Impact of triethylamine as a mobile phase additive on the resolution of racemic amino acids on an (+)-18-crown-6-tetracarboxylic acid-derived chiral stationary phase

Recently, a new HPLC chiral stationary phase (CSP) prepared by covalently bonding (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid on silica gel was successfully employed in resolving various racemic natural and unnatural amino acids containing a primary amino group. Current work details on-going efforts to improve the effectiveness of this type of material. The analytes used in this study included various substituted phenylalanines, phenylglycine homologues and other primary amino acids. In an attempt to increase enantioselectivity, the effect of methanol and triethylamine modifiers was evaluated in an aqueous mobile phase containing sulfuric acid. In general, retention time increased with increasing methanol and triethylamine concentration. In addition, highest enantioselectivities were obtained with high methanol and high triethylamine; however, these conditions produced excessively long retention. All of the analytes were well resolved on the CSP with a mobile phase of 20% methanol containing 14.3 mM triethylamine and 10.0 mM sulfuric acid.