a) What is SRM?
SRM means selected reaction monitoring. It is a tandem MS technique that observes exclusively ions formed by a specified fragmentation process. For example, a constant neutral loss scan can be set up to selectively observe ions formed by loss of 31 u neutrals which most probably correspond to loss of OCH3 radicals in the above case.
b) Suggest a SRM setup to identify the ethyl esters in that mixture.
A precursor ion scan of m/z 101 would identify the ethyl esters, because the ion of m/z 101 results from gamma-cleavage of the ethyl ester molecular ions. Alternatively, a precursor ion scan of m/z 88, the product ion of the ester molecular ions’ McLafferty rearrrangement, could be employed.
c) Why gives SRM improved signal-to-noise ratio as compared to the extraction of RICs from repetitive scanning data?
Instead of simply monitoring a set of fragment ions as done by extracting RICs or acquiring SIM traces, SRM allows for the detection of a signal at a certain m/z only if the corresponding ion has been created from a precursor ion by a predefined process. Thus, SRM distinguishes the analyte ion from those of unspecific matrix compounds by classifying it due to a characteristic fragmentation process.
d) In many cases, MS/MS techniques can offer an alternative to chromatography-mass spectrometry coupling. Give an example where the pure MS/MS approach fails.
Isomeric ions are also isobaric ions, and therefore, tandem MS alone will fail to separate them.