Ion trap mode

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 quadrupole platform where Q3 can be operated either in normal RF/DC mode or in the LIT ion trap mode (Fig. 1.24). 

Figure 3.13. MS/MS spectra of tamoxifen (m/z 372) obtained using the ion trap mode in a QTRAP with (bottom panel) and without (top panel) the TDF scan function. 

To improve upon the approach employed by King et al. [164], Li et al. [327] used SRM-triggered information-dependent acquisition (IDA) to acquire both parent drug quantification data and qualitative metabolite MS/MS data. To validate the IDA approach, Li et al. [327] tested both the conventional triple quadrupole mode as well as in the ion trap mode and showed that the cycle time improved from 2.78 to 1.14 s with the IDA approach. The longer cycle time in the triple quadrupole mode of operation would have resulted in possibly missing some the metabolites. 

Many similar applications of quadrupole linear ion trap instruments have been reported [320-322,329,330], As discussed above, the Q-Trap is a triple quadrupole mass spectrometer capable of performing QMF type and 2D ion trapping experiments. This mass spectrometer can be operated exclusively in the QMF mode, as with a conventional QMF, or it can be operated exclusively in the ion trapping mode similar to a conventional 2D ion trap mass spectrometer. Advantages of using a Q-Trap mass spectrometer over a conventional QMF mass spectrometer come into play when one is attempting to perform both quantitative and qualitative metabolite detection/identification experiments from a single injection rather than separate... 

In addition to this simultaneous quantitative and qualitative function, the QTrap can be used under full-scan MS in the ion trap mode and/or CNLS, PSI as survey scans to trigger product ion scan (MS2) and MS3 experiments to obtain structural information of drug metabolites on-the-fly. Xia et al. [43] reported five metabolites of gemfibrozil were detected in a single injection. Bramwell and colleagues [44] utilized different scan functions in the QTrap to identify GSH conjugates in human liver microsome incubations. 

Most mass analyzers operate under high-vacuum conditions to ensure that charged particles do not deviate from their trajectories due to collision with residual gas. Mass spectrometers can be grouped into different types of operation mode continuous mode (magnetic sector, quadrupole), pulsed mode (time of flight), and ion trapping mode (quadrupole traps, Fourier transform ion cyclotron, orbitrap). 

A further extension of the DFG S19 method was achieved when polar analytes and those unsuitable for GC were determined by LC/MS or more preferably by liquid chromatography/tandem mass spectrometry (LC/MS/MS). Triple-quadrupole MS/MS and ion trap MS" have become more affordable and acceptable in the recent past. These techniques provide multiple analyte methods by employing modes such as time segments, scan events or multiple injections. By improving the selectivity and sensitivity of detection after HPLC separation, the DFG S19 extraction and cleanup scheme can be applied to polar or high molecular weight analytes, and cleanup steps such as Si02 fractionation or even GPC become unnecessary. 

A triple-quadrupole mass spectrometer with an electrospray interface is recommended for achieving the best sensitivity and selectivity in the quantitative determination of sulfonylurea herbicides. Ion trap mass spectrometers may also be used, but reduced sensitivity may be observed, in addition to more severe matrix suppression due to the increased need for sample concentration or to the space charge effect. Also, we have observed that two parent to daughter transitions cannot be obtained for some of the sulfonylurea compounds when ion traps are used in the MS/MS mode. Most electrospray LC/MS and LC/MS/MS analyses of sulfonylureas have been done in the positive ion mode with acidic HPLC mobile phases. The formation of (M - - H)+ ions in solution and in the gas phase under these conditions is favorable, and fragmentation or formation of undesirable adducts can easily be minimized. Owing to the acid-base nature of these molecules, negative ionization can also be used, with the formation of (M - H) ions at mobile phase pH values of approximately 5-7, but the sensitivity is often reduced as compared with the positive ion mode. 

Specificity is unsurpassed. Traditionally, MS was performed on very large and expensive high-resolution sector instruments operated by experienced specialists. The introduction of low-resolution (1 amu), low-cost, bench-top mass spectrometers in the early 1980s provided analysts with a robust analytical tool with a more universal range of application. Two types of bench-top mass spectrometers have predominated the quadrupole or mass-selective detector (MSD) and the ion-trap detector (ITD). These instruments do not have to be operated by specialists and can be utilized routinely by residue analysts after limited training. The MSD is normally operated in the SIM mode to increase detection sensitivity, whereas the ITD is more suited to operate in the full-scan mode, as little or no increase in sensitivity is gained by using SIM. Both MSDs and ITDs are widely used in many laboratories for pesticide residue analyses, and the preferred choice of instrument can only be made after assessment of the performance for a particular application. 

Tables 6.27 and 6.31 show the main characteristics of ToF-MS. ToF-MS shows an optimum combination of resolution and sensitivity. ToF-MS instruments provide up to 40000 spectra s-1, a mass range exceeding 100000 (in principle unlimited), a resolution of 5000, and peak widths as short as 200 ms. This is better than quadruples and most ion traps can handle. Unlike the quadrupole-type instrument, the detector is detecting every introduced ion (high duty factor). This leads to a 20- to 100-times increase in sensitivity, compared to QMS used in scan mode. The mass range increases quadratically with the time range that is recorded. Only the ion source and detector impose the limits on the mass range. Mass accuracy in ToF-MS is sufficient to gain access to the elemental composition of a molecule. A single point is sufficient for the mass calibration of the instrument. ToF mass spectra are commonly calibrated using two known species, aluminium (27 Da) and coronene (300 Da). ToF is well established in combination with quite different ion sources like in SIMS, MALDI and ESI.

For the characterization of compounds extracted from plants, wool and dye baths, acquisition in the NI mode is used. The main signals in the mass spectra of each colourant are attributed to deprotonated molecular ions [M H]. More detailed studies can be performed by ESI MS" with a quadrupole ion trap mass analyzer, and such a set-up was used e.g. for the investigation of photo-oxidation processes of components of weld and onion skins.[29]... 

Mass spectrometric analysis was performed with a hybrid triple quadrupole/ linear ion trap Applied Biosystem MSD Sciex 4000QTRAP (Applied Biosystems, Foster City, USA) instrument equipped with a Turbospray ESI interface. For target quantitative analyses, data acquisition was performed in SRM, recording the transitions between the precursor ion and the two most abundant fragment ions. The developed instrumental method display excellent LODs in SRM mode between 0.5 and 1.2 pg (Table 2). 

The mass-selective instability mode of operation permits the selection and trapping of all ions created over a specified period with subsequent ejection to the detector.26 Ions with different m/z values can be confined within the ion trap and scanned singly by application of voltages that destabilize the orbits of the ions and eject them to the detector. Ion trap instruments interface readily with liquid chromatography, ESI,15 and MALDI.27 The motions of the ions and the dampening gas (e.g., helium) concentrate around the middle of the ion trap, thereby diminishing ion loss through collisions with electrodes. 

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