6.11: Tandem Mass Spectrometry
Tandem mass spectrometry is a technique that uses multiple mass analyzers in series to obtain a higher selectivity and signal-to-noise ratio for the analyte. Instruments with multiple analyzers separated by an interaction cell enable secondary fragmentation and selected study of the fragment ions.
Secondary fragmentations occur in the interaction cell and can be induced by various factors. Fragmentation induced by collision with inert gases, such as N2, Ar, He, etc., is called collision-induced dissociation (CID). Other techniques involve interaction with the surface through surface-induced dissociation (SID); interaction with low-energy electrons, called electron-capture dissociation (ECD); or through an intense laser beam, called photo-induced dissociation (PID).
Tandem mass spectrometry can be used for various experiments by varying the mode of operation of the two mass analyzers. When the first mass analyzer selects a particular precursor ion and the second mass analyzer scans all the fragmented product ions, it is called a product-ion scan, and the spectra obtained by this experiment are called product-ion spectra. Alternately, when the first mass analyzer scans all the precursor ions and the second mass analyzer selects a particular product ion, it is called 'precursor-ion mode', and the spectra are termed precursor-ion spectra. Another experiment can be performed by scanning both the analyzers simultaneously with an offset of a mass equal to a neutral fragment lost by the ion to be monitored. Such a mode is called 'neutral-loss scanning mode', and the corresponding spectra are called neutral-loss spectra.
The instrumentation of tandem mass spectrometry can be varied. Tandem-in-space spectrometers use two independent mass analyzers in two different regions in space, for example, a triple quadrupole mass spectrometer and TOF–TOF spectrometer. On the other hand, a tandem-in-time spectrometer, such as a quadrupole ion-trap instrument, uses the same spatial region to fragment the ions and dissociate and analyze a selected ion of interest, but at different times by expelling the unwanted ions. Tandem-in-time spectrometers can be used to perform MSn experiments.
