Electrokinetic separations in fused silica capillaries

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Virginia Tech


Methods of co-optimizing resolution and detection in Capillary Zone Electrophoresis (CZE) and Micellar Electrokinetic Chromatography (MEKC) are examined by deriving mathematical expressions which illustrate the relative importance of various experimental parameters.

For CZE, expressions are derived to show the interrelationship between efficiency, capillary dimensions and sample size. The interrelationship shows that resolution and detectability cannot be optimized simultaneously. Efficiency and, therefore, resolution are maximized when small sample sizes and capillaries with small internal diameters are employed. Detection is more favorable when large sample sizes and capillaries with large internal diameters are used.

To achieve a favorable compromise between resolution and detection, the Influence of pH, electrolyte concentration and forced air convection are examined. A decrease in pH or an increase in electrolyte concentration reduces electroosmotic flow. This increases the relative velocity difference between two zones and, thereby, minimizes the efficiency required for unit resolution. Forced air convection minimizes the loss in efficiency observed as capillaries with larger internal diameters are employed.

In MEKC, the importance of efficiency is minimized by employing a micellar phase which provides adequate selectivity for the separation. The separation of ASTM test mix LC-79-2 obtained in sodium dodecyl sulfate, sodium decyl sulfate, and sodium dodecyl sulfate modified with Brij 35 indicates that selectivity is governed by the nature of the surfactant's polar head group. Beyond selectivity optimization, resolution may be improved by increasing efficiency or decreasing electroosmotic flow. Of these approaches, increasing capillary length, to improve efficiency, is more time effective.

Using the guidelines described herein, several practical applications were developed. The methods are examined with respect to migration time and quantitative reproducibility.