In this project, using discovery-based mass spectrometry approach and adopting data-dependent LC-MS/MS, we detected and identified modified peptides in complex clinical plasma samples and developed first-level biomarker verification strategy. Further, using liquid chromatography-multiple reaction monitoring mass spectrometry (LC-MRM-MS) based targeted approach which virtually quantifies irreversible oxidation at any targeted cysteine, we quantified cysteine tri-oxidation in HSA in diabetes patients. Site-specific tri-oxidized HSA could be a potential biomarker of diabetes and provide directive of disease mechanism.