Resonant ejection frequencies

Over the years, several methods of isolating ions in a QIT have been implemented. Methods include RF/DC isolation, forward and reverse RF resonance ejection isolation, " and various forms of TWF isolation. " The RF/DC isolation methods position the ion of interest near the boundaries of the stability diagram for isolation. In this case, the parameter is set to a nonzero value (see RF/DC isolation point in Figure 9.3b). The RF resonance ejection method sweeps the main RF amplitude and/or the resonance ejection frequency in both the forward and/or reverse directions to eject all but the ion of interest. This technique has been shown to yield high-resolution isolations and can be used to analyze multiply charged ions. ... 

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... 

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... 

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. 

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. 

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. 

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. 

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. 

Although the mass resolution of commercial instruments is poor (unit mass resolution), substantial improvements can be realized on research instruments by reducing the scan speed and the frequency and amplitude of the resonance ejection signal [44,45]. For example, a resolution of 1.13 x 10 was achieved for a cluster of Csl ions at 3510 u with a 2000-fold decrease in the scan speed [44]. The reduction of the scan rate to 0.1 miq unit/s has led to a further improvement in resolution to 1.2 x 10 for mIq 614 [45]. A zoom scan, in which a narrow window of masses is scanned, is another alternative to enhance resolution. 

Waveforms Operation of a mini-CIT requires five waveforms drive rf (2.000 MHz) and rf amplitude modulation potentials, ion isolation waveform, and ac potentials for ion excitation and for mass-selective axial ejection. Typical ac amplitude used in the mini-CIT for CID is 2Vo p and is applied in dipolar mode to the end-cap electrodes. The frequency used for resonant ejection is an octapole resonance that causes rapid ejection of ions. In Figure 8 is shown a typical timing diagram of scan function by which an El mass spectrum may be obtained. A scan function is a diagram showing the temporal relationships of the various potentials applied during... 

A dipole signal (Vres cos(2nf. esi)) is applied to the end caps as shown in Figure 9.3a, for resonance excitation and/or resonance ejection. For either of these processes to occur efficiently, however, the frequency used must equal the fundamental resonant frequency of the ion, fz-Ks- For example, during the mass selective instability scan supplemented with resonance ejection (see Figure 9.3b), ions are ejected from the trap axially as they are scanned into resonance at a frequency that corresponds to a 2 eject = 0.83, Pz-eject = 0.736. Resolution, peak height, and sensitivity are greatly improved when using the mass selective instability scan mode supplemented with resonance ejection. 

Figure 9.6. Scan functions for the SIM and high-resolution SIM inodes of operation for the XQIT. Only the (1) ion injection, (2) ion isolation, and (3) mass analysis steps are required. In this example a SOS-TWF is used during high q isolation step to ejected unwanted ions from the trap. The optional ion injection TWF has been turned off in this example. A frequency-domain spectrum (FFT) of the SOS-TWF reveals the notch in frequencies applied as shown in the inset. The resonance ejection amplitude is increased during mass analysis for optimum resolution. The multiplier is turned on during the mass analysis segment, step 4, to collect the ion signal.

Figure 9.15. MALDI QIT MS ftom three different QITs. (a) Mass spectrum of 250fmol of tetradecapeptide renin substrate in a 1 2000 ratio with 2,5-dihydroxybenzoic acid [M + H] = 1758.9 (average) using external inj ection. The exclusion limit was 95 Da and resonant ejection occurred at a frequency of 89 kHz. (With permission from Jonscher, K. Yates, J. Rapid Commim. Mass Spectrom. 1993, 7, 20-26.) (b) Extemal-MALDI QIT mass spectrum of alpha-lactalbumin with a MW 14175, 3 pmol, 15 laser shots. (Reprinted with permission from Schwartz, J. Bier, M. E. Rapid Comm. inMass Spectrom. 1993,7,27-32) (c) Intemal-MALDIQIT mass spectrum of IgG (MW 148,500Da) with 10 pmol on target. (Reprinted with permission from Schlimegger, U. Caprioli, R. Rapid Commm. Mass Spectrom. 1999, 13, 1792-1796.)...

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