The 3D ion trap

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. 

Historically, the first ion traps were 3D ion traps. They were made up of a circular electrode, with two ellipsoid caps on the top and the bottom that creates a 3D quadrupolar field. These traps were also named quadrupole ion traps (QITs). To avoid confusion, this term should not be used but should be replaced preferably with Paul ion trap. The acronym QUISTOR derived from quadrupole ion storage is also largely used but not recommended. 

Besides 3D ion traps, 2D ion traps have also been developed. They are based on a four-rod quadrupole ending in lenses that reflect ions forwards and backwards in that quadrupole. Therefore, in these 2D ion traps, which are also known as LITs, 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. 

Conceptually, a Paul ion trap can be imagined as a quadrupole bent in on itself in order to form a closed loop. The inner rod is reduced to a point at the centre of the trap, the outer rod is the circular electrode, and the top and bottom rods make up the caps. 

In quadrupole instruments, the potentials are adjusted so that only ions with a selected mass go through the rods. The principle is different in this case. Ions of different masses are present together inside the trap, and are expelled according to their masses so as to obtain the spectrum. 

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With quadrupole CID all fragments are recorded in one experiment, while in the case of the 3D ion trap MS, MS and experiments are required to ob-... 

Mass selective ejection of the ions in a radial direction occurs by applying an AC voltage between the two cut rods. As for the 3D ion trap, an AC frequency corresponding to qz = 0.88 is used. Ions of successively higher masses are brought to this qz value by increasing V. An ejection efficiency of about 50 % is achieved at 5 Th s scan rate [25]. 

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. ... 

Anrcjl) are the occupied volumes of the same instruments, respectively. For collisionally focused ion clouds at a common rf potential and radial frequency, rA = = fc = 1.0 mm based on ion tomography experiments, the ratio Nzd,a N2D,b N3d = 95 22 1. This ratio suggests an ion capacity for the linear ion trap two orders of magnitude greater than that of the 3D ion trap. 

QqQ in MRM mode and enables use of shorter chromatography columns and shorter run times (and thus increased throughput), while the additional selectivity provided by MRM detection can permit simplification of sample preparation procedures (Chapter 3). In practice, usually for each target analyte just one reaction channel (sometimes referred to as an MRM transition) is used to provide the quantitative data while one or two others are sometimes monitored simultaneously in order to provide confirmation of analyte identity via the relative responses (essentially a check on the selectivity of the analytical method. Section 9.4.3b). In contrast with the QqQ, the 3D ion trap has a poor duty cycle in MRM mode and the full scan product ion scan method is the method of choice if this analyzer is used for quantitation, since it is often possible to acquire 10 such scans across a chromatographic peak with adequate S/B values. Additional post-acquisition data processing is required to obtain quantitation data from such full scan MS/MS experiments. 

TABLE 20.3 MS/MS Fragmentation Energy Index for the 3D Ion Trap MS Analysis of Active Chemical Agents... 

An alternative to the 3D quadrupole ion trap (Paul trap) is the linear quadrupole ion trap. The linear ion trap is akin to a hybrid of the quadrupole mass filter and the 3D ion trap in that it consists of a four-rod assembly, like the quadrupole filter, but also it has entrance and end electrodes like the 3D ion trap. Confinement of ions along the axial direction is provided by DC potentials applied to the end electrodes. The quadrupole rods produce radial motion of the ions through application of an RF electric field, in a similar manner to that already described for the quadrupole mass filter. To record a mass spectrum axial ion ejection, initiated by RF excitation, can be used in a procedure similar to that used for the 3D ion trap. 

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