The Quadrupole Ion Trap

The QIT consists of two hyperbolic electrodes serving as end caps along with a ring electrode that replaces two of the linear quadmpole rods, i.e., it could theoretically be obtained by rotating a linear quadmpole with hyperbolic rods by 360° (Fig. 4.43, 4.44). Thus, a section through the rz-plane of the QIT closely resem- 

Electrodes of the Finnigan MAT ITS40 quadmpole ion trap. By courtesy of Thermo Electron (Bremen) GrribH. 

For the QIT, the electric field has to be considered in three dimensions. Let the potential Oq be applied to the ring electrode while the hyperbolic end caps are grounded. The axial coordinate of the trap is designated as the z-axis, the value zo defines the physical dimension of the trap (center to cap), and z presents the actual position of an ion with respect to the z-axis. The x,y-plane is resolved into the cell radius ro where r analogously defines the actual radial position of an ion. With the pair of end caps grounded the potential inside the trap is given as 

Solving these differential equations which are again of the Mathieu type, finally yields the parameters and 

Stability as plotted in the aJq plane are represented as envelopes of characteristic shape. Ions with their m/z value inside the boundaries are stored in the QIT. The low-mass limit of the trapped m/z range is strictly defined by = 0.908. 

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Asano K, Goeringer D and McLuckey S 1998 Dissociation kinetics in the quadrupole ion trap Proc. 46th Conf Am. Soc. Mass Spectrom. 

The quadrupole ion-trap, usually referred to simply as the ion-trap, is a three-dimensional quadrupole. This type of analyser is shown schematically in Figure 3.5. It consists of a ring electrode with further electrodes, the end-cap electrodes, above and below this. In contrast to the quadrupole, described above, ions, after introduction into the ion-trap, follow a stable (but complex) trajectory, i.e. are trapped, until an RF voltage is applied to the ring electrode. Ions of a particular m/z then become unstable and are directed toward the detector. By varying the RF voltage in a systematic way, a complete mass spectrum may be obtained. 

In mass spectrometers, ions are analysed according to the ml7. (mass-to-charge) value and not to the mass. While there are many possible combinations of technologies associated with a mass-spectrometry experiment, relatively few forms of mass analysis predominate. They include linear multipoles, such as the quadrupole mass filter, time-of-flight mass spectrometry, ion trapping forms of mass spectrometry, including the quadrupole ion trap and Fourier-transform ion-cyclotron resonance, and sector mass spectrometry. Hybrid instruments intend to combine the strengths of the component analysers. 

Figure 2.16. A cross-section schematic of a quadrupole ion trap mass spectrometer (a), with a three-dimensional perspective view of the quadrupole ion trap (b). Reprinted from A. Westman-Brinkmalm and G. Brinkmalm (2002). In Mass Spectrometry and Hyphenated Techniques in Neuropeptide Research, J. Silberring and R. Ekman (eds.) New York John Wiley Sons, 47-105. With permission of John Wiley Sons, Inc.

The quadrupole ion trap still suffers from mass accuracy problems (in comparison with the high resolution attainable) but <30 ppm has been obtained for peptides [240]. This reference nicely addresses performance features of the linear QIT in particular, but mentions other instrument types as well. As with the resolution, to obtain better mass accuracy, sensitivity and speed have to be reduced. 

Figure 6.11. Product ion spectrum used in Example 1. The spectrum was obtained on the quadrupole ion trap mass spectrometer with ESI source and low energy fragmentation. Precursor mass is 574.3 Da.

There are two kinds of Type (2) ion trap mass spectrometers the quadrupole ion trap and the ion cyclotron resonance instruments. 

The quadrupole ion trap traps ions in an electric field generally in the presence of a buffer gas (He). The theory and some of its uses have been discussed by March (26). Ion molecule reactions in ion traps have been reviewed (27). The quadrupole ion trap is a relatively new instrument commercialized <20 years ago and the new generation of instruments has only been available since 1995. Thus gas-phase inorganic chemistry using a quadrupole ion trap is as yet relatively hard to find, but on the increase. 

Jonscher K.R. and Yates J.R. (1997), The quadrupole ion trap mass spectrometer — a small solution to a big challenge, Anal. Biochem. 244, 1-15. 

SID over CID because losses of resolution due to high background pressure are avoided. SID has been successfully employed for structure elucidation of proto-nated peptides, [131] and a SID mode of operation has even been implemented with a quadrupole ion trap. [132] However, apart from the quadrupole ion trap SID requires substantial modifications of the instrumental hardware circumstances that made SID lag behind the countless applications of CID. 

Note With the exception of the quadrupole ion trap, which uses buffer gas to damp ion trajectories, a reduced background pressure, i.e., better vacuum, is also beneficial for all other types of mass analyzers. 

The quadrupole ion trap (QIT) creates a three-dimensional RF quadrupole field to store ions within defined boundaries. Its invention goes back to 1953, [103-105] however, it took until the mid-1980s to access the full analytical potential of quad-mpole ion traps. [137-140] The first commercial quadmpole ion traps were incorporated in GC-MS benchtop instruments (Finnigan MAT ITD and ITMS). Electron ionization was effected inside the trap by admitting the GC effluent and a beam of electrons directly into the storage volume of the trap. Later, external ion sources became available, and soon a large number of ionization methods could be... 

Kaiser, R.E., Jr. Louris, J.N. Amy, J.W. Cooks, R.G. Extending the Mass Range of the Quadrupole Ion Trap Using Axial Modulation. Rapid Commun. Mass Spectrom. 1989, 5, 225-229. 

Fig. 1.20 The quadrupole ion trap. A fundamental RF potential is applied onto the ring electrode to trap ions. The gray circles represent...

The ion trap is a device that utilizes ion path stability of ions for separating them by their m/z [53]. The quadrupole ion trap and the related quadrupole mass filter tvere invented by Paul and Steinwedel [57]. A quadrupole ion trap (QITor 3D-IT) mass spectrometer operates with a three-dimensional quadrupole field. The QIT is formed by three electrodes a ring electrode with a donut shape placed symmetrically between two end cap electrodes (Fig. 1.20). 

McClellan, J. F. Murphy, J. R, III Mulholland, J. J. Yost, R. A. Fffects of Fragile Ions on Mass Resolution and on Isolation for Tandem Mass Spectrometry in the Quadrupole Ion Trap Mass Spectrometer. Anal. Chem. 2002, 74, 402-412. 

The quadrupole ion trap, time-of-flight (QitTofMS) analyzer was first developed by Lubman [12,13] and co-workers. QitTofMS components were first sold commercially by R. M Jordan Company and a full commercial instrument first... 

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