Tandem mass spectrometric analysis of the E1 tryptic digest. A. Base peak ion chromatogram obtained by HPLC/ESI-qTOF MS and MS/MS analysis of the E1 tryptic digest. B. Selected ion retrieval for m/z 1088.8, which corresponds to the doubly-charged tryptic phosphopeptide LDLIDEEEDpSEEDGDSMR. C. Full scan mass spectrum at RT 53.1 min showing the presence of several tryptic peptides including the doubly and triply-charged phosphopeptide LDLIDEEEDpSEEDGDSMR.