Chiral Separations by Capillary Electrochromatography on Multiple-Interaction Based Stationary Phases

Summary

The direct resolution of neutral amino acid and amino alcohol derivatives by pressurized capillary electrochromatography (p-CEC) is reported. Separations were performed on columns packed with multiple-interaction based chiral stationary phases (CSPs) in reversed-phase mode, using borate buffer/acetonitrile mixtures as the eluent. Two CSPs were used: 3,5-dinitrobenzoyl-(R)-phenylglycine (DNB-PGLY)- and 3,5-dinitrobenzoyl-(R)-naphthylglycine (DNB-NGLY)-silica bonded, respectively. Baseline separations of N-benzoyl-β-naphthylamide derivatives of phenylalanine and leucine were accomplished on DNB-PGLY-CSP in less than three minutes, and on DNB-NGLY-CSP within about 3.5 minutes, with a comparable efficiency of 160.000 plates per meter. As expected, DNB-NGLY-CSP showed higher selectivity for the compounds under study. The R-isomer eluted before the S-isomer on both CSPs. The effect of the buffer pH on the efficiency of the columns was also studied. With buffer pH over the values of 8–8.5, free residual γ-aminopropyl groups on the silica particles seemed do affect neither selectivity nor resolution. At these mobile phase conditions calculated efficiency against electroosmotic flow plot is consistent with that generally obtained in CEC, providing a reduced plate height of about 1.8 at a linear velocity of 0.5 mm s⁻¹. With buffer pH values under 7.0, protonation of the free aminopropyl groups strongly affected the resolution with the result of higher retention and lower solute mass transfer.