Figure 4:

Peptide quantitation using iTRAQ. Digested protein samples are modified with isobaric tags on the N-terminus of all peptides. All tagged peptide groups are mixed together and undergo LC-MS/MS analysis. After fragmentation and peptide identification, the low mass region of the MS/MS spectrum displays the reporter ions produced by the fragmentation of the isobaric reporter tag. The ratio of reporter ions in the MS/MS spectrum provides relative abundance of the peptide in each biological sample.