Figure 1.

Scheme of SRM/MRM useful for metabolic research. SRM or MRM for quantitative assays consists of: (A) following, for example, ionization ESI type, (B) 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 (C) whose signal abundances are indicative of the abundance of the peptide in the sample. SRM can be carried out on a triple quadrupole, 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. A precursor/product pair is often referred to as a transition.