SRM - Selected reaction monitoring (SRM) is a non-scanning mass spectrometry technique, performed on triple quadrupole-like instruments and in which collision-induced dissociation is used as a means to increase selectivity. In SRM experiments two mass analyzers are used as static mass filters, to monitor a particular fragment ion of a selected precursor ion. The specific pair of m/z values associated to the precursor and fragment ions selected is referred to as a "transition" and can be written as parent m/z > fragment m/z (e.g. 673.5 > 534.3). Unlike common MS based proteomics, no mass spectra are recorded in a SRM analysis. Instead, the detector acts as counting device for the ions matching the selected transition thereby returning an intensity value over time.
MRM - Multiple SRM transitions can be measured within the same experiment on the chromatographic time scale by rapidly toggling between the different precursor/fragment pairs (multiple reaction monitoring, MRM). Typically, the triple quadrupole instrument cycles through a series of transitions and records the signal of each transition as a function of the elution time. The method allows for additional selectivity by monitoring the chromatographic coelution of multiple transitions for a given analyte. Although broadly used, the term multiple reaction monitoring to indicate the parallel acquisition of multiple SRM transitions might be in the future deprecated by the IUPAC nomenclature (IUPAC Current provisional recommendations, Murray et al. August 2006 (pending approval).