Accurate quantification is essential in the fields of proteomics, clinical assay, and biomarker discovery. Popular methods for absolute protein quantitation by mass spectrometry (MS) involve the digestion of target protein and employ isotope-labeled peptide internal standards to quantify chosen surrogate peptides. Although these methods have gained success, syntheses of isotope-labeled peptides are time-consuming and costly. To eliminate the need for using standards or calibration curves, herein we present a coulometric mass spectrometric (CMS) approach for absolute protein quantitation, based on the electrochemical oxidation of a surrogate peptide combined with mass spectrometric measurement of the oxidation yield. To demonstrate the utility of this method, several proteins were analyzed such as model proteins β-casein, and apomyoglobin as well as circadian clock protein KaiB isolated from
