Integration of liquid chromatography mass spectrometry with a heavy peptide response curve accurately measures unprocessed C-terminal lysine during peptide mapping analysis of therapeutic antibodies in a single run

•Novel, simple, accurate and cost-effective quantification of mAb unprocessed C-terminal K.•Incorporates heavy standards for both native and modified peptides in a single LC–MS2 run.•Applies to IgG1, IgG2, IgG3 and IgG4 antibodies.•<10% error evaluated across different mass spectrometers and therapeutic antibodies.•Similar response curve approach can be developed for other PTMs.