Effect of analyte lipophilicity on the resolution of α- and β-amino acids on liquid chromatographic ligand exchange chiral stationary phases

Separation of the two enantiomers of racemic alpha- and beta-amino acids on two ligand exchange chiral stationary phases (CSPs) prepared previously by covalently bonding sodium N-((S)-1-hydroxymethy-3-methylbutyl)-N-undecylaminoacetate or sodium N-((R)-2-hydroxy-1-phenylethyl)-N-undecylaminoacetate on silica gel was studied with variation of the organic modifier (methanol) concentration in the aqueous mobile phase. In particular, the variation of retention factors with changing organic modifier concentration in the aqueous mobile phase was found to be strongly dependent on both the analyte lipophilicity and the stationary phase lipophilicity. In general, the retention factors of relatively lipophilic analytes on relatively lipophilic CSPs tend to increase with increasing organic modifier concentration in the aqueous mobile phases while those of less lipophilic or hydrophilic analytes tend to increase. However, only highly lipophilic analytes show decreasing retention factors with increasing organic modifier concentration in the aqueous mobile phase on less lipophilic CSPs. The contrasting retention behaviors on the two CSPs were rationalized by the balance of the two competing interactions, viz. hydrophilic interaction of analytes with polar aqueous mobile phase and the lipophilic interaction of analytes with the stationary phase.