Comparison of polarimetry and crown ether-based HPLC chiral stationary phase method to determine (l)-amino acid optical purity

Although various pharmacopoeias provide titration methods to assay (l)-amino acid content, none of these methods distinguish between (l)- and (d)-amino acids and do not consider the presence of enantiomeric impurities. Consequently, these methods are limited in scope to describe the relationship between content and specific rotation, [α]. In this study, the US Pharmacopoeia method was compared with the crown ether-based high performance liquid chromatographic (HPLC) chiral stationary phase (CSP) method to determine (l)-amino acid content and specific rotation. The (l)-amino acid content specified by the US Pharmacopoeia method was not consistent with the specific rotation in the presence of enantiomeric impurities, whereas the HPLC-CSP method was very effective for determining the (l)-amino acid content and the optical purity. The other advantage is that the HPLC-CSP method requires amino acid samples of quite low concentration (as low as 1 μg/mL), whereas the pharmacopoeia method requires higher concentrations (20–110 mg/mL).