3.

Identification of the HSA peptide LK­[+395.138]­C­[+57.021]­ASLQK based on the tandem mass spectra acquired for peptide LK­[+312.089]­C­[+57.021]­ASLQK. Extracted ion chromatograms (XICs) of the doubly protonated precursor ion (A) and selected b- and y-ion signals (B, y₆ highlighted in red). Arrows indicate fragment ions shared by both peptides. The drift times of the precursor and y₆ ions in IMS are given below the retention times. Mirror plots of the fragment ion spectra of the doubly protonated modified peptides (C). High-intensity signals are clipped and marked with double dashed lines.