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.
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