High-efficiency chiral separations of N-derivatized amino acids by packed-capillary electrochromatography with a quinine-based chiral anion-exchange type stationary phase

Abstract

An anion-exchange type chiral stationary phase (CSP) was evaluated regarding applicability for the separation of enantiomers of chiral amino acids in packed-capillary electrochromatography (CEC). Thus, 5-μm porous silica particles were modified with a basic tert.-butyl carbamoyl quinine chiral selector, and this modified chiral sorbent was packed into fused-silica capillaries of 75 and 100 μm I.D., respectively, with a packed bed of 25 cm. When an electric field is applied across the capillary the electroosmotic flow generated by this new packing material may be reversed compared to bare silica, depending on the buffer or mobile phase pH. At pH values below ca. 6.3 the net charge of the chirally modified silica surface is positive and thus the electroosmotic flow is directed towards the anode. Accordingly, electrophoretic migration of the anionic analytes and electroosmotic flow have same directions. This new chiral anion-exchange CEC separation technique was used to separate the enantiomers of N-derivatized α-amino acids, e.g. N-(9-fluorenylmethoxycarbonyl) α-amino acids and N-(3,5-dinitrobenzyloxycarbonyl) α-amino acids. Enantioselectivity values were as high as in HPLC and efficiency was typically by a factor of 2 to 3 higher than in HPLC regarding theoretical plate numbers per meter. The influence of mobile phase parameters (pH, organic modifier, buffer concentration) on electroosmotic flow behaviour as well as on effective retention and separation of the analytes has been investigated.