Selected reaction monitoring principles

Figure 22-21 Principle of selected reaction monitoring, also called tandem mass spectrometry, mass spectrometry/mass spectrometry, or MS/MS.

The HPLC/MS/MS assays of other CYP enzymes are very similar in principle and use the identical instrumentation but employ different internal standards. As a consequence of the high degree of specificity of MS/MS selected reaction monitoring, batteries of CYP assays can be robotically programmed for high throughput with little additional manpower. 

Multiple reaction monitoring (MRM) and selected reaction monitoring (SRM) are two terms for the same operation. The general goal is to quantify analytes whereas the modes presented below are dedicated to structural analysis. They are not really adapted to quantification analysis because they generally supply limits of detection insufficiently low for trace analysis. We saw at the beginning of this chapter that a quadrupole supplies detection limits that are high when used in the full scan mode due to the principles of operation. 

Because process mixtures are complex, specialized detectors may substitute for separation efficiency. One specialized detector is the array amperometric detector, which allows selective detection of electrochemically active compounds.23 Electrochemical array detectors are discussed in greater detail in Chapter 5. Many pharmaceutical compounds are chiral, so a detector capable of determining optical purity would be extremely useful in monitoring synthetic reactions. A double-beam circular dichroism detector using a laser as the source was used for the selective detection of chiral cobalt compounds.24 The double-beam, single-source construction reduces the limitations of flicker noise. Chemiluminescence of an ozonized mixture was used as the principle for a sulfur-selective detector used to analyze pesticides, proteins, and blood thiols from rat plasma.25 Chemiluminescence using bis (2,4, 6-trichlorophenyl) oxalate was used for the selective detection of catalytically reduced nitrated polycyclic aromatic hydrocarbons from diesel exhaust.26... 

Selected ion flow tube mass spectrometry (SIFT-MS) is an analytical technique used for direct and quantitative determination of VOCs in mixtures of gases. SIFT-MS was introduced in 1976 by N. G. Adams and D. Smith. The technique can be applied for parallel real-time monitoring of a few substances [126]. A scheme of the SIFT-MS system is presented in Fig. 14.9 [127]. The principle of gas mixture analysis is based on the reaction of reagent ions with molecules of analyte within a specific time (a few milliseconds). In this method, chemical ionization is applied reagent ions are generated in the ion source by a suitable ionization gas (nitrogen, oxygen, or water vapor). Of aU the obtained ions, only cations of the desired m/z... 

In principle, any measurable property of a reacting system that is proportional to the extent of reaction may be used to monitor the progress of the reaction. The most common techniques are spectrophotometric (UV-visible, fluorescence, IR, polarimetry and NMR) or electrochemical (pH, ion-selective electrodes, conductivity and polarography). Either a "batch" method can be used, in which samples are withdrawn from the reaction mixture and analyzed, or the reaction may be monitored in situ. By far the most widely used technique involves UV-visible spectrophotometry. 

In principle spectroscopic methods have turned out to be preferable in kinetic analysis. Nowadays a large variety of spectroscopic methods exist which allow the progress of the photochemical reaction to be monitored. However, none of them perfectly satisfies under all conditions the requirements stated above. Even though it is difficult to state a general procedure, some generalised ideas are discussed next to find arguments for the selection of certain analytical tools. 

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