Figure 2: Components of the spectrum illustrated are (i) MS and MS spectrum of the precursor ( [ M + H ] + , m/z 1259.67 Da) (ii) low-mass region showing the reporter ions used for quantitation. The peptide is labeled by isobaric tags at both the N-terminus and C-terminal lysine side chain. The precursor ion and all the fragment ions therefore contain all four members of the tag set, but remain isobaric. The MS and MS spectrum was obtained from the singly charged [ M + H ] + peptide using a 4800 MALDI TOF-TOF Analyzer.