Selected reaction monitoring
Selected reaction monitoring (SRM) is a method used in tandem mass spectrometry in which an ion of a particular mass is selected in the first stage of a tandem mass spectrometer and an ion product of a fragmentation reaction of the precursor ion is selected in the second mass spectrometer stage for detection.
SRM in proteomics 
SRM can be used for targeted quantitative proteomics by mass spectrometry. Following ionization in an electrospray source, a peptide precursor is first isolated to obtain a substantial ion population of mostly the intended species. This population is then fragmented to yield product ions whose signal abundances are indicative of the abundance of the peptide in the sample. This experiment is primarily performed on triple quadrupole mass spectrometers, where mass-resolving Q1 isolates the precursor, q2 acts as a collision cell, and mass-resolving Q3 is cycled through the product ions which are detected upon exiting the last quadrupole by an electron multiplier. A precursor/product pair is often referred to as a transition. Much work goes into ensuring that transitions are selected that have maximum specificity. By spiking in heavy-labeled (e.g., D, 13C, or 15N) peptides to a complex matrix as concentration standards, SRM can be used to construct a calibration curve that can provide the absolute quantification (i.e., copy number per cell) of the native, light peptide, and by extension, its parent protein.
SRM has been reported to identify the proteins encoded by wild-type and mutant genes (mutant proteins) and quantify their absolute copy numbers in tumors and biological fluids, thus answering the basic questions about the absolute copy number of proteins in a single cell, which will be essential in digital modelling of mammalian cells and human body, and the relative levels of genetically abnormal proteins in tumors, and proving useful for diagnostic applications.
Consecutive reaction monitoring 
The serial application of three or more stages of selected reaction monitoring.
Multiple reaction monitoring (MRM) 
The application of selected reaction monitoring to multiple product ions from one or more precursor ions.
Parallel reaction monitoring (MRM) 
The application of selected reaction monitoring with parallel detection of all transitions in a single analysis using a high resolution mass spectrometer.
See also 
- E. de Hoffmann (1996). "Tandem Mass Spectrometry: a Primer". Journal of Mass Spectrometry 31: 129–137. doi:10.1002/(SICI)1096-9888(199602)31:2<129::AID-JMS305>3.0.CO;2-T.
- Wang Q, Chaerkady R, Wu J, et al. (February 2011). "Mutant proteins as cancer-specific biomarkers.". Proc. Natl. Acad. Sci. U.S.A. 108 (6): 2444–9. Bibcode:2011PNAS..108.2444W. doi:10.1073/pnas.1019203108. PMC 3038743. PMID 21248225.
- Cooks, Robert (1978). "Multiple Reaction Monitoring in Mass Spectrometry/Mass Spectrometry for Direct Analysis of Complex Mixtures". Analytical Chemistry 50 (14): 2017–2021.
- Peterson A C et al.. Parallel reaction monitoring for high resolution and high mass accuracy quantitative, targeted proteomics. doi:10.1074/mcp.O112.020131. PMID 22865924.