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What is mass spectrometry/mass spectrometry (MS/MS)??

      Coupling two stages of mass analysis (MS/MS) can be very useful in identifying compounds in complex mixtures and in determining structures of unknown substances. In product ion scanning, the most frequently used MS/MS mode, product ion spectra of ions of any chosen m/z value represented in the conventional mass spectrum are generated. From a mixture of ions in the source region or collected in an ion trap, ions of a particular m/z value are selected in the first stage of mass analysis. These "parent" or "precursor" ions are fragmented and then the product ions resulting from the fragmentation are analyzed in a second stage of mass analysis (Figure 17). If the sample is a pure compound and fragment-forming ionization is used, the product spectra obtained from the fragment ions in the normal mass spectrum can provide much additional information for structural analysis. If the sample is a mixture and soft ionization is used to produce, for example, predominantly [M+H]⁺ ions, the second stage of MS can be used to obtain an identifying mass spectrum for each component in the mixture. Mass spectrometry/mass spectrometry can also be useful in eliminating interferences in SIM experiments when ion signal at the m/z of interest is produced by more than one compound. For sector, quadrupole and time-of-flight instruments, each stage of mass analysis requires a separate mass analyzer. For quadrupole ion trap or ICR mass spectrometers, the MS/MS experiment can be conducted sequentially in time within a single mass analyzer. In Figure 16, only the signals at m/z 158 and m/z 160 were monitored during a gas chromatographic run. This procedure is known as selected ion monitoring (SIM) and is different from the situation shown schematically in Figure 15 where compete mass spectra are obtained and used to construct selected ion profiles.