The limits of detection (LOD) for capillary electrophoresis (CE) are constrained by the dimensions of the capillary. For example, the small volume of the capillary limits the total volume of sample that can be injected into the capillary. In addition, the reduced pathlength hinders common optical detection methods such as UV detection. Many different techniques have been developed to improve the LOD for CE. In general these techniques are designed to compress analyte bands within the capillary, thereby increasing the volume of sample that can be injected without loss of CE efficiency. This on-line sample preconcentration, generally referred to as stacking, is based on either the manipulation of differences in the electrophoretic mobility of analytes at the boundary of two buffers with differing resistivities or the partitioning of analytes into a stationary or pseudostationary phase. This article will discuss a number of different techniques, including field-amplified sample stacking, large-volume sample stacking, pH-mediated sample stacking, on-column isotachophoresis, chromatographic preconcentration, sample stacking for micellar electrokinetic chromatography, and sweeping.

Many decisions in drug development and medical practice are based on measuring blood concentrations of endogenous and exogenous molecules. Yet most biochemical and pharmacological events take place in the tissues. Also, most drugs with few notable exceptions exert their effects not within the bloodstream, but in defined target tissues into which drugs have to distribute from the central compartment. Assessing tissue drug chemistry has, thus, for long been viewed as a more rational way to provide clinically meaningful data rather than gaining information from blood samples. More specifically, it is often the extracellular (interstitial) tissue space that is most closely related to the site of action (biophase) of the drug. Currently microdialysis (microD) is the only tool available that explicitly provides data on the extracellular space. Although microD as a preclinical and clinical tool has been available for two decades, there is still uncertainty about the use of microD in drug research and development, both from a methodological and a regulatory point of view. In an attempt to reduce this uncertainty and to provide an overview of the principles and applications of microD in preclinical and clinical settings, an AAPS-FDA workshop took place in November 2005 in Nashville, TN, USA. Stakeholders from academia, industry and regulatory agencies presented their views on microD as a tool in drug research and development.

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A separation-based biosensor has been developed that is capable of near-real-time analysis of aspartate and glutamate with a temporal resolution of less than 2 min in anesthetized or awake, freely moving animals. The instrument consists of a microdialysis sampling system, an on-line reactor, an injection interface, and a CE-LIF system. Primary amine analytes are derivatized with NDA/CN following microdialysis sampling using an on-line reactor to produce fluorescent CBI derivatives. The reaction takes approximately 1 min. The derivatized sample then travels to a microinjection valve which alternately sends CE running buffer and reacted microdialysis sample to the CE column via an injection interface. The interface allows a controllable volume of 10-20 nL to be injected onto the CE separation capillary. Separation of aspartate and glutamate from the other amino acids present in the microdialysis sample was achieved within 70 s. Detection limits for glutamate and aspartate using laser-induced fluorescence detection were 0.1 microM. The linear dynamic range was acceptable for the determination of aspartate and glutamate in dialysate samples where the levels are between 1 and 10 microM. Full automation of the system was achieved by computer control of the valve, the interface, and the data collection system. The performance of this system was demonstrated in an anesthetized rat by monitoring ECF levels of aspartate and glutamate released in brain after stimulation with high concentrations of K+.