Linear Ion Trap LIT

Fig. 1.30 Schematic of the linear ion trap (LIT)-orbitrap (LTQ orbitrap, Thermo). One of the specificities of the system is that the LIT has two detectors. Therefore the LIT can perform various experiments at the same time. Adapted with permission from reference [76].

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). 

I hasten to add that this system does not exist at the moment. You probably do not want to include it in your equipment proposal for this year. But, it is rapidly becoming a viable option for development. And, if successful, would the portable linear ion trap (LIT) based LC/MS/MS, for definitive compound identification by searching a MS database, be far behind ... 

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. 

Figure 2.37 shows the scheme of the first commercially available orbitrap instrument. It is somewhat different from the one previously described. First, there is an linear ion trap (LIT) that can be used for ion storage and ejection to the orbitrap by axial ejection, or independently used as a linear trap by radial ejection. It is possible to inject all the ions from the LIT, or selected ones, or product ions from the MS" operations of the LIT. The normal acquisition cycle time in the orbitrap is 1 second. 

Some mass spectrometers combine several types of analysers. The most common ones include two or more of the following analysers electromagnetic with configurations EB or BE, quadrupoles (Q), ion traps (ITs) with Paul ion traps or linear ion traps (LITs), time-of-flight (TOF), ion cyclotron resonance (ICR) or orbitrap (OT). These are named hybrid instruments. The aim of a hybrid instrument is to combine the strengths of each analyser while avoiding the combination of their weaknesses. Thus, better performances are obtained with a hybrid instrument than with isolated analysers. Hybrids are symbolized by combinations of the abbreviations indicated in the order that the ions travel through the analysers. 

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],... 

FT-ICR-MS systems equipped with external API sources are conunercially available from Bruker Instruments, lonSpec, and Thermo Fimiigan. In order to control the number of ions in the ICR cell, hybrid systems have been developed. Bruker offers a FT-ICR-MS hybrid with a quadrapole front-end (APEX-Qh) [121], whereas the LTQ-FT instrument from Thermo Fimiigan features a linear-ion-trap (LIT, Ch. 2.4.2) front end [122]. In this way, MS-MS can be performed prior to ion introduction into the ICR-cell, avoiding problems with CID in the ICR-cell. 

The quadrupole ion trap (QJT) is about the size of a small fist and consists of a ring electrode and two hyperbolic end electrodes (see March and Todd68 for a detailed theory of operation and history of development). Like the linear ion trap (LIT, see below), the QJT operates at relatively high pressure (10 3 torr) with a helium buffer gas that assists the ions to maintain a stable orbital frequency. The buffer gas also serves as the collision gas for collision-induced dissociation (CID) during MS/MS experiments. 

More recently, a linear ion trap (LIT) on a triple-quadrupole platform... 

Shevchenko and colleagues have developed a somewhat different form of shotgun lipidomics which is particularly suitable to applications utilizing a hybrid linear ion trap (LIT) Fourier transform (FT) instrument, for example, LTQ-Orbitrap, called top-down lipidomics [18,24]. In a top-down lipidomics experiment, the survey scan is recorded at high resolution (100,000) (full width at half maximum height [FWMH]) in the FT analyzer. Mass accuracy on FT instruments is typically <5 ppm and often with internal calibration <2 ppm. This degree of mass accuracy... 

An important configuration of quadrupoles is the triple quadrupole (QqQ), in which there are two analytical quadrupoles (Q) separated by a transmission quadrupole (q). While the predominant use of the QqQ is in quantification, this very versatile format has several scanning modes that enable multiple MS/MS approaches to obtain structural information (Section 3.3.3.1). Extensions of the quadrupole technology are the quadrupole ion trap (QIT) and the more recent linear ion trap (LIT) that has higher ion capacity. The resolutions of these ion traps are similar to those of single quadrupoles. However, an advantage of the traps is the ability to store and manipulate ions prior to their detection, thus enabling MS/MS experiments (Section 2.3.2). 

In tandem-in-time mass spectrometers all operations take place within a single-ion storage device, but at different times. The most common tandem-in-time instrument is the quadrupole ion trap (QIT). The ongoing development of a higher-sensitivity counterpart, the linear ion trap (LIT), will presumably lead to an eclipsing of the QIT. 

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