The 2D ion trap

The 2D ion trap, also known as the linear ion trap (LIT), is an analyser based on the four-rod quadrupole ending in lenses that repel the ions inside the rods, and thus at positive potentials for positive ions, and vice versa. In these traps, ions are confined in the radial dimension by means of a quadrupolar field and in the axial dimension by means of an electric field at the ends of the trap. 

The LIT principles were recently reviewed by Douglas [19], Once in the LIT, the ions are cooled by collision with an inert gas and fly along the z axis between the end electrodes, 

MS2 and MS3 mass spectra from two identical oligosaccharides except for the position of the first galactose residue on the N-acetylglucosamine. These same disaccharides originating from more complex oligosaccharides produce these same fragmentations. Reproduced (modified) from Viseux N., de Hoffmann E. and Domon B., Anal. Chem., 

These voltages repel the ions inside the linear trap, and this repulsion is higher when the ions are closer to the ends. Ions are thus repelled towards the centre of the quadrupole if the same repelling voltages are applied at each end. Thus, the ion cloud will be squeezed at the centre of the quadrupole if the applied voltages are symmetrically applied, but can be located at closer to one end if the repelling voltage at that end is smaller. 

Ions trapped within an LIT can be mass selectively ejected either along the axis of the trap (axial ejection) or perpendicular to its axis (radial ejection). Therefore, in commercial 

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An ion trap is a device that uses an oscillating electric field to store ions. The ion trap works by using an RF quadrupolar field that traps ions in two or three dimensions. Therefore, ion traps can be classified into two types the 3D ion trap or the 2D ion trap. 

What is one of the significant advantages of a linear (2D) ion trap mass analyzer when compared to a 3D (Paul) ion trap (increased ion capacity, which leads to improved sensitivity). 

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. 

In recent years, linear (or 2D) ion trap mass spectrometers have been developed to improve detection limits over the traditional ion traps (Hager, 2002 Schwartz et al., 2002). In addition, LITs have greater ion storage capacities that increase the number of ions that can be trapped, and hence detected, without space charge effects. 

Senko, M.W. and Schwartz, J.C. (2002) Trapping efficiency measured in a 2D ion trap mass spectrometer. Proceedings of the 50th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, Florida, June. 

Many similar applications of quadrupole linear ion trap instruments have been reported [320-322,329,330], As discussed above, the Q-Trap is a triple quadrupole mass spectrometer capable of performing QMF type and 2D ion trapping experiments. This mass spectrometer can be operated exclusively in the QMF mode, as with a conventional QMF, or it can be operated exclusively in the ion trapping mode similar to a conventional 2D ion trap mass spectrometer. Advantages of using a Q-Trap mass spectrometer over a conventional QMF mass spectrometer come into play when one is attempting to perform both quantitative and qualitative metabolite detection/identification experiments from a single injection rather than separate... 

Quadrupole mass filters are one of the most common and cost effective mass analyzers. Although they have limited mass resolution (Table 10.2) and are less sensitive than other mass analyzers, they are durable and suitable for high-throughput analyses. To perform a tandem mass analysis (MS/MS), a triple quadrupole MS is used in which three quadrupoles are placed in series to select, fragment, and analyze ions of interest. Quadrupole ion-trapping devices such as linear 2D ion traps are... 

Thermo Scientific has introduced the 2D (or linear) liquid chromatography/mass spectrometry (LC/MS) and liquid chromatography/tandem mass spectrometry (LC/ MS/MS) ion trap mass spectrometer [11], known as the LTQ. The ion injection efficiency has been improved dramatically in the linear ion trap. In addition, ions in a linear ion trap are distributed along the central axis, rather than concentrated in the ion trap center for a 3D ion trap thus, both charge capacity and sensitivity in a linear ion trap have been increased significantly. Although at the present time there is neither commercial linear ion trap/GC/MS nor commercial linear ion trap/GC/ MS/MS instruments, a commercial version of a linear ion trap/GC/MS is expected to appear in the near future. 

The overall trapping efficiency of the 3D ion trap is only about 5% or less for externally-created ions, and this relatively low trapping efficiency has contributed to the popularity of the 2D LC/MS ion trap among ion trap users. There is little doubt that 2D GC/MS instruments soon will be popular with practitioners of GC ion trap mass spectrometry. ... 

