Two-dimensional ion trap

Figure 2.7 Mass spectra recorded at different resolutions. Mass spectrum obtained by a two dimensional ion trap at low resolution (a) and by an Orbitrap at resolving power 50000 (b). Mass spectrum of a mixture of three isobaric species [C19H7N]+, [C20H9]+, [C13H19N302]+ obtained at low resolution (black line) and at resolving power 50000 (grey line) (c). It is noteworthy that at low resolution the three peaks are completely unresolved...

Belov, M. E., Nikolaev, E. N., Alving, K., and Smith, R. D. (2001). A new technique for unbiased external ion accumulation in a quadrupole two-dimensional ion trap for electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Rapid... 

A recent innovation is the commercial availabihty of linear two-dimensional ion traps [54], The linear ion trap (LIT) is found to be less prone to space-charging effects, enabling a higher number of ions to be accumulated, which results in enhanced sensitivity. In the conunercial instrument, the linear ion trap is the third quadrapole in a triple-quadrapole arrangement, i.e., (J-q o -LlT. In that setup, it can be used to accumulate product ions generated by CID in a LINAC colhsion cell, providing enhanced sensitivity and lack of low-mass cut-off. Further stages of MS-MS can be performed in the linear ion-trap, which then has similar features as the three-dimensional ion-trap. Early reports described the application of the hnear ion trap in metabolite identification and quantitative bioanalysis [55-56],... 

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. 

More recently, instruments with a hnear two-dimensional ion trap (LIT), i.e., a linear quadrupole as ion trap, have become commercial available [24, 62, 63]. As a LIT is less prone to space charging effects, a higher number of ions can be accumulated, and enhanced sensitivity can be achieved. Initially, LITs were apphed in hybrid Q-LIT [24] a LIT and hybrid LIT-FT-ICR-MS [64] instruments, but later on stand-alone versions of an LIT were introduced too, thus competing the three-dimensional ion traps. A dual-pressme two-stage LIT has been reported as well the first high-pressure ion trap serves to capture, select, and fragment ions, whereas the second low-pressure ion trap is used to perform fast scamiing of product ions, eventually at enhanced resolution [65]. 

An alternative to three-dimensional Paul traps, linear (two-dimensional) ion traps have demonstrated great utility as mass... 

A three-dimensional ion trap is formed by three electrodes, two end caps and a ring electrode in the linear ion trap different electrodes form each of the four edges (Figure 2.11). 

An important development in quadnipole technology is the three-dimensional ion trap [38-39]. A quadmpole ion trap consists of a cylindrical ring electrode to which the quadnipole field is apphed, and two end-cap electrodes (Figure 2.3). One end-cap contains holes for the introduction of electrons or ions into the trap, while the other has holes for ions ejected out of the trap towards the electron multiplier. In LC-MS systems, ions are generated in an external ion source. The ions are introduced to the trap in a pulsed mode and stored there. A helium bath gas (0.1 Pa) is present in the trap to stabilize the ion trajectories. 

Other mass spectrometers are equipped with three-dimensional ion traps of which the geometry is much different to the quadrupoles previously described. In an ion-trap, the ions are confined between three electrodes (one toroidal and two end-caps), whose particular shape appears to result from a sort of anamorphosis of the four-bar set-up of a classic quadrupole. As in the previous category they operate under the effect of a variable electric field (with or without a superimposed fixed field). Although they are, in appearance, physically simple devices, the fundamental principle of ion trap is complex. These ion trap detectors are sensitive, less costly than quadrupoles and compatible with different ionization techniques. The volume defined by the electrodes, named superior, inferior and annular, is simultaneously the ion source and the mass filter (Figure 16.11). These analysers are almost exclusively linked with a separative technique (GC/MS). 

A typical ion trap, also called quadmpole ion trap or three-dimensional ion trap, consists of a cylindrical ring electrode to which a quadmpole RF field is applied, and two end-cap electrodes [60,61] (Fig. 4.4). The end-cap electrodes contain holes for the introduction of ions from an external ion source and for the ejection of ions out of the trap towards the external electron multiplier detector. The ion trajectories in the trap are stabilized by a He bath gas ( 1 mbar). With respect to ion trap actions, the individual steps in the mass analysis process are performed... 

A three-dimensional ion trap (e.g., Q ion trap) bears the same physical principles as the quadrupole mass analyzer, but the ions are trapped and sequentially ejected mainly by using an RF field, within a space defined by a ring electrode between two end-cap electrodes (Figure 2.6b). A linear quadrupole ion trap is similar to a three-dimensional ion trap, but it traps ions in a two-dimensional, instead of a three-dimensional, quadrupole field. Some of the figures of merit of the ion-trap instrument are as follows ... 

Schley, C., Altmeyer, M.O., Swart, R., Muller, R., Huber, C.G. (2006). Proteome analysis of Myxococcus xanthus by off-line two-dimensional chromatographic separation using monolithic poly-(styrene-divinylbenzene) columns combined with ion-trap tandem mass spectrometry. J. Proteome Res. 5, 2760-2768. 

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). 

Fig. 8.1.1 Simple illustrations of a various mass spectrometers, a The triple-quadrupole tandem mass spectrometer (top panel). The middle set of quadrupoles are part of the collision cell (CC) and do not perform mass separation. MSI and MS2 indicate the first and second quadrupole mass separation devices, respectively. The bold arrow shows the path of ions, b Ion-trap mass spectrometer (middle left). The charged sections of the ion trap are not elliptical as drawn, but rather hyperbolic. The diagram is also two-dimensional, whereas the ion trap is three-dimensional. The ion path is such that ions enter the device and are trapped until a specific voltage ejects these ions, c Time of Flight mass spectrometer with a Reflectron (middle left). Ions are separated by the time it takes to pass through the instrument. The Reflectron improves/focuses the ions, d Hybrid Tandem mass spectrometer (bottom). The diagram shows that a quadrupole instrument can be combined with a different type of mass spectrometer, forming a tandem hybrid instrument...

QIT The quadrupole ion trap (QIT) utilizes a cylindrical ring and two end-cap electrodes to create a three-dimensional (3D) quadrupolar field for mass analysis. These instruments are capable of selectively trapping or ejecting ions and are often used for the sequential fragmentation and analysis experiments of product ion MS/MS. Also known as a 3D trap due to the configuration (March, 1997). 

LIT The linear ion trap (LIT) (also referred to as a two-dimensional, or 2D, trap) is a variation on the transmission quadrupole mass analyzer. In the LIT, the quadmpole is constructed such that either ions can be analyzed immediately or, ions can be trapped and held in the quadrupole region and then analyzed (Hager, 2002 Schwartz et al., 2002). Various types of MS/MS can be performed, as described in Chapter 3. 

The two-dimensional (2-D) or linear ion trap (LIT) emerged in the 2000s as an effective alternative to the 3-D trap. Before 1995, linear traps were used primarily as ion storage/transfer/ion-molecule reaction devices in combination with FTICR (Senko et al., 1997 Belov et al., 2001), TOF (Collings et al., 2001), 3D ion trap (Cha et al., 2000), and triple-quadrupole (Dolnikowski et al., 1988) mass spectrometers because LITs offer better ion storage efficiencies in comparison to 3D quadrupole ion traps of the same dimensions (Hager, 2002 Schwartz et al., 2002). In 2002, commercial LITs were introduced as either stand-alone mass spectrometers (Schwartz et al., 2002) or as part of a triple quadrupole mass spectrometer (Hager, 2002). 

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