3D traps

QIT The quadrupole ion trap (QIT) utilizes a cylindrical ring and two end-cap electrodes to create a three-dimensional (3D) quadrupolar field for mass analysis. These instruments are capable of selectively trapping or ejecting ions and are often used for the sequential fragmentation and analysis experiments of product ion MS/MS. Also known as a 3D trap due to the configuration (March, 1997). 

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. 

As no DC voltage is applied, the 3D trap will be operated along the qu axis, because in the absence of DC voltage, au = 0. As already explained, qz is given by the following equation ... 

If we remember that the secular frequency at which an ion oscillates in the 3D trap is given by... 

Principle of resonant ejection. Upper ions are stored in the 3D trap at a voltage Va of the fundamental RF. An additional RF is applied to the end caps corresponding to qz = 0.8. On increasing V (lower panel) ions are moved to higher qz values. In the figure, the smallest ion has reached qz = 0.8 and is ejected by resonance. This ejection occurs at a lower value of V than the one needed to eject by instability at qz = 0.908. 

Operations similar to the 3D traps can be performed, as for example to expel ions of all masses except one and observe the fragmentation, with or without ion excitation at the secular frequency. Then the fragments are analysed. This can be repeated several times for MS" experiments. All the other operations of a 3D trap can be applied, but it also has similar limitations, for example MS/MS is limited to fragmentation scans. Thus precursor ion scan or neutral loss scan that are available with triple quadrupole instruments cannot be used on ion traps (Figure 2.11). 

Although 3D traps have been evaluated for simultaneous quantification of drugs and identification of their metabolites, the mainstay of the 3D traps within... 

Although 3D traps have been extensively used, during the early to mid-2000s, for structural elucidation of metabolites, overall a slower scan rate compared to TOF mass analyzers, in combination with limited ion capacity and trapping efficiency are the limitations associated with the QITs for becoming the mass analyzer of choice for quantitative/qualitative bioanalysis. Most importantly, 3D traps can only simulate SRM by acquiring full-scan MS data, true SRM scan modes can only be... 

Xia et al. [328] demonstrated the utility of IDA approach to collect maximum amount of information with the minimum number of analytical runs during the course of identification of in vitro formed metabolites of gemfibrozil. The Q-Trap tandem mass spectrum contained fragment ions at mlz 113 and 85 and they were absent in the MS/MS spectrum generated using a 3D iontrap due to inherent low mass cutoff. With the 3D trap, fragment ions with mlz values lower than approximately one-third of the precursor ion were not detected. 

Feature QqQ 3D Trap 2D (linear) trap QqTOF qoQiqa/rQs/r ( QTRAP )... 

An early account (Dawson 1976) of the bridge between the subsequent developments of the device by physicists (who refer to the device as the Paul Trap ) and chemists (who have used several names, most often simply ion trap but sometimes quadrupole ion storage trap, QUISTOR , 3D trap and, in one commercially available form, ion trap mass spectrometer ), was followed by extensive reviews written by and for chemists (e.g., Todd 1991 March 1992) and a three-volume set (March 1995) covering theory, practicalities and applications. More recently an excellent first introduction for chemists (March 1997) was updated (March 1998) and followed by a comprehensive treatise on the subject (March 2005). An interesting personal perspective by one of the leading contributors to the field (Stafford 2002) describes the additional problems faced in producing a commercial instrument. 

Another reason for limited performance of 3D traps in quantitation can also be traced to this same need to control space charge. The same number of ions is always allowed into the trap (with differences in the ion trap fill time corrected for after the fact), and this leads to longer scan times at the beginning and the end of chromatographic peaks where the numbers of ions are lower. 

When used as a linear ion trap (2D trap) many of the same techniqnes nsed with Paul traps can be used with only minor modifications. Trapping of ions with selected mJz values can be achieved, as in 3D traps, by resonant excitation methods or hy methods that exploit the boundaries of the stahihty diagram. Selective ion trapping is necessary if the hap is to be used also to create fragment ions by in-hap collision induced dissociation for subsequent analysis by a Panl hap or by FTICR or time of flight analyzers. Also, particnlarly in the case of downsheam trapping analyzers, excess unwanted ions that cause space charge problems can be selectively ejected. Resonant excitation at the ions secnlar frequencies (Eqnation [6.33])... 

While the inherent trapping capability provides enhanced sensitivity, and the tandem-in-time mode of operation provides enhanced ion structure informing power (MS ) relative to linear Q-based instruments, these same features also lead to decreased quantitative performance of 3D traps compared to QqQs in MRM mode. This is largely due to the fact that the total scan times associated with full scan MS/MS on a trap are inherently longer than those required for MRM on a QqQ-In addition, unlike a QqQ that provides constant scan times over the elution of the chromatographic peak, scan times on an ion trap vary due to the need to control the population of ions stored in the trap to avoid the decreased performance (resolution and sensitivity) that comes with space charge effects. These variations in scan times leads to poorer precision of chromatographic peak areas across replicate injections. Further, should a stable isotope labeled SIS be used in the analysis, the 3D... 

In the remainder of this Chapter, Stark deceleration of a molecular beam is presented in more detail, followed by a description of the process of trapping neutral polar molecules. An overview of the applications of the slow beams and trapped samples of molecules that can be produced is given, before the Chapter is concluded. The experiments described in this Chapter to exemplify the operational characteristics of the various components, have been performed in different molecular beam machines and using different molecules. The deceleration and 3D trapping of molecules in low-field seeking states is explained using the OH radical as a model... 

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