The figure shows a schematic illustration of the principle of a triple quadrupole mass spectrometer (QQQ). In the first quadrupole (Q1), a specific precursor ion of a PTP is selected based on its mass-to-charge (m/z) ratio. The precursor ion is fragmented in the second quadrupole (Q2) by collision-induced dissociation, which allows for the selection of a specific fragment of the target peptide ion, according to its m/z ratio, in the third quadrupole (Q3). The signal intensity of this fragment is reported over time. The pair of m/z ratios for the precursor and fragment ions is a so-called SRM transition. A series of the best SRM transitions for the target peptide in combination with its retention time and instrument parameters, serve as a fingerprint for a PTP and constitute a definitive SRM assay.