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Triple Quadrupole Linear Ion Trap

In a linear ion trap one of the most efficient ways to perform mass analysis is to eject ions radially. Hager [60] demonstrated that, by using fringe field effects, ions can also be mass-selectively ejected in the axial direction. There are several benefits for axial ejection: (i) it does not require open slits in the quadrupole, (ii) the device can be operated either as a regular quadrupole or a LIT using one detector. A commercial hybrid mass spectrometer was developed based on a triple quadru-pole platform where Q3 can be operated either in normal RF/DC mode or in the LIT ion trap mode (Fig. 1.24).

Sciex Trap Mass Spectrometer
Fig. 1.24 Schematic of the triple quadrupole linear ion trap (AB/MDS Sciex). Q3 can be operated in quadrupole or trap mode. In both modes ions are detected in the axial direction.

In the triple quadrupole linear ion trap, tandem MS2 is performed in space where the LIT serves only as a trapping and mass-analyzing device. Figure 1.25 illustrates the difference between quadrupole CID spectra and trap CID spectra for trocade.

With quadrupole CID all fragments are recorded in one experiment, while in the case of the 3D ion trap MS2, MS3 and MS4 experiments are required to ob-

Iontraps Ms3
Fig. 1.25 Quadrupole CID spectra and ion trap CID spectra for trocade (Mr 403): (A) MS/MS on QqQLm (B) MS, (C) MS2, (D) MS3, (E) MS4. Spectra B-E were recorded on a 3D ion trap).

tain the low mass fragments. In the triple quadrupole linear ion trap MS3 is performed in the following manner. The first stage of fragmentation is accomplished by accelerating the precursor ions chosen by Q1 into the pressurized collision cell, q2. The fragments and residual precursor ions are transmitted into the Q3 linear ion trap mass spectrometer and are cooled for approximately 10 ms. The next generation precursor ion is isolated within the linear ion trap by application of resolving DC near the apex of the stability diagram. The ions are then excited by a single frequency of 85 kHz auxiliary signal and fragmented. The particularity of the QqQLIT is that the instrument can be operated in various ways, as described in Table 1.4 [61, 62]. MS2 spectra are obtained in the quadrupole CID mode while MS3 spectra are obtained in the trap CID mode.

The major advantage of this instrument is that qualitative and quantitative analysis can be performed in the same LC-MS run. As an example in a data-dependent experiment, the selected reaction monitoring mode can be used as a survey scan and the enhanced product ion mode (EPI) as a dependent scan. The consequence is that for each quantified analyte a confirmatory MS/MS spectrum can be obtained.

Table 1.4 Mode of operation of the triple quadrupole linear ion trap (QqQUT).

Mode of operation Q1 q2 Q3

Q1 scan

Resolving (scan)

RF only

RF only

Q3 scan

RF only

RF only

Resolving (scan)

Product ion scan (PIS)

Resolving (fixed)

Fragment

Resolving (scan)

Precursor ion scan (PC)

Resolving (scan)

Fragment

Resolving (fixed)

Neutral loss scan (NL)

Resolving (scan)

Fragment

Resolving (scan offset)

Selected reaction monitoring mode (SRM)

Resolving (fixed)

Fragment

Resolving (fixed)

Enhanced Q3 single MS (EMS)

RF only

No fragment

Trap/scan

Enhanced product ion (EPI)

Resolving (fixed)

Fragment

Trap/scan

MS3

Resolving (fixed)

Fragment

Isolation/fragment trap

Time delayed fragmentation (TDF)

Resolving (fixed)

Trap/no fragment Fragment/trap/scan

Enhanced resolution Q3 single MS (ER)

RF only

No fragment

Trap/scan

Enhanced multiply charged

RF only

No fragment

Fig. 1.26 Schematic of the simplest form of a time of flight mass spectrometer. After ionization the ions are accelerated with a strong electric field.

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