Tandem mass spectrometry (MS/MS) spectrum of labeled lysine peptide. MS/MS of the H3 73-83 peptide unmodified on K79 and labeled with 13C615N2-lysine isotope. Underlined nominal masses above and below the sequence denote the b and y ions respectively that were annotated from the spectrum. The expected and observed mass to charge ratio (m/z) for the [M+2H+]2+ precursor ion is provided. pr = Propionyl (heavy, D5-labeled).