Finally in this section, it is noted that single quadrupole mass filters (as opposed to triple quadrupole instruments) are generally used in trace quantitative analyses only for GC-EI(CI)-MS methods for thermally stable analytes (e.g. pesticides in various matrices) in SIM mode. (It is interesting that GC/MS instruments still appear to account for a large fraction of mass spectrometer units sold worldwide.) The same linear quadrupole devices can also be operated as 2D ion trap mass spectrometers, discussed in Sections 6.4.5 and 6.4.6. 

The preceding comments were made with no detailed justifications. These can only be made via a reasonably detailed consideration of the fundamental principles underlying operation of 3D quadrupole ion traps. The limitations outlined above were in fact the motivation behind the recent development of the newer 2D ion traps. The remainder of this section attempts to provide a description of these fundamental principles at a level that at least permits an understanding of the strengths and limitations of ion traps in chemical analysis, particularly when compared with triple quadrupoles in the context of trace level quantitation. 

Linear (2D) ion traps have more recently been introduced for use as ion storage devices, as stand-alone mass spectrometers and as components of hybrid tandem instruments. Although no example of their use in rigorously validated analytical methods has been published to the best of the knowledge of the present authors, there are reasons to believe that this situation might change. For... 

Key among these are quadrupole based instruments, single quadrupoles and triple quadrupoles as well as the closely related quadrupole ion trap instruments (3D and 2D ion traps). Their widespread use in trace quantitative analyses has been facilitated by the fact that these instruments can also be readily coupled to GC and/or LC because of inherent features of the mass analyzer fields, i.e. they operate at or near ground potential. 

Quadrupole mass spectrometry (Q-MS) has wide functionality making it the most popular form of MS. The family of quadrupole mass analyzers includes a variety of devices, such as quadrupole mass filter (QMF), two-dimensional (2D) ion trap (IT) or linear ion trap (LIT), and three-dimensional (3D) IT. The 3D IT device is also called the Paul trap in honor... 

The commercially available stand-alone LITs, marketed under the name LTQ, are made of four hyperbolic cross-sectional rods (Fig. 1.25). Since ions are trapped in an axial mode as opposed to central trapping on 3D ion traps, LTQs have been successfully coupled with Orbitrap and FTICR for achieving high-resolution capabilities (Peterman et al., 2005 Sanders et al., 2006) (Chapter 5). Functional improvements in 2D traps over 3D traps include 15 times increase in ion storage capacity, 3 times faster scanning, and over 50% improvement in detection efficiency and trapping efficiency. 

The quadrupole analyser is a device which uses the stability of the trajectories in oscillating electric fields to separate ions according to their m/z ratios. The 2D or 3D ion traps are based on the same principle. 

A third type of MS/MS instruments is a hybrid of tandem-in-space and tandem-in-time devices, including the Q-trap (QQ-2D-linear trap) [45] and the ion trap-FT-ICR (2D-linear ion trap-FT-ICR) [46]. The Q-trap takes the configuration of triple quadrupole, with the third quadrupole replaced by a 2D-linear ion trap. The uniqueness of this design is that the 2D-linear ion trap component can be used to perform either (a) a normal quadrupole scan function in the RF/DC mode or (b) a trap scan function by applying the RF potential to the quadrupole. It is well-suited for both qualitative and quantitative studies. In the case of ion Trap-FT-ICR, it combines ion accumulation and MS" features of a 2D-linear ion trap with excellent mass analysis capability (mass resolution, mass accuracy, and sensitivity) of FT-ICR. 

The methods for each study are divided into the initial protein separation step, a second separation step if applicable, the type of mass analysis, and the software used for peptide identification. ID = one dimensional polyacrylamide gel electrophoresis, 2D = two dimensional polyacrylamide gel electrophoresis, MS = mass spectrometry (peptide mass fingerprinting), MS/MS = tandem mass spectrometry, MALDI-TOF = matrix assisted laser desorption/ionization-time of flight, MS FIT = software from Protein Prospector, http //prospector.ucsf edu/, ESI = electrospray ionization, Q-TOF = quadrupole-time of flight, PPSS2 =Protana s Proteomic Software Suite (ProtanaEngineering, Odense, Denmark), Mascot = Matiix Science, http //www.matrixscience.com/, TOF-TOF = MALDI plus TOF tandem mass spectrometry, RP-HPLC = reverse phase high performance liquid chromatography, Q-IT = quadrupole ion trap, LIT = linear ion trap. Bioworks = Thermo Electron Corporation. 

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