Quadrupole linear trap

One of the latest mass analyzer is the linear-trap quadrupole (LTQ) Orbitrap mass spectrometer. In this, the commercial LTQ is coupled with an ion trap, developed by Makarov [73, 74]. Due to the resolving power (between 70000 and 800000) and the high mass accuracy (2-5 ppm), Orbitrap mass analyzers, for example, cab be used for the identification of peptides in protein analysis or for metabolomic studies. In addition, the selectivity of MS/MS experiments can be greatly improved. However, the coupling is not useful with UHPLC for rapid chromatographic pre-separation, as the data acquisition rate is too low for a reproducible integration of the narrow signals produced with UHPLC. 

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). 

The LIT introduced as part of a triple-quadrupole mass spectrometer is marketed under the name QTRAP. As shown in Fig. 1.26, the ion path and the differentially pumped region of QTRAP are similar to a triple quadrupole (API 3000, API 4000, and API 5000), except the Q3 is capable of functioning as a linear trap. QTRAP and its capabilities are described in detail in Chapter 3. Table 1.2 compares some of the advantages and limitations of QTRAP and LTQ mass spectrometers. 

Once a targeted list is assembled, the appropriate LC-MS instrument can be set up to acquire both MS and MS/MS data (or MS" data for traps) in an automated fashion. The MS/MS acquisitions would only be triggered by detection of a targeted precursor ion (from the list) at a minimum specified intensity. Linear ion trap quadrupole instruments are increasingly popular for this type of work (Hopfgartner and Zell, 2005) and are discussed in Chapter 3 of this book. 

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. 

The linear trap with axial ejection was invented by Hager, from MDS Sciex, in 2002 [20], Figure 2.31 displays a scheme of such an ion trap included in the ion path of a triple quadrupole mass spectrometer. 

View of the linear trap included in a triple quadrupole at q2. This instrument can be operated as a regular triple quadrupole or with a trap. Reproduced from Hager J.W., Rapid Comm. Mass Spectrom., 16, 512-526, 2002, with permission. 

Linear trap with slots cut in two opposite rods. Sizes are 12 mm for sections A and C and 37 mm for B. Detectors D are placed off-line and ions are attracted by the conversion dynodes. The slots are 30 x 0.25 mm. Drawn according to the data from Schwartz J.C., Senko M.W. and Syka J.E.P., A Two-Dimensional Quadrupole Ion Trap Mass Spectrometer , Proceedings of the 50th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, Florida, 2002. 

Vallverdu-Queralt, A. Jauregui, O. Medina-Remon, A. Andres-Lacueva, C. Lamuela-Raventos, R.M. 2010. Improved characterization of tomato polyphenols using liquid chromatography/electrospray ionization linear ion trap quadrupole Orbitrap mass spectrometry and liquid chromatography/electrospray ionization tandem mass spectrometry. Rapid Comm. Mass Spectrom. 24 2986-2992. 

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 first idea was to build a trap from the linear quadrupole mass filter structure but, rapidly, the properties of multi-pole potentials were exploited also. For a quadrupole linear trap, the RF electric field [19] is transverse to the z-axis of the ion trap near this axis, the time potential, ( ), in the x- and y-directions can be expressed by... 

In the first evolutionary stage in the development linear traps, two traps resembling quadrupole mass filters were used to create an extended linear trap [23],... 

The role of the mass analyzer is to separate ions according to their miz values and to focus and transfer these ions onto a detector, or into a collision cell in multianalyzer instruments (see later). The mass analyzer is the heart of all mass spectrometers (block 3 in Figure 1.2). The choice of which analyzer to use is critical as it affects multiple aspects of the data generated, including mass resolution, mass measurement accuracy, and available dynamic range. There are several types of analyzer quadru-pole (Q), ion trap (quadrupole (QIT) or linear (LIT) ion trap), time-of-flight (TOF),... 

A mass analyzer separates and mass-analyzes the ionic species. Magnetic and/or electric fields are used in mass analyzers to control the motion of ions. A magnetic sector, quadrupole, time-of-fiight, quadrupole ion trap, quadrupole linear ion trap, orbitrap, and Fourier transform ion cyclotron resonance instrument are the most common forms of mass analyzers currently in use (discussed in Chapter 3). 

In the operation of 2D and 3D quadrupole instruments, the modification of ion trajectories by ion/ neutral collisions must be considered. In rf quadrupole fields, an ion/neutral collision reduces both ion kinetic energy and ion excursions such that the ions are cooled and focused to the center of each field. Collisional cooling is an important aspect of the behavior of ions in a quadrupole field for example, in a 2D quadrupole device, collisional cooling is employed to limit the excursions of ions so as to form a tightly focused ion beam of diminished kinetic energy constrained close to the central axis. A focused beam of ions may be transmitted through a relatively small orifice from one section of an instrument to the next such that pumping requirements are reduced, and a focused ion beam can be accelerated with reduced ion loss. When the axial motion of a focused ion beam is arrested within a rod array such that the confined ions can be excited resonantly, a linear ion quadrupole trap is obtained. 

Trapping efficiency The efficiency of trapping in Qc approaches 100% for ions with 5-15 eV of kinetic energy over the pressure range 1-17 x 10 Torn The high acceptance of ions into a linear trap is attributed to the absence of a quadrupole field along the z-axis. 

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