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.

A general case of SRM can be represented by

where the precursor ion ABCD⁺ is selected by the first stage of mass spectrometry (MS1), dissociates into molecule AB and product ion CD⁺, and the latter is selected by the second stage of mass spectrometry (MS2) and detected. The precursor and product ion pair is called a SRM "transition." Consecutive reaction monitoring (CRM) is the serial application of three or more stages of mass spectrometry to SRM, represented in a simple case by

where ABCD⁺ is selected by MS1, dissociates into molecule AB and ion CD⁺. The ion is selected in the second mass spectrometry stage MS2 then undergoes further fragmentation to form ion D⁺ which is selected in the third mass spectrometry stage MS3 and detected. Multiple reaction monitoring (MRM) is the application of selected reaction monitoring to multiple product ions from one or more precursor ions, for example

where ABCD⁺ is selected by MS1 and dissociates by two pathways, forming either AB⁺ or CD⁺. The ions are selected sequentially by MS2 and detected. Parallel reaction monitoring (PRM) is the application of SRM with parallel detection of all transitions in a single analysis using a high resolution mass spectrometer.

SRM can be used for targeted quantitative proteomics by mass spectrometry. Following ionization in, for example, 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 can be 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.

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