Application of achiral-chiral two-dimensional HPLC for separation of phenylalanine and tryptophan enantiomers in dietary supplement.

This study developed a two-dimensional heart-cutting LC method for the separation of amino acid enantiomers. Two approaches for achiral separation of amino acids, phenylalanine and tryptophan, were selected. Amino acids were separated on C18 or hydrophilic interaction liquid chromatography (HILIC) columns in first dimension after their enantiomer separation on a teicoplanin chiral column in second dimension. Mobile phases for both separation systems were optimized by testing different types of organic modifiers, concentrations of ion-pair agent (sodium 1-octanesulfonate), and ionic modifier (ammonium acetate). The resolution of enantiomers higher than 1.5 for both amino acids was achieved using a C18-teicoplanin coupled column separation system with mobile phases methanol/2 mM sodium 1-octanesulfonate (10:90 and 75:25, step gradient between achiral and chiral columns, respectively). The lower resolution of amino acid enantiomers (R ˃ 0.9), but higher column efficiency, was achieved on a HILIC-teicoplanin separation system with mobile phases acetonitrile/50 mM ammonium acetate (90:10 and 80:20, step gradient between achiral and chiral columns, respectively). The developed heart-cutting 2D-LC methods were validated in terms of linearity, limit of detection, limit of quantification, precision, and accuracy. The results suggested that the developed methods were applicable for the simultaneous determination of amino acid enantiomers in the dietary supplement. The sample contained l-phenylalanine and l-tryptophan.