Pulsed electrochemical detection of nonchromophoric compounds following capillary electrophoresis

Cyclic voltammetric detection in capillary electrophoresis with application to metal ions

Fast cyclic voltammetry (CV) was evaluated over sweep rates of 20-1000 V/s at Au disk electrodes (25 and 10 μm) for end-capillary detection in capillary electrophoresis with metal ions as test analytes; some studies were also done with 25-μm Pt disk electrodes. The waveform applied to the electrode consisted of a preconcentration period (55-330 ms) followed by cyclic voltammetry (2-100 ms). Maximum signal-to-noise was obtained with the integrated CV current as the analytical signal, and this was linearly proportional to sweep rate; maximum response was obtained at sweep rates of >100 V/s for 10-μm electrodes and >200 V/s for 25-μm electrodes; sweep rates of >400 V/s caused peak tailing due to trapping of the analyte at the electrode. With this CV detection approach, comigrating analytes could be identified and determined. Reproducibilities for six analytes over the range 1.0 × 10(-)(7)-1.0 × 10(-)(5) mol/L were 2%-5%, and calibration curves were linear, with response factors in the range of 2%-6%. Detection limits (2 × peak-to-peak baseline noise) were in the range of 5 × 10(-)(9)-4 × 10(-)(8) mol/L, which are 1-2 orders of magnitude better than results obtained previously with square-wave pulsed amperometric detection of metal ions.

Improving MCE with electrochemical detection using a bubble cell and sample stacking techniques

Two efforts to improve the sensitivity and limits of detection for MCE with electrochemical detection are presented here. One is the implementation of a capillary expansion (bubble cell) at the detection zone to increase the exposed working electrode surface area. Bubble cell widths were varied from 1x to 10x the separation channel width (50 mum) to investigate the effects of electrode surface area on detection sensitivity, LOD, and separation efficiency. Improved detection sensitivity and decreased detection limits were obtained with increased bubble cell width, and LODs of dopamine and catechol detected in a 5x bubble cell were 25 and 50 nM, respectively. Meanwhile, fluorescent imaging results demonstrated approximately 8 and approximately 12% loss in separation efficiency in 4x and 5x bubble cell, respectively. Another effort at reducing the LOD involves using field amplified sample injection for gated injection and field amplified sample stacking for hydrodynamic injection. Stacking effects are shown for both methods using amperometric detection and pulsed amperometric detection. The LODs of dopamine in a 4x bubble cell were 8 and 20 nM using field amplified sample injection and field amplified sample stacking, respectively. However, improved LODs were not obtained for anionic analytes using either stacking technique.

Analysis of alkyl polyglucosides in industrial products by capillary electrophoresis with pulsed amperometric detection

Pulsed amperometric detection following micellar electrokinetic chromatography has been applied successfully to the direct detection of alkyl polyglucosides (APGs) in shampoos and other industrial products without prior conversion to highly absorbing or fluorescing derivatives. For electrochemical detection, it is necessary to dissociate the hydroxyl groups of the APGs. Thus, we used 0.1 M NaOH in the outlet vial to dissociate the APGs. The main problems associated with the combination of electrochemical detection and capillary electrophoresis are the need to isolate the detector from the electric field used in the capillary electrophoresis separation and the difficulty of aligning the working electrode with the end of the capillary. To overcome these problems, a simple capillary-electrode holder was constructed. This holder automatically aligns the capillary and the electrode in a wall-jet configuration without the aid of micropositioners and facilitates the replacement of electrodes and capillaries without reconstruction of the entire capillary/electrode setup. Special microcylindrical gold electrodes have been produced by sealing 300-microm-diameter gold wire into borosilicate-glass capillaries.

A pulsed potential waveform displaying enhanced detection capabilities towards sulfur-containing compounds at a gold working electrode

Pulsed electrochemical detection of sulfur-containing compounds was successfully investigated by applying a four-step potential waveform at a gold working electrode. This potential waveform called APAD, which stands for activated pulsed amperometric detection, is composed of an activation potential step added to a conventional three-step potential waveform. A key advantage of the APAD at the Au electrode is the ability to enhance sensitivity through the use of a short potential pulse (E(ACT) = +750 mV versus Ag/AgCl and tACT approximately 90 ms) during which the formation of redox active species (presumably OH*) are able to efficiently oxidize organosulfur compounds. The APAD waveform parameters were optimized to maximize the signal-to-noise ratio (S/N) and successfully applied for the sensitive detection of lipoic acid, biotin, iminobiotin, methionine, cystine, cysteine, homocysteine, homocystine, N-acetylcysteine and glutathione, following their separations by high-performance anion-exchange chromatography (HPAEC) using alkaline mobile phases. The detection limits (S/N = 3, 10 microL injected) ranged from 0.3 for cysteine (400 pg) to 0.02 micromol/L for biotin (50 pg) and methionine (30 pg). The response of sulfur-, amine- and alcohol-based compounds was compared by using four selected pulsed potential waveforms. It was found that the APAD exhibits excellent sensitivity for thiocompounds outperforming all other pulsed potential waveforms. Ratios of the peak areas for APAD and the six-step potential integrated waveform increased from 3.2 +/- 0.4 to 13.5 +/- 0.6 for lipoic acid and biotin, respectively.

Determination of biogenic amines by capillary electrophoresis with pulsed amperometric detection

The biogenic amines, putrescine, cadaverine, spermidine and spermine were separated and quantified by capillary electrophoresis with pulsed amperometric detection. Detection potential of the pulsed amperometric detection was optimized as 0.6 V. Optimal separation of the biogenic amines was achieved using a separation buffer of 30 mM citrate at pH 3.5, while keeping the buffer in the detection cell as 20 mM NaOH. Using these conditions, the four biogenic amines were baseline separated. Extrapolated limits of detection for putrescine, cadaverine, spermidine and spermine were 400, 200, 100 and 400 nM for the standard mixture (polyamines dissolved in running buffer), respectively. These are lower than ultraviolet detection and comparable or even lower than laser-induced fluorescence detection results as reported in the literature. The number of theoretical plates was maintained at the 10(5) level, which is absolutely higher than any reported method. When applying capillary electrophoresis-pulsed amperometric detection to milk analysis, only spermidine was found in amounts varying between 0.1 and 0.5 mg/kg.

Non-aqueous capillary electrophoresis of the positional isomers of a sulfated monosaccharide

A non-aqueous capillary electrophoresis (NACE) method coupled to indirect absorbance detection has been developed for the separation of the three positional isomers of monosulfated fucose. The optimized electrolyte was composed of 12 mM ethanolamine, 2 mM trimesic acid buffer in a methanol-ethanol (1:1, v/v) mixture. As the retained electrolyte entails no separating agent other than the pH buffer, the NACE separation of the positional isomers has been ascribed mainly to selective ion-pairing with the electrolyte counter-ion and the possibility of a selective solvation effect in the alcohol mixture. In the absence of pure isomeric standards, peak identification was completed by MS and NMR spectroscopy and selective enzymatic desulfation. This method should be of interest for the structure elucidation of monosulfated fucose-based polysaccharides and for the screening of sulfoesterase of unknown activity.

Applications of capillary electrophoresis with electrochemical detection in pharmaceutical and biomedical analyses

As a high efficiency separation technique, capillary electrophoresis has been widely used in various fields of analytical science. This review discusses the applications of electrochemical detection systems combined with capillary electrophoresis in pharmaceutical and biomedical analysis. These detection methods mainly involve amperometric detection but also include conductivity detection and potentiometric detection. Its applications in the field are divided into six parts, including catechol compounds, thiols, amino acids and peptides, carbohydrates, general pharmaceuticals, and other related compounds. A relatively detailed discussion is described for each compound under the current studied. On this basis, we have suggested several conceivable directions for capillary electrophoresis with electrochemical detection in the future.

Electrochemical determination of carbohydrates: enzyme electrodes and amperometric detection in liquid chromatography and capillary electrophoresis

In recent years, electrochemical detection (EC) methods have become increasingly important for the determination of carbohydrate compounds in a variety of biological and pharmaceutical samples. In this work, recent advances in the design and application of EC approaches are reviewed, with the goal of providing the non-electrochemist with a basic understanding of the most important EC approaches to carbohydrate detection and an overview of their current applications. Two specific EC detection strategies are considered in detail: enzyme electrodes and electrodes used for HPLC or capillary electrophoresis detection.

A tabulated review of capillary electrophoresis of carbohydrates

This review summarizes publications on capillary electrophoresis (CE) of carbohydrates, covering almost all hitherto published papers on this topic. It is designed to be a convenient tool for the literature search by providing a comprehensive table. Since CE analysis of carbohydrates is generally complicated due to the structural diversity of carbohydrate species, an attempt is made in this table to supply detailed information on the analyzed form (underivatized or derivatized, type of derivative) and analytical conditions (capillary size, state of the inner wall, composition of the electrophoretic solution, applied voltage, detection method, etc.), for each combination of carbohydrate species to be analyzed. In addition, a brief overview is presented to help in the literature search.

Advances in separation science relating to clinical nutrition

Advances in separation science are leading to more sensitive and more specific analysis of the complex mixtures of metabolites, vitamins and nutrients found in biofluids. Recent developments include capillary electrokinetic separations such as capillary electrophoresis, and the linking of separation methods to new forms of mass spectrometry.

Recent developments in capillary electrophoresis of carbohydrate species

Carbohydrates are ubiquitous species involved in many life processes. Because of the multilateral roles of carbohydrates, their analysis has come to have increasing importance. As shown in this review, capillary electrophoresis in its various modes of operation has proved very useful in the analysis of carbohydrate species including mono- and oligosaccharides, glycoproteins, glycopeptides and glycosaminoglycans. Advances in separation approaches and applications as well as advances in detection including sensitive and selective pre-column derivatization are described. In summary, this comprehensive review is a supplement to previous reviews and covers the published work in 1996 and the first half of 1997.

Developments in amino acid analysis using capillary electrophoresis

The current status in the analysis of amino acids using capillary electrophoresis is addressed. This area of biological analysis has received increased attention with more than 200 articles being published in the last five years. This review discusses pre-, post-, and on-column derivatization techniques used to tag amino acids providing a detectable moiety. Several separation methodologies which provided resolution for large sets of amino acids are presented. An overview of advances in the enantiomeric resolution methodologies for amino acids is given. Both direct and indirect enantiomeric separation schemes are summarized. Recent advances in detection strategies for both derivatized and underivatized amino acids are presented. Applications utilizing amino acid analysis by capillary electrophoresis are described. This review covers articles published between 1991 and 1996.

Electrochemical detection in capillary electrophoresis

Recent advances in the design and application of electrochemical detection (EC) systems in capillary electrophoresis (CE) are reviewed, with the objective of providing the non-electrochemist with a state-of-the-art picture of CEEC instrumentation and an overview of the primary analytes and samples for which the technique is best suited. In particular, instrument innovations designed to aid in decoupling the CE and EC systems electrically and in aligning them physically are described in detail. In addition, CEEC applications are summarized for four specific analyte groups: catecholamines, thiols and disulfides, amino acids, and carbohydrates. On this basis, it is clear that EC techniques have reached a stage where they are already having a significant impact on CE usage in selected areas of analysis. Continued developments with respect to new electrode materials and electrode configurations promise to broaden this impact further.

Pulsed electrochemical detection of thiols and disulfides following capillary electrophoresis

Pulsed electrochemical detection (PED) following capillary electrophoresis (CE) has been applied to the direct detection of thiocompounds. Both reduced and oxidized thiol moieties are detected without the need of derivatization. Thiocompounds can be detected over a wide range of pH conditions (i.e., pH 0-14), and except for pH, their response is relatively unperturbed by buffer composition. Integrated pulsed amperometric detection (IPAD) results in more stable baselines, eliminates oxide-induced artifacts, and yields lower limits of detection than other PED waveforms. Mass detection limits using optimized IPAD waveforms are typically 2 pg (5 fmol) or less. The high selectivity of PED for thiocompounds reduces sample preparation and produces simpler electropherograms of complