Head-column field-amplified sample stacking in binary system capillary electrophoresis. 2. Optimization with a preinjection plug and application to micellar electrokinetic chromatography

In capillary electrophoresis, head-column field-amplified sample stacking (FASS) provides the largest sensitivity enhancement of all electrokinetic concentration techniques (Zhang, C.-X.; Thormann, W. Anal. Chem. 1996, 68, 2523). Application of head-column FASS to the analysis of closely related opioids by capillary zone electrophoresis in binary systems with ethylene glycol is described. It is shown that sample condensation is further increased about 2-fold by introduction of a preinjection plug, i.e., introduction of a short solution plug of high conductivity, high pH, and high viscosity at the capillary tip prior to injection. The preinjection plug acts as a temporary trap for solutes. Its effective length is shown to be limited to a few millimeters. The highest sample stacking efficiency in head-column FASS is obtained via optimization of the electric field strength and the effective electrophoretic mobility of the solutes within the sample and the adjacent zones and is thus strongly dependent on the compositions of both the sample matrix and the preinjection plug. Binary sample solutions of low conductivity and low viscosity containing small amounts of a weak acid are demonstrated to be most effective for the stacking of positively charged opioids. The procedure developed for capillary zone electrophoresis of opioids in binary systems with ethylene glycol and UV absorbance detection is documented to provide a 3 orders of magnitude sensitivity enhancement, exhaustive sample injection from an external reservoir of up to 20 mu L (i.e., from a volume that is more than 20-fold the volume of the capillary employed), and a lowest detectable concentration of 0.1 ng/mL (S/N = 3). Using internal calibration, typical intraday and interday imprecisions of solute concentrations between 3 and 10 ng/mL and between 0.5 and 1.5 ng/mL are less than or equal to 5% and less than or equal to 15, respectively. The stacking approach has been successfully applied to the analysis of dihydrocodeine in extracts of 20 mu L of human plasma and is shown to permit the precise (imprecision less than or equal to 10%) determination of dihydrocodeine plasma levels of pharmacological interest (3-300 ng/mL or 10-1000 nM). Furthermore, using a modified protocol for micellar electrokinetic chromatography, head-column FASS is shown to provide a 400-fold sensitivity enhancement for a number of opioids. The stacking procedure is based on insertion of a surfactant-free preinjection plug and temporary application of reversed voltage after electroinjection.