Figure 2.

Signal exclusivity of isotopically labeled histone H4 peptides. Two tissue sections underwent tryptic digestion. One was afterward sprayed with four different versions of the target molecule, and the second acted as control. The presence of the target histone H4 peptide at m/z 1325.7535 was confirmed in both control and multilabel sprayed experiments (a). Comparison of the skyline spectra between control and multilabel tissues exhibited the evidence for the tissue-wide exclusivity of the respective [M + H]⁺ mass channels for the labels L1–L4 (b–e). In another experiment, a concentration range of label L1 was spotted onto another tryptic digested tissue to estimate the concentration range of the native histone H4 peptide (f). This enabled to determine the concentrations 62, 15, and 3.8 fmol/mm² to best cover the mass range of the target histone H4 peptide.