Targeted precursor ion

Once a targeted list is assembled, the appropriate LC-MS instrument can be set up to acquire both MS and MS/MS data (or MS" data for traps) in an automated fashion. The MS/MS acquisitions would only be triggered by detection of a targeted precursor ion (from the list) at a minimum specified intensity. Linear ion trap quadrupole instruments are increasingly popular for this type of work (Hopfgartner and Zell, 2005) and are discussed in Chapter 3 of this book. 

The width of the TIS to be utilized should be determined by the proximity of adjacent protein ion peaks. Typically, +50 Da is the narrowest window for protein ion isolation that does not significantly affect signal intensity of the target precursor ion. Like the TIS, the metastable suppressor (also referred to as the precursor ion suppressor) functions as a mass gate. The purpose of the metastable suppressor is to defiect the slower velocity precursor ions, exiting the second source, from... 

Any subset of the above target precursor ions may be selected for confirmation of a given analyte or group of analytes. Collision energy for a selected subset may also be adjusted based upon previously determined optimizations typically range from 25 to 45% (see Table 2). 

Product ion scan, where Ql is set to aUow only the target precursor ion mass to enter the cell, while Q2 scans to measure all the product ions formed in the cell, including controUed cluster ion analysis. An example of this is the use of NH3 gas to create cluster ions of an analyte such as titanium. By allowing only Ti through Ql, only titanium cluster complex ions are formed in the cell, and not other potentially interfering transition metal cluster ions as with a traditional coUision/reaction cell. 

The TIC trace from the LC-MS analysis of an extracted river water sample, spiked with 3 p.g dm of atrazine and three of its degradation products, is shown in Figure 3.30. The presence of significant levels of background makes confirmation of the presence of any materials related to atrazine very difficult. The TIC traces from the constant-neutral-loss scan for 42 Da and the precursor-ion scan for m/z 68 are shown in Figure 3.31 and allow the signals from the target compounds to be located readily. 

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). 

Precursor ion and neutral loss scan functions are unique to a triple quadrupole mass spectrometer. These are powerful techniques for targeted detection of compounds and their related impurities that produces a characteristic neutral loss or a fragment ion corresponding to a unique structural feature. Both neutral loss and... 

Selected Reaction Monitoring (MS/MS) Selected reaction monitoring (SRM) is the process by which the first mass analysis selects a specific m/z (the precursor ion) to be fragmented in the collision cell and the second mass analysis selects and detects a specific product ion. Most commonly used in the quantitative analysis of well-characterized, targeted species for which optimized precursor-product pairs can be established. In SRM-based LC-MS assays no qualitative information can be obtained. However, SRM can be used to trigger product ion, neutral loss, or precursor ion scans. 

QqQ MS) are, on the other hand, a good option for sensitive, quantitative targeted analysis using multiple reaction monitoring (MRM), and they can also be used for class-specific detection through precursor ion and neutral loss scanning (4-6). 

An overall view shows the CID process as a sequence of two steps. The first step is very fast (10 14 to 10 16 s) and corresponds to the collision between the ion and the target when a fraction of the ion translational energy is converted into internal energy, bringing the ion into an excited state. The second step is the unimolecular decomposition of the activated ion. The collision yield then depends on the activated precursor ion decomposition probability according to the theory of quasi-equilibrium or RRKM. This theory is explained elsewhere. Let us recall that it is based on four suppositions ... 

SRM is a version of the product ion scan and is used in experiments designed to identify and quantify targeted analytes (Figure 7(e)). Both mass analyzers, Qj and Q3, are set to pass predetermined masses. These correspond, first, to a specific precursor ion (Qj) and, second, to a fragmentation or transition (Q3) that is characteristic of the selected analyte. Typically, the MS will be rapidly switched between several sets of such transitions representing different analytes, internal standards (ISs), or possibly an alternative confirmatory... 

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