Electrophoresis zyxwvutsrqponmlkji 1996, 17, 1931-1937 Optimized MEKC zyxwvu of corticosteroids with mixed SDS and SC 1931 Susanne zyxwvutsrq K. Wiedmer' Juho H. Jumppanen' Heikki Haario3 Marja-Liisa Riekkola' 'Laboratory of Analytical Chemistry, Department of Chemistry, University of Helsinki, Finland 'Finnsugar Ltd., Finnsugar Development, Kantvik, Finland 'University of Helsinki, Department of Mathematics, University of Helsinki, Finland Optimization of selectivity and resolution in micellar electrokinetic capillary chromatography with a mixed micellar system of sodium dodecyl sulfate and sodium cholate Selectivity and resolution were studied for the separation of seven corticoste- roids by micellar electrokinetic capillary chromatography (MEKC) using a mixed micellar solution of sodium dodecyl sulfate (SDS) and sodium cholate (SC), buffered with 3-(N-morpholino)propanesulfonic acid (MOPS) or 3-[(l,l- dimethyl-2-hydroxyethyl)amino]-2-hydroxypropane sulfonic acid (AMPSO). The changes in selectivity were compared for the AMPSO-SDS-SC system by varying the pH and the concentrations of AMPSO, SDS and SC. The experi- mental design started with the central composite design and continued in a sequential manner. The optimum selectivity for the separation of the cortico- steroids was calculated from the analyte migration times and the analyte velo- cities, by using empirical quadratic regression models. Satisfactory regression fits and coefficients of determination for prediction were obtained with cross- validated models. To optimize the resolution, the physical parameters of capil- lary length and analysis time were varied under the conditions optimal for the selectivity. In both the selectivity and the resolution, optimization the overall optimum was determined by using the desirability function technique. Ana- lysis times were controlled by using 1,3-diaminopropane to influence the elec- troosmotic flow velocity zyxwv (v,,). The voltage was kept constant, which resulted in higher electric field strength in shorter capillaries. No changes in the selectivity were observed when 1,3-diaminopropane was used to control the electroos- motic flow velocity. Such an optimization technique, where the chemical and physical factors affecting the separation are treated independently, seemed to be effective for finding the best possible resolution for the corticosteroids. 1 Introduction In micellar electrokinetic capillary chromatography (MEKC) [l, 21, the separation of analytes is based on interactions between the analytes and the pseudosta- tionary phase formed by the micelles. The mechanism of the interaction is a combination of hydrophobic-hydro- philic and dipolar interactions [3, 41. With proper choice of the micellar solution, compounds of different polari- ties can easily be separated. Although several different surfactants are now available, the search for the best or optimum micellar solution for the separation of analytes is not easy, due to the difficulty of predicting the solute- micellar interactions. The situation becomes even more complex when two or more micelles are mixed. In these cases, not only solute-micelle interactions occur, but also strong micelle-micelle interactions are possible. In an earlier study we have shown that the optimal con- ditions for the selectivity and the resolution of a system Correspondence: Professor Marja-Liisa Riekkola, Laboratory of Analyt- ical Chemistry, Department of Chemistry, POB 55, FIN-00014 Univer- sity of Helsinki, Finland (Tel: +358-0-19140268; Fax: +350-0-19140253; E-mail: Riekkola@kumpu.helsinki.fi) Nonstandard abbreviations: AMPSO, 3-[(l,l-dimethyl-2-hydroxyethyl) amino]-2-hydroxypropane sulfonic acid; SC, sodium cholate Keywords: Micellar electrokinetic capillary chromatography zyxwvutsr / Mixed micellar system / Corticosteroids / Optimization / 1,3-Diaminopro- pane zyxwvutsrqp 0 VCH Verlagsgesellschaft mbH, 69451 Weinheim, 1996 are not necessarily the same zyx [5]. Due to the lack of knowledge about the mechanism of solute-micelle inter- actions, the optimization of selectivity and resolution can be troublesome without the use of mathematical optimization schemes. The analyst can, of course, per- form systematic, time-consuming experiments by varying, one at a time, the pH, buffer concentration, micelle concentration(s), the physical parameters of capillary length, temperature, voltage and injection pres- sure, and so on. But the question still remains whether the global optimum point for the selectivity or the reso- lution of a system can be found in this way. Often, the system will display drastic changes or even discontinui- ties within the experimental area. Such situations are particularly common when micelles are introduced. In such cases it might be difficult to find reliable statistical models that describe the separation in terms of the experimental variables. The optimum selectivity and resolution for the separa- tion of seven corticosteroids were determined mathema- tically in this study. The biological buffers 3-(N-morpho- 1ino)propanesulfonic acid (MOPS) and 3-[(l,l-dimethyl- 2-hydroxyethyl)amino]-2-hydroxypropane sulfonic acid (AMPSO), each having excellent buffer capacity in the pH range studied, were used together with a mixed micellar system of sodium dodecyl sulfate (SDS) and sodium cholate (SC). The selectivity of the system was studied by monitoring the normalized velocity ratios of analyte pairs. The optimum was calculated by using desi- rability functions zyxw [6]. Recently we showed that the reso- 0173-0835/96/1212-1931 $10.00+.25/0