Triple-quadrupole

Multiple-reaction monitoring is the most commonly used function for quantification and confirmation. The first quadrupole is set to only filter the selected precursor ion into the collision cell. This ion is then collisionally dissociated to form product ions. A small number of these product ions, which are chosen by the analyst for their structural significance, are then allowed to pass through the third quadrupole to the detector. MRM filters out chemical matrix noise, which results in superior selectivity and great sensitivity. Table 6.1 lists the MRM transitions of 

Detection of class of compounds with common product ion 

Tandem LC-MS/MS is the method of choice for both quantification and confirmation. In routine practice, two or more transitions are monitored (Table 6.1). The most intense transition is used for a quantitative work, whereas the second or third transition is utilized for confirmatory purposes. An LC-MS/MS instrument is able 

Decision 2002/657/EC has set relative abundance criteria that are dependent on the relative intensities of two transitions (Table 6.2), and has also established an identification points (IPs) system in order to confirm organic residues and contaminants in live animals and animal products (Table 6.3). For instance, where the relative intensity of the confirmation MRM is 50%, the maximum permitted tolerance is 20% relative. Therefore, at 60% relative abundance, the acceptable range would be 48-72%. With regard to the assignment of IPs, one precursor ion and one transition product are assigned I IP and 1.5 IPs, respectively, from a low-resolution (unit mass) mass spectrometer. Therefore, one precursor and two transitions from a QqQ mass spectrometer earn a total of 4 IPs. For the confirmation of banned substances listed in group A in Council Directive 96/23/EC, a minimum of 4 IPs are required. For substances listed in the group B in the directive, a minimum of 3 IPs are mandatory. 

Qx allows to select the ion of interest. If a collision gas is injected into Q2, (operating in rf only, i.e., with only yrf and with U = 0), M+ will collide with the target gas molecules. Its internal energy will increase, promoting the occurrence of the fragmentation processes. 

The product ions so formed can be analyzed by scanning the U, V values imposed on Q3. By using this description, it is easy to understand that the QQQ systems must be considered a MS/MS in space device. 

The instrumental arrangement allows a wide series of collisional experiments to be performed, among which the most analytically relevant are (schematized in Fig. 3.5), the following  

Product ion scan Identification of the decomposition products of a selected ionic species (Qi fixed, Q2 in rf only mode, Q3 scanned). 

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Linked Scanning with Triple Quadrupole Analyzers... 

An example of linked scanning on a triple quadrupole instrument. A normal ion spectrum of all the ions in the ion source is obtained with no collision gas in Q2 all ions scanned by Q1 are simultaneously scanned by Q3 to give a total mass spectrum (a). With a collision gas in Q2 and with Q1 set to pass only m+ ions in this example, fragment ions (f, fj ) are produced and detected by Q3 to give the spectrum (b). This CID spectrum indicates that both f, and fj are formed directly from m+. 

Metastable and collisionally induced fragment ions can be detected efficiently by a triple quadrupole instmment. By linking the scanning regions of the first and third quadrupoles, important information about molecular structure is easily obtained. 

There are a variety of possible linked scanning methods, but only those in more frequent use are discussed here. They differ from the linked scanning methods used in triple quadrupole instruments and ion traps in that two of the three fields (V, E, and B) are scanned simultaneously and automatically under computer control. The most common methods are listed in Table 34.1, which also defines the type of scanning with regard to precursor and product ions. 

Linked scanning is particularly easy with a triple quadrupole instrument. 

Triple quadrupole instruments can be used to detect metastable ions or can be used for linked scanning to obtain information about molecular structure. 

Scanning techniques are carried out differently with such hybrid instruments as the triple quadrupole analyzer, the Q/TOF (quadrupole and time-of-flight), and double magnetic-sector instruments. 

Q/TOF, used for two mass analyzers (quadrupole and time-of-flight) used in combination QQQ (or QqQ)- a triple quadrupole analyzer (if q is used, it means the central quadrupole is also a collision cell)... 

Table 2. Scan Functions for a Triple Quadrupole Mass Spectrometer...

MS/MS Instrumentation As was noted previously, a variety of instrument types can perform MS/MS experiments, but because of their popularity, we only discuss MS/MS experiments using triple quadrupole instruments. The principles can be applied to other types of instrumentation. 

The triple quadrupole instrument consists of two mass analyzers separated by an rf-only quadrupole. In the rf-only mode, ions of all masses are... 

When the first quadrupole of a triple quadrupole is replaced by a double-focusing mass spectrometer, the instrument is termed a hybrid (i.e. a hybrid of magnetic sector and quadrupole technologies). Figure 3.9 shows the MSi unit as a forward-geometry instrument although there is no reason why this could not be of reversed- or even tri-sector geometry. 

Figure 3.8 Schematic of a triple quadrupole mass spectrometer.

Figure 5.40 Product-ion spectrum of the (M + H)+ ion (m/z 614) of Indinavir with the proposed origins of the ions observed. Reprinted by permission of Elsevier Science from Identification of in vitro metabolites of Indinavir by Intelligent Automated LC-MS/MS (INTAMS) utilizing triple-quadrupole tandem mass spectrometry , by Yu, X., Cui, D. and Davis, M. R., Journal of the American Society for Mass Spectrometry, Vol. 10, pp. 175-183, Copyright 1999 by the American Society for Mass Spectrometry.

This is not always the case, and the ability to use accurate mass measurements to confirm that certain ions do, or do not, have the same atomic composition would certainly be an advantage. As discussed earlier in Chapter 3, the instruments most widely used for MS-MS studies, i.e. the triple-quadrupole and the ion-trap, do not routinely have accurate mass capability for product ions. 

Figure 5.69 Calibration curves obtained from (a) LC-ToF-MS and (b) LC-MS-MS using selected-reaction monitoring for Idoxifene in human plasma, fortified from 5 to 2000 ngml for LC-ToF-MS and 0.5 to 1000 ngml for LC-MS-MS with a triple quadrupole is the correlation coefficient, a measure of the quality of calibration (see...

Table 5.21 Intra-assay precision and accuracy of theLC-MS-MS determination of Idoxifene using a triple-quadrupole mass spectrometer. Reprinted from J. Chromatogr., B, 757, Comparison between liquid chromatography-time-of-flight mass spectrometry and selected-reaction monitoring liquid chromatography-mass spectrometry for quantitative determination of Idoxifene in human plasma , Zhang, H. and Henion, J., 151-159, Copyright (2001), with permission from Elsevier Science...

Triple quadrupole A mass spectrometer consisting of three sets of quadrupole rods in series, which is used extensively for studies involving MS-MS. 

APCl in positive mode ionization and triple quadrupole detection was used for determination of free and bound carotenoids in paprika, obtaining the [M + H]+ and losses of fatty acids as neutral molecules from the [M + H]+ with MeOH, MTBE, and H2O as eluent from the C30 column. The positions of the fatty acids on unsymmetrical xanthophylls could not be established by the MS data. 

Different types of mass analyzers have been used for anthocyanin analysis single or triple quadrupole mass analyzers, TOP mass analyzer,ion trap mass analyzers,and the combination of analyzers cited above. " ... 

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. 

At the present time, LC/MS/MS with triple-quadrupole instruments is the analytical method of choice for the determination of residues of sulfonylurea herbicides. We can expect to see improved triple-quadrupole instrumentation become more available and affordable as time passes, so that more analytical laboratories will have this capability. Time-of-flight (TOP) instrumentation may also play an increasingly important role in sulfonylurea analysis. Even though the metabolites are innocuous, stricter regulatory requirements may mandate that they be monitored, and LC/MS/MS is the method of choice for these compounds also. 

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