Oxidation of M257 and M187 was below 2.50% in all cases, with values between 0.75% and 1.48% for all DP batches and DS. Oxidation is known to be easily introduced by excessive sample handling and causes low reproducibility of results across laboratories and analysts . Low reproducibility could also be justified by the fact that peptides containing oxidised methionine showed complex elution behaviour and certain variability is due to oxidation processes produced in-sample, in-column or insource .