Fig. 4.

HCD spectra of low abundance oxidized filgrastim variants labeled in red in Fig. 3. (a) The diagnostic fragment ions for M127ox + M138ox (y47 + 16 Da and y50 + 32 Da) were unambiguously identified in the HCD spectra despite representing only 0.7 % of the total peak area. (b) Due to the low abundance of the triply oxidized variant, which represents 0.12 % of the total peak area, the diagnostic fragment ion y78 is missing. However, the combination of native mass (18,788.01 + 48.3) and diagnostic ions y47, y50 + 16 Da, and b97 + 16 Da allows the sites of oxidation to be elucidated