When authentic samples of impurities are not available. A stressed drug product can be analyzed to show separation of the most significant related substances. In addition, the peak homogeneity of the stressed sample should be investigated by PDA or mass spectrometry. Alternatively, one may use
an orthogonal procedure to verify the method specificity. The orthogonal method can be a different technique (e.g., capillary electrophoresis, thinlayer chromatography) or different type of HPLC analysis (e.g., reversed phase versus normal phase). For example, compare the related substance
profile in the original HPLC method and that of the orthogonal method. To demonstrate method specificity, the significant related substances should be consistent in these methods.