Abstract: An analytical method for detecting presence of oxidizing substances by measuring degradation of peptides may include preparing a test preparation that includes a peptide and a sample. The peptide degrades in the presence of oxidizing substances. The method may include detecting impurities of the peptide in the test preparation in which detected impurities indicate presence of oxidizing substances in the sample. Examples include using high performance liquid chromatography to detect, identify, and quantify impurities of the peptide indicating presence of oxidizing substances in the sample. Exemplary oxidizing substances include peroxide-containing, chlorine-containing compounds, and bromine-containing compounds. Exemplary peptides include vasopressin.