Targeted metabolomics - multi reaction monitoring mass spectrometry
Targeted metabolomics is able to accurately identify compounds and when calibration standards are used, able to absolute quantify the targeted compounds. Absolute quantification gives the true abundance of a compound in a sample (not its relative value). A draw back is that only compounds for which methods have been established can be identified and quantified, which means unknown compounds of interest are not detected.
High Performance Liquid Chromatography coupled to Multi Reaction Monitoring Mass Spectrometry (HPLC-MRM-MS) is a common method for targeted metabolomics. After metabolite extraction from samples, the extracted metabolite species are separated by liquid chromatography. Here the strength of interaction with solid phase depends on the physical/chemical properties of the molecule species and the gradient of the liquid phase. This leads to differing retention for the molecule species. Eluting molecules are ionized so that they can be accelerated and manipulated by the triple quadrupole mass spectrometer. The first quadrupole (Q1) is set, so that only ions of a certain mass to charge ratio are able to pass through it. In the second quadrupole the filtered ions are fragmented and passed to the third quadrupole (Q3). The Q3 is set so that only fragment ions of a specific mass to charge ratio have a stable trajectory and are able to reach the detector.