A, Spectral pair found by PAPP. Two spectra have peaks with a 58 mass difference (tentative y ions, marked with dashed elbow connectors) and peaks with the same m/z value (tentative b ions, marked with solid lines). B, Fragment ions found in the spectrum of the larger peptide and the corresponding possible amino acid residues. C, Mass errors of all possible sequences. D, MS/MS spectrum of the synthetic peptide ELEGERPL-NH2.