Selected-reaction monitoring example

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 spectmm can be obtained. 

Selected reaction monitoring is extremely selective for the analyte of interest. An example is provided by the analysis of human estrogens in sewage at parts per trillion levels (ng/L). Estrogens are hormones in the ovarian cycle. The synthetic estrogen 17a-ethinylestradiol (designated EE2) is a contraceptive. Even at parts per trillion levels, some estrogens can provoke reproductive disturbances in fish. 

MS/MS A process in which mass (m/z) selection or analysis is typically performed in two distinct serial steps. Operational examples include selected reaction monitoring or constant neutral loss scanning (see below). 

Quantitation of drags and their metabolites in biological matrices currently is one of the most important applications of LC-MS. This can be attributed to the greatly enhanced selectivity and reliability of the analysis, as compared to LC-UV. Selective reaction monitoring (SRM) in tandem mass spectrometry (MS-MS) has become the method-of-choice in quantitative bioanalysis. Ample examples are described in literatnre. Even a larger number of successful examples are hidden in the archives of pharmaceutical companies and contract research organizations. 

MS/MS has also become established in biochemistry and genetics, both as a discovery tool and a diagnostic technique. The high sensitivity of contemporary tandem and multidimensional mass spectrometers means they can be used to detect disease-specific molecules in tissue and blood samples down to the femtomole level. For example, MS/MS neutral loss scanning is used to detect increased levels of phenylalanine in the blood of newborn infants, to increase the detection rate and lower the false-positive rate associated with screening for phenylketonuria. Selected reaction monitoring is used widely to detect drug residues, such as steroids in athletes and domestic animals and in adulterated foods. 

Finally, the use of linked scan or MS/MS techniques can impart even greater selectivity. In this approach, the use of collisionally activated decomposition of a parent ion to give a suitable daughter ion is monitored. For instance, in a typical MS/MS experiment on t-BDMS ethers, the first mass spectrometer MSI would pass the parent ion whilst MS2 would be set to pass the M — 57 ion. This technique is known as selective reaction monitoring. An example is shown in Figure 2, which shows the analyses of a plasma extract of oestradiol-17/1 obtained using the... 

Selected reaction monitoring is extremely selective for the analyte of interest. For example, we can monitor traces of caffeine in natural waters to detect contamination by domestic wastewater. The global daily average consumption of caffeine in Europe and North America is 200-400 mg per person, much of which ends up in sewage. If caffeine shows up in a municipal water supply, it probably migrated from wastewater. To measure parts per trillion levels of caffeine, 1 L of water was passed through a column containing 10 mL of adsorbent polystyrene beads that retain caffeine and a host of other... 

Where tandem mass spectrometry is available, even more security can be achieved by use of selected reaction monitoring (SRM) instead of SIM. Tandem mass spectrometry is the use of successive steps of MS to select or scan ions, create fragments from them, and scan the fragments. It can be carried out in time or space. Triple quadrupole systems are an example of tandem-in-space. The masses of parent ions are filtered and scanned by the first quadrupole, fragmentation occurs in the... 

A scan-by-scan calibration correction method used, for example, in high resolution selected reaction monitoring, H-SRM. 

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