“…In parallel with ECD/ETD, a variety of methods that introduce peptide/protein radical ions in the gas phase have been developed. Siu and coworkers discovered that peptide radical cations, M •+ , could be generated by low energy CID of ternary metal complexes with peptide and auxiliary ligands (Chu et al, ; Bagheri‐Majdi et al, ; Wee, O'Hair, & McFadyen, ; Barlow, McFadyen, & O'Hair, ; Barlow et al, ; Ke et al, ; Laskin, Yang, & Chu, ; Hopkinson, ; Chu & Laskin, ; Kalli & Hess, ). UV photo‐excitation followed by H atom loss provides another way to generate radical peptide ions (Tabarin et al, ; Lemoine et al, ; Park et al, ).…”