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

An anion-exchange type chiral stationary phase (CSP) was evaluated regardin g applicability for the separation of enantiomers of chiral amino acids in packed-capillary electrochromatography (CEC). Thus, 5-mu m porous silica pa rticles were modified with a basic tert.-butyl carbamoyl quinine chiral sel ector, and this modified chiral sorbent was packed into fused-silica capill aries of 75 and 100 mu m I.D., respectively, with a packed bed of 25 cm. Wh en an electric field is applied across the capillary the electroosmotic flo w generated by this new packing material may be reversed compared to bare s ilica, 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 t hus the electroosmotic flow is directed towards the anode. Accordingly, ele ctrophoretic migration of the anionic analytes and electroosmotic flow have same directions. This new chiral anion-exchange CEC separation technique w as used to separate the enantiomers of N-derivatized alpha-amino acids, e.g . N-(9-fluorenylmethoxycarbonyl) alpha-amino acids and N-(3,5-dinitrobenzyl oxycarbonyl) alpha-amino acids. Enantioselectivity values were as high as i n HPLC and efficiency was typically by a factor of 2 to 3 higher than in HP LC regarding theoretical plate numbers per meter. The influence of mobile p hase parameters (pH, organic modifier, buffer concentration) on electroosmo tic flow behaviour as well as on effective retention and separation of the analytes has been investigated. (C) 1998 Elsevier Science BN. All rights re served.