Molecular dissociation, which is also called fragmentation, enables more complete sequence and structural information to be obtained from the mass spectra of ionized samples. Sequential dissociation may even be employed on particularly large or complex molecules such as proteins and lipids.
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To better understand the structural composition of a molecule, and especially a larger molecule such as a peptide, a dissociation technique is used to break it up (i.e., induce fragmentation) into its smaller constituents prior to mass spec. As a result, the one large m/z peak that would have resulted from mass spectral analysis of the molecule instead becomes a set of two, three or more peaks. These additional peaks act as the molecule's "fingerprint", helping to confirm characteristics such as molecular groups and tags, oxidation states and degradation products.
In diagnostics and applied laboratories, several sequential dissociation processes may be used to confirm the identity of a compound. For example, biological samples being tested for cocaine metabolites typically undergo several rounds of molecular dissociation in forensic toxicology laboratories.
Over the years, various mass spectrometry dissociation techniques have developed. Most of these techniques rely on a specific mass analyzer. Some techniques are coupled with distinct ionization processes.
Different molecule sizes and ionization states, as well as available technologies such as mass analyzers, have led to different dissociation processes to best induce fragmentation. Smaller molecules can often be successfully fragmented via CID, while larger molecules such as peptides and proteins are better suited to analysis by ETD. In fields such as proteomics, several dissociation methods must be used to characterize complex proteins that contain extensive branching and linkages.