Selected reaction monitoring (SRM) is synonymous with triple-quadrupole mass spectrometry. SRM is a method where the following processes occur in the order listed:

  1. The first quadrupole mass analyzer (Q1) selects and isolates a precursor ion of a specific mass-to-charge (m/z) value.
  2. The collision cell (Q2) fragments the precursor ion to produce product ions.
  3. The third quadrupole mass analyzer (Q3) sequentially selects and isolates the resulting product ions as they exit the collision cell.
  4. The electron multiplier detects the product ions after they exit Q3.

In a similar way to SIM, SRM provides for very rapid analysis of trace components in complex mixtures. Additionally, the SRM provides greater selectivity, which gives greater sensitivity compared to SIM. This is because the SRM monitors a specific product ion from a specific precursor ion, which virtually eliminates any ions, but the specific precursor/product combination being monitored. Any interfering compound would have to form a precursor ion of the same m/z value as the selected precursor ion from the target compound. Furthermore, that precursor ion would have to fragment to form a product ion of the same m/z value as the selected product ion from the target compound.