An Introduction to Mass Spectrometry (cont.)

V. Conclusion

An advantage of this approach is that each peptide tandem mass spectrum represents a unique piece of information, consequently matching one or more tandem mass spectra to sequences in the same protein provides a high level of confidence in the identification and enables the identification of proteins present in mixtures. This process has been automated in the software SEQUEST.

Definitions:

• Mass-to-charge ratio (m/z): Mass spectrometers measure the mass to charge ratios of ions. In MALDI and electrospray ionization, peptides are typically ionized by the addition of one or more protons. Thus, a peptide of molecular weight 1000 daltons will have a m/z value of 1001 after ionization by the addition of one proton and 501 with the addition of two (M+2H)+2.
• Collision-induced dissociation (CID): One method of energetically activating ions to dissociate. Typically, a gas-phase collision cell filled with argon gas is used to subject ions to low energy collision (10-50 eV) causing energetic excitation. As ions become energetically excited, covalent bonds dissociate to produce structurally informative fragment ions. Often the molecular structure of the ion can be postulated from the fragmentation pattern, or in the case of peptides, the amino acid sequence deduced.

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