resonant ejection of all other ions. Once isolated, the ions can be stored for variable periods of time (even hours) and allowed to react with neutral reagents that are introduced into the trapping cell. In this maimer, the products of bi-molecular reactions can be monitored and, if done as a fiinction of trapping time, it is possible to derive rate constants for the reactions [47]. Collision-induced dissociation can also be perfomied in the FT-ICR cell by tlie isolation and subsequent excitation of the cyclotron frequency of the ions. The extra translational kinetic energy of the ion packet results in energetic collisions between the ions and background... 

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... 

Example Tandem mass spectrometric experiments in quadmpole ion traps 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. 4.45). [156]... 

Greaser, C.S. Stygall, J.W. A Comparison of Overtone and Fundamental Resonances for Mass Range Extension by Resonance Ejection in a Quadrupole Ion Trap Mass Spectrometer. Int. J. Mass Spectrom. 1999,190/191, 145-151. 

Williams, J.D. Cox, K.A. Cooks, R.G. McLuckey, S.A. Hart, K.J. Goeringer, D.E. Resonance Ejection Ion Trap Mass Spectrometry and Nonlinear Field Contributions The Effect of Scan Direction... 

Makarov, A.A. Resonance Ejection From the Paul Trap A Theoretical Treatment Incorporating a Weak Octa-pole Field. Anal. Chem. 1996, 68, 4257-4263. 

In contrast to triple quadrupole instruments, where MS-MS experiments can be conducted in space in separate regions of the instrument, ion traps enable MS-MS sequentially in the same physical space, and thus, occur tandem in time. After the ions have been formed an trapped, a parent ion is selected by resonance ejection of all ions except those of the selected m/z ratio. This is done by applying a resonance ejection radiofrequency voltage to the end-cap electrodes which stimulates motion of the ions in the axial direction. The next step in the MS-MS sequence is to effect collisionally... 

It should be noted that if an ion fragments during the analysis, it is possible that its m/z ratio is such that its qz value is higher than the resonant ejection value. If, later on, by increasing V, it reaches the stability limit (qz = 0.908) and is ejected, it will then be detected at a wrong m/z, as the data system expects it to be expelled by resonance. Its apparent m/z will be higher than the true one. These ghost peaks will occur more if the resonance frequency corresponds to a lower qz value. 

Figure 2.23 shows the sequence of events for a resonance ejection analysis in a pictorial way [15]. Note that in this figure the supplementary applied voltage on the end caps is designated as AC . 

Principle of resonant ejection. Upper ions are stored in the 3D trap at a voltage Va of the fundamental RF. An additional RF is applied to the end caps corresponding to qz = 0.8. On increasing V (lower panel) ions are moved to higher qz values. In the figure, the smallest ion has reached qz = 0.8 and is ejected by resonance. This ejection occurs at a lower value of V than the one needed to eject by instability at qz = 0.908. 

Analyse the ions by one of the described scanning methods stability limit or resonant ejection. 

A second example describes the use of resonant ejection of ions by selected-waveform inverse Fourier transform (SWIFT). Figure 2.26 describes an MS/MS experiment with an instrument using RF voltages applied to the caps, but no DC voltage. In this example, the final analysis of the fragments is performed by the stability limit method. 

The IT is used to accumulate ions and to perform ion selection and activation in MS/MS experiments before analysis in the TOF analyser. All the ions accumulated in the trap are then ejected in the RTOF analyser. Therefore, the TOF analyser is used for mass analysis instead of the classical ion ejection methods used with ITs, namely mass selective ion ejection at the stability limit or resonant ejection. In comparison with TOF instruments, higher sensitivity is achieved by ion accumulation in the IT. In comparison with IT instruments, the analysis by TOF reduces the time as the TOF analyser allows faster mass analysis, extends the mass range, and gives a better resolution and much better accuracy. 

The trapped ions possess characteristic oscillation frequencies. The stable motion of ions in the trap is assisted by the presence of a helium buffer gas (1 mtorr) to remove kinetic energies from ions by collisions. When a supplementary AC potential, corresponding to the frequency of a certain m/z ion, is applied to the end-cap electrode, ions are resonantly ejected from the trap. This method of resonance ejection is used to effectively extend the mass-to-charge ratio of the ion trap. Some other characteristic features of a 3-D ion trap include high sensitivity, high resolution with slow scan rate, and multiple-stage MS capability (see the section on tandem MS). In addition, it is inexpensive and small in size. As a result, a 3-D ion trap is widely used in LC/MS and LC/MS/MS applications. 

The tandem-in-time instruments are mostly ion-trapping devices, including ion trap and FT-ICR. They operate in a time sequence in the scan function to yield MS/MS data, mostly product ion spectra. No additional mass analyzer is required. In the case of an ion trap, the scan function begins with the isolation of ions of interest with ejection of all other ions from the ion trap, followed by (a) translational excitation of ions by applying a supplementary RF voltage to the trap and (b) mass analysis of the product ions using resonant ejection. 

Dynamic range extension in GD quadrupole/ion-trap MS based on selective ion-accumulation (e.g. by mass-selective instability, single-frequency resonance ejection, combined rf-dc and entrance end-cap dc methods) allows the selective accumulation of the analyte ions and enables the dynamic range to be increased by a factor of 105 [233]. The linearities and relative trapping efficiencies of the previous methods were assessed with respect to the injection time and the methods were used for the GD ion-trap MS determination of major and minor constituents in NIST SRM 1103 Free Cutting Brass. 

Using mass selective instability with resonance ejection, ions are scanned out of the trap through slits in the center of two opposite center section rods and focused onto two separate conversion dynodes. In the case of the QIT, where ions are scanned out of both end cap electrodes, the only place for a detector is behind the end cap opposite the ion entrance, so that only half of the ions scanned out of the trap are detected. Both the QJT and LIT operate at unit mass resolution with similar scan rates and both have the capacity to generate higher resolution spectra at slower scan rates. 

One of the principle uses of the ion trap is as a tandem-in-time mass spectrometer. Ions with a particular m/z ratio formed in the ion trap, or injected into the trap from an external source, can be isolated by resonantly ejecting all other... 

A supplementary RE voltage, the axial modulation, is applied during the scan program to improve the resolution by increasing the efficiency with which the ions are resonantly ejected from the ion trap during the RE analytical ramp. 

Multiple-frequency resonance ejection methods have been developed to eject specifically one or more ion species simultaneously. Digital waveform generators have been used for this purpose and have been shown to provide greater control over the excitation processes [7]. Most of these methods are derived from ICR mass spectrometry. 

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