Three dimensional ion trap

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). 

If the instrument allows for multistage fragmentation, the MS3 spectra of the most intense MS/MS fragment ions should also be acquired. There is never too much data in a de novo sequencing experiment. Even if the MS2 spectrum provides the complete sequence, data obtained from additional stages can still be used for sequence validation. In case of three-dimensional ion-traps that suffer from low mass cut-off, MS" spectra are indispensable to cover low mass fragments. For further explanation of this phenomenon, please refer to Chapter 3. 

Three-dimensional ion traps that operate on the principle of the quadrupole are another type of mass analyser (with or without a DC component). In ion traps, the ions are confined between the electrodes which have a particular shape that resembles the set-up of a quadrupole. Although they are physically simple devices, the fundamental principle of ion traps is very complex and they are more sensitive yet less expensive than quadrupoles. The volume determined by the so-called annular, superior and inferior electrodes is simultaneously the ion source and the mass-filter (see Fig. 16.12). Ion traps are usually coupled to separation techniques (GC/ MS, LC/MS). 

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. 

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],... 

Most results with ion-trap instrument were obtained nsing three-dimensional ion traps. A Q-LIT was recently introduced as well. Such a system shows great promise in oligosaccharide characterization. It provides collision-cell CID with ion-trap sensitivity but without low m/z cut-off, CID in the LIT giving MS possibilities, and... 

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). 

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. 

An LIT has the advantage over a three-dimensional ion trap of much-improved trapping efficiency, increased ion-storage capacity, improved ion-ejection efficiency, faster scan speeds, and improved detection sensitivity. 

S.2.3 Ion Trap and Linear Ion Trap Mass Spectrometry Full-scan MS experiments followed by data-dependent MS" acquisition with three-dimensional ion trap or linear ion trap mass spectrometers are commonly carried out to identify metabolites [9]. This... 

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... 

Fig. 4.4 Explanatory photo of a three-dimensional ion trap featuring essential elements of the device. ( 2013, hyphen MassSpee)...

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]. 

It allows reaction products to be observed from reactions under either kinetic or thermodynamic control. Numerous reports are available, where ion trap MS is applied in ion chemistry studies [68], e.g., involving reactions between 1,4-benzodiazepines and dimethyl ether ions [77], dissociation of [Alanine + Alkali cation]+-ions to study the role of the metal cation [78], or regioselective ion-molecule reactions to enable MS differentiation of protonated isomeric aromatic diamines [79]. The three-dimensional ion trap mass spectrometer has even been described as a complete chemical laboratory for fundamental gas-phase studies of metal-mediated chemistry [80]. 

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 ... 

Three-dimensional ion trap mass spectrometer, usually reflected with this term is the typical internal ionization capability modern 3D traps also use external ion sources providing ion injection into the trap for storage and mass analysis. 

Weber-Grabau, M. Method of operating a three-dimensional ion trap with enhanced sensitivity. U.S. Patent 4,818,869, 1989. 

Three-dimensional ion traps have problems with both poor ion collection efficiencies and space-charge effects, where the latter provides an upper limit on the number of ions that can be stored within the trap. The much larger spatial volume within a linear ion trap reduces the space-charge problem and therefore a larger quantity of ions can be stored in this trap when compared to the 3D quadrupole ion trap. 

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