Chiral capillary electrophoretic separation of amino acids derivatized with 9-fluorenylmethylchloroformate using mixed chiral selectors of β-cyclodextrin and sodium taurodeoxycholate

Chiral separation of 19 pairs of amino acid (AA) enantiomers derivatized with 9-fluorenylmethylchloroformate (FMOC) was successfully conducted by capillary electrophoresis using the mixture of beta-CD and sodium taurodeoxycholate (STDC) as selectors. Resolution was considerably superior to that obtained by using either beta-CD or STDC alone. After a systematic inspection, a buffer composed of 150 mM borate and 18% v/v isopropanol at pH 8.0, dissolved with 30 mM beta-CD and 30 mM STDC, was adopted and able to generate baseline resolution (> 1.50) for 17 pairs of FMOC-AA enantionners and somewhat lower resolution for arginine (1.36) and alanine (1.18), respectively. Experimental data revealed that the addition of the second selector did not increase the mobility difference between a pair of enantionners (Delta mu = mu(D) - mu(L)) and the number of theoretical plates (N), but decreased the summed apparent mobility of a pair of enantiomers (Sigma mu = mu(D) + mu(L)), which was mainly due to the decrease of the electroosmotic flow. The variation of Sigma mu was thus the major reason responsible for the improvement of chiral resolution in this study. The result demonstrated that not only the intrinsic selectivity of the selectors was the basis of the chiral separation, but also the non-chiral effect of the selectors, the change of the electroosmotic flow, was an important factor in enhancing the enantioseparation resolution. This study could probably help to explain the reasons for resolution improvement in some dual selectors systems, which are not very clear at present.