Amino Acid Analysis (AAA) by RP-HPLC is used in protein characterization to analyse peptide or protein concentration, content and molar ratio or, alongside UV spectrophotometry, to gain an extinction coefficient. This analysis is used throughout all stages of drug discovery, or to demonstrate comparability and consistency between batches for release during the manufacturing phase. Amino acid analysis is a necessary part of protein characterisation and can also be used for estimation of extinction coefficient, total amino acids including cysteine, tryptophan, hydroxyproline & free amino acids, including asparagine & glutamine. By determination of extinction coefficient the method is validated for the product and includes molecular weight measurement by MALDI-TOF MS. The validated method may be used for batch release. Protein concentration may be calculated by relating extinction coefficient to Optical Density measurement, which is performed using a UV spectrometer at 280nm. Amino acid analysis is a key part of the ICH Q6B guidelines for characterisation and confirmation of biopharmaceuticals in support of new marketing applications.