Selected reaction monitoring metabolite identification

In both tandem-in-space and tandem-in-time instruments, the most common experiment is for the first analyzer to select specific ions from the total ion beam arriving from the ion source. Next, the selected ions undergo collision-induced dissociation (CID) in a pressurized cell followed by the analysis of the product ions in the second analyzer. In tandem-in-time the same analyzer is used for both scans, but at different times. The resulting product ion spectra (or precursor and neutral loss spectra in other forms of MS/MS analysis) provide vital structural information for the identification of small molecules (such as drug metabolites) as well as complex biomolecules. Selected reaction monitoring (SRM), another mode of MS/MS operation, provides highly specific and sensitive quantification of target analytes. 

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