Susan M. Lunte

Microchip capillary electrophoresis/electrochemistry.

Microfabricated fluidic devices have generated considerable interest over the past ten years due to the fact that sample preparation, injection, separation, derivatization, and detection can be integrated into one miniaturized device. This review reports progress in the development of microfabricated analytical systems based on microchip capillary electrophoresis (CE) with electrochemical (EC) detection. Electrochemical detection has several advantages for use with microchip electrophoresis systems, for example, ease of miniaturization, sensitivity, and selectivity. In this review, the basic components necessary for microchip CEEC are described, including several examples of different detector configurations. Lastly, details of the application of this technique to the determination of catechols and phenols, amino acids, peptides, carbohydrates, nitroaromatics, polymerase chain reaction (PCR) products, organophosphates, and hydrazines are described.

Recent trends in microdialysis sampling integrated with conventional and microanalytical systems for monitoring biological events: A review

Microdialysis (MD) is a sampling technique that can be employed to monitor biological events both in vivo and in vitro. When it is coupled to an analytical system, microdialysis can provide near real-time information on the time-dependent concentration changes of analytes in the extracellular space or other aqueous environments. Online systems for the analysis of microdialysis samples enable fast, selective and sensitive analysis while preserving the temporal information. Analytical methods employed for online analysis include liquid chromatography (LC), capillary (CE) and microchip electrophoresis and flow-through biosensor devices. This review article provides an overview of microdialysis sampling and online analysis systems with emphasis on in vivo analysis. Factors that affect the frequency of analysis and, hence, the temporal resolution of these systems are also discussed.

Capillary electrophoresis with electrochemical detection employing an on-column Nafion joint

Abstract The construction and evaluation of an on-column joint utilizing Nafion tubing for the isolation of the electrical circuit from the detection end of a capillary zone electrophoresis system is described. The joint enables electrochemical detection to be performed without adverse effects from the applied high voltage. The joint is both simple to construct and durable. The electrochemical detector employing a carbon fiber working electrode exhibited high coulometric efficiencies and a detection limit of 6 · 10−9M or 34.8 amol for hydroquinone. A high efficiency, of the order of 185 000 theoretical plates, was achieved for this compound. This system was evaluated for the detection of phenolic acids in apple juice and for the determination of naphthalene-2,3-dicarboxaldehyde derivatized amino acids in a brain homogenate. The use of voltammetry as a method of compound verification was also demonstrated.

Recent developments in amperometric detection for microchip capillary electrophoresis.

The interest in microfluidic devices has increased considerably over the past decade due to the numerous advantages of working within a miniature, microfabricated format. This review focuses on recent advances in coupling amperometric detection with microchip capillary electrophoresis (CE). Advances in electrochemical cell design, isolation of the detector from the separation field, and integration of both pre‐ and postseparation reaction chambers are discussed. The use of microchip CE with amperometric detection for enzyme/immunoassays, clinical and environmental assays, and the determination of neurotransmitters is described.

Analytical considerations for microdialysis sampling

Adaptations in microdialysis probe designs have made it possible to obtain samples from the extracellular fluid of a variety of tissues with high temporal resolution. The resulting small volume samples, often with low concentration of the analyte(s) of interest, present a particular challenge to the analytical system. Rapid separations can be coupled on-line with microdialysis to provide near real-time data. By combining microdialysis sampling with a liquid chromatographic or capillary electrophoretic separation and a highly sensitive detection method, a separation-based sensor can be developed. Such sensors have been applied to the investigation of drug entities as well as to study endogenous analytes.

Fabrication and evaluation of a carbon‐based dual‐electrode detector for poly(dimethylsiloxane) electrophoresis chips

The first carbon‐based dual‐electrode detector for microchip capillary electrophoresis (CE) is described. The poly(dimethylsiloxane) (PDMS)‐based microchip CE devices were constructed by reversibly sealing a PDMS layer containing separation and injection channels to another PDMS layer containing carbon fiber working electrodes. End‐channel amperometric detection was employed and the performance of the chip was evaluated using catechol. The response was found to be linear between 1 and 600 μM with an experimentally determined limit of detection (LOD) of 500 nM and a sensitivity of 30 pA/μM. Collection efficiencies for catechol ranged from 36.0 to 43.7% at field strengths of 260—615 V/cm. The selectivity that can be gained with these devices is demonstrated by the first CE‐based dual‐electrode detection of a Cu(II) peptide complex. These devices illustrate the potential for a rugged and easily constructed microchip CE system with an integrated carbon‐based detector of similar scale.

Monitoring excitatory amino acid release in vivo by microdialysis with capillary electrophoresis- electrochemistry

Capillary electrophoresis (CE) with electrochemical detection (ED) was used to determine extracellular levels of aspartate, glutamate and alanine in samples from the frontoparietal cortex of the rat which were obtained by microdialysis. The method was used to monitor the effect on the overflow of the excitatory amino acids aspartate and glutamate of an influx of high concentrations of potassium ion. Samples were derivatized with naphthalenedialdehyde-cyanide prior to analysis. Detection limits for aspartate and glutamate were 80 and 100 nM, respectively. CE-ED is extremely useful for the analysis of microdialysis samples because of the very small sample volumes required by this analytical technique. The use of ED provides the requisite sensitivity and allows verification of peak purity by voltammetry.

Carbon paste-based electrochemical detectors for microchip capillary electrophoresis/electrochemistry

The first reported use of a carbon paste electrochemical detector for microchip capillary electrophoresis (CE) is described. Poly(dimethylsiloxane) (PDMS)-based microchip CE devices were constructed by reversibly sealing a PDMS layer containing separation and injection channels to a separate PDMS layer that contained carbon paste working electrodes. End-channel amperometric detection with a single electrode was used to detect amino acids derivatized with naphthalene dicarboxaldehyde. Two electrodes were placed in series for dual electrode detection. This approach was demonstrated for the detection of copper(II) peptide complexes. A major advantage of carbon paste is that catalysts can be easily incorporated into the electrode. Carbon paste that was chemically modified with cobalt phthalocyanine was used for the detection of thiols following a CE separation. These devices illustrate the potential for an easily constructed microchip CE system with a carbon-based detector that exhibits adjustable selectivity.

Comparison of the performance characteristics of poly(dimethylsiloxane) and Pyrex microchip electrophoresis devices for peptide separations

A comparative study of electrophoretic separations of fluorescently labeled peptides and amino acids on poly(dimethylsiloxane) (PDMS) and Pyrex microchips is presented. The separation parameters for each microchip substrate were compared, including electroosmotic flow, plate numbers, resolution, and limits of detection. The effect of buffer composition on the separation was also investigated. Acceptable separations were obtained for most peptides with both substrates; however, PDMS chips exhibited much lower separation efficiencies and longer analysis times.

Detection of homocysteine by conventional and microchip capillary electrophoresis/electrochemistry

A method based on capillary electrophoresis (CE) with electrochemical (EC) detection for the determination of both total homocysteine (tHcy) and protein‐bound homocysteine (pbHcy) in plasma is described. Both end‐column and off‐column amperometric detection were investigated. Off‐column detection resulted in a more sensitive assay for the determination of homocysteine (Hcy). The detection limit for homocysteine was 500 nM using off‐column EC detection and the response was linear over the range 1–100 νM. Therefore, this assay is appropriate for the quantification of Hcy over the physiological concentration ranges found in all disease states. Methodologies for the determination of tHcy and pbHcy in human plasma were investigated and optimized and the concentrations of both pbHcy and tHcy in plasma obtained from a healthy individual were determined to be 2.79 ± 0.31 νM (n = 4) and 3.37 ± 0.15 νM (n = 3), respectively. The methodology was then transferred to a microchip CE‐EC format and Hcy and reduced glutathione (GSH) were detected. Future work will focus on the development of ancillary methodologies to identify the other forms of Hcy in vivo.