Charged cyclodextrin-mediated sample stacking in micellar capillary electrophoresis - A simple method for enhancing the detection sensitivity of hydrophobic compounds

The development of on-line sample stacking techniques for enhancing limits of detection of neutral analytes in micellar capillary electrophoresis (MCE) has recently gained much attention. Utilizing high-conductivity sample matrices to invoke sample stacking is promising, but requires the limited use of sample solubilizing agents such as alcohols in the sample matrix. In this study, we show how simple replacement of the sample solvent (methanol) with a solution of sulfated beta-cyclodextrin (s beta-CD) allows a significant increase in the sensitivity of detection of model hydrophobic analytes. This increase in sensitivity is accompanied by significant peak sharpening. Sulfated CDs in the sample matrix allow for effective solubilization of hydrophobic analytes without the use of organic solvents such as methanol. The testing of various sample matrix s beta-CD concentrations for their effect on peak sharpening identified 3 to 5% as optimal for the estrogen standards. The use of a s beta-CD sample matrix allowed for hydrostatic injections (3.5 kPa) of 297 s, compared with 4 s when the analytes were dissolved in methanol. A mechanism explaining the sp-Cn-induced effect involves an analyte transfer mechanism where the s beta-CDs, despite providing anodic mobility to analytes in the sample zone, are able to transfer analytes to trailing separation buffer micelles for "recycling" back into the sample zone without compromising the stacking process. The overall improvement in sensitivity allows detection of estrogens in the parts-per-billion range and stands to improve the utility of MCE as a bioanalytical technique. (C) 1999 Elsevier Science B.V. All rights reserved.