Yes.
The purity stated on the ADS for peptides containing free Cys residues signifies the monomer content at the time the analytical HPLC was recorded. As air oxidation cannot be completely prevented in the meantime, we suggest reducing the peptide before use by treatment with dithiothreitol (DTT).
For a short description of the cystine reduction using DTT please see our Technical Note “Reversal of Inadvertent Oxidation of Cys-containing Peptides”, which can be downloaded from the “Service & Support” section of our homepage under “Technical Library”.