FIG 1.

Diagram representing the operation of a triple quadrupole mass spectrometer in an MRM assay. First, peptides are filtered in the first quadrupole (Q1) according to the mass to charge ratio (m/z) of the precursor ion. In the collision cell (second quadrupole [Q2]), peptides are fragmented by collision-induced dissociation using nitrogen as the collision gas. Finally, predefined peptide-specific fragments are selected in the second mass filter (third quadrupole [Q3]), which is followed by measurement of the intensity of the transitions. Transitions are the precursor/product ion pairs, and several transitions are monitored over time for each peptide. LC-ESI, liquid chromatography coupled to electrospray ionization.