The selected reaction monitoring (SRM) experiment is a specific and sensitive mass spectrometry technique that can selectively quantify compounds within complex mixtures.

SRM is a method where the following processes occur in the order listed:

  1. The quadrupole mass analyzer selects and isolates a precursor ion of a specific mass-to-charge (m/z) value.
  2. The ICRM collision cell fragments the precursor ion to produce product ions.
  3. The high-pressure cell traps the product ions and transfers them to the low-pressure cell for the ion scan-out (the ion detection step). Mass analysis is performed in windows over each product ion range, with a jump up to the next product ion. Product ion windows with nearby m/z are merged into the same analysis window.
  4. The resulting product ions exit the low-pressure cell and are detected by the ion detection system.

In a similar way to SIM, SRM provides very rapid analysis of trace components in complex mixtures. Additionally, SRM provides greater selectivity, which gives greater sensitivity than SIM. This is because SRM monitors a specific product ion from a specific precursor ion, which virtually eliminates any ions except 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.

While SRM is the experiment of choice for triple-quadrupole instruments, it has less utility with the Stellar MS, because full-scan MS/MS spectra can be acquired in about the same amount of time and yield vastly more information and utility. The full-scan nature of the MSn spectra allows for increased selectivity and experimental flexibility, because if one precursor-to-product transition has a chemical interference from another compound, it might be possible to use another transition for analysis. For full-scan MSn experiments, you can select the transitions to analyze during post-processing; however, for SRM you have to decide which transitions to use for analysis before you begin the experiment. Therefore, there is essentially no reason to use SRM with the Stellar MS unless it was to transfer an existing TSQ Series II instrument method to the Stellar MS platform.