Oxidation is a commonly observed post-translational modification (PTM) in biotherapeutic proteins which is identified via a mass shift of 15.99 Da. The oxidation of methionine can have various effects on protein structure and protein interaction and hence a reliable and reproducible method is critical for the monitoring of this PTM . Methionine (M) oxidation was observed for M81, M187 and M257 for all samples assayed. The relative abundance of oxidation is visualised in Figure . Oxidation of M81 residue showed consistent levels for all the DP batches and DS between 2.18% and 2.31 %.