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In order to understand the role of stacked micelles in sample preconcentration, it is necessary to understand the factors that contribute to the micelle stacking phenomenon. Various MEKC background electrolyte (BGE) solutions were prepared in the presence of Sudan III in order to monitor the micelle stacking phenomenon in the anionic sodium dodecyl sulfate and sodium cholate micelle systems. The data show that micelle stacking is a dynamic process that is strongly dependent upon the relative conductivities of the sample matrix and BGE, the relative column length of the sample plug, and the mobilities of the ions involved in the stacking process regardless of electric field conditions (i.e., field-amplified stacking, sweeping, or high-salt stacking). Conditions under which micelle stacking can be expected to occur are presented, and the extent of micelle stacking is quantified. The micelle stacking phenomenon is correlated to the separation performance of a series of neutral alkaloids. It is shown that neutral analytes migrate rapidly through the evolving stacked micelle region in the initial moments of the separation. As a consequence of this transient interaction, analytes with small retention factors spend less time in the stacked micelle region and experience lower stacked micelle concentrations than analytes with large retention factors that spend more time in the growing stacked micelle region. It is also demonstrated that the extent of analyte enrichment generally increases with injection length, by facilitating greater interaction time with stacked micelles; however, enrichment will eventually plateau with increasing injection length as a function of an analyte's affinity for the micelle. Finally, it is shown that, in contrast to conventional wisdom, a range of long injection plugs exist where separation efficiency can be dramatically improved due to analyte interaction with an actively growing stacked micelle region.

In capillary electrokinetic chromatography, neutral analytes can be injected by electroosmotic flow directly from a sample matrix into a separation buffer containing an electrokinetic vector with an opposite mobility. Analytes are injected at the velocity of electroosmotic flow but are retained at the interface of the sample matrix co-ion and separation buffer micelle zones as analyte/micelle complexes. A simple electrokinetic chromatography system containing sodium dodecyl sulfate as the micellar agent with borate as the buffering electrolyte included in the separation buffer and in the sample matrix to provide continuous conductivity was investigated. Concentrations of the micelle, methanol, and borate in the separation buffer were explored to increase maximum injection length of neutral analytes. Reducing the analyte velocity in the separation buffer without substantially decreasing the velocity of the analyte during injection from the sample vial allowed greatly extended sample plug injection lengths. It is presently possible to inject sample solvent volumes equivalent to ~7 effective capillary lengths (180 cm) with a 50-[micro]m-i.d. capillary (24.5 cm effective capillary length), total volume of sample injection ~3.5 [micro] L. Equations describing the injection process and maximum injection lengths for this mode of stacking in electrokinetic capillary chromatography are introduced. The result of this work leads to a postulated generalization of electrokinetic stacking injection maximums for electrophoretic processes, and the concept of orthogonal analyte stacking/ injection systems is discussed.

A large volume sample stacking for negative species is performed using an electro-osmotic flow modifier which avoids polarity switching. This method is based on the condition that the electrophoretic mobility of the ions are opposite the electro-osmotic flow. Sample loading does not alter the electrode configuration. The experiment demonstrates the ability of whole-column stacking to enhance the detection of fast-moving ions by 100%.

A method for the narrow band stacking of very large sample volumes is introduced. This involves the utilization of electroosmotic flow for pumping out the sample buffer during the sample stacking process. Experimental applications utilized tetradecyltrimethylammonium bromide as its organic modifier for applications with positive ions. The utility of the technique for characterizing analytes in minute amounts was demonstrated.

Sample stacking in capillary zone electrophoresis (CZE) may be done by creating a concentration boundary between two different buffer strengths of the same electrolyte in a single capillary column. This field amplification method raises the sensitivity of CZE over a hundredfold, matching the best detection limits attained by high performance liquid chromatography. Several techniques are discussed, each with its own usefulness depending on required resolutions, the method of sample introductionand the type of ionic species to be analyzed.

An on-column mechanism for electrokinetically injecting long sample plugs with simultaneous stacking of neutral analytes in capillary electrokinetic chromatography is presented. On-column stacking methods allow for the direct injection of long sample plugs into the capillary, with narrowing of the analyte peak width to allow for an increase in the detected signal. Low-pressure injections (~50 mbar) are commonly used to introduce sample plugs containing neutral analytes. We demonstrate that injection can be accomplished by applying an electric field from the sample vial directly into the capillary, with neutral analytes injected by electroosmotic flow at up to 1 order of magnitude faster than the corresponding pressure injections. Since stacking occurs simultaneously with electrokinetic injection, stacking is initiated at the capillary inlet, resulting in an increased length of capillary remaining for separation. Reproducibility obtained for peak height and peak area with electroosmotic flow injection is comparable to that obtained with the pressure injection mode, while reproducibility of analysis time is markedly improved. Electrokinetic stacking of neutral analytes utilizing electroosmotic flow is demonstrated with discontinuous (high conductivity, high mobility) as well as continuous (equal conductivity, equal mobility) sample electrolytes. Injecting neutral analytes by electroosmotic flow affords a 10-fold or greater decrease in analysis times when capillaries of 50-[micro]m i.d. or smaller are used. This stacking method should be exportable to dynamic pH junction stacking and electrokinetic chromatography with capillary arrays. Equations describing this electrokinetic injection mode are introduced and stacking of a neutral analyte on a microchip by electrokinetic injection using a simple cross-T channel configuration is demonstrated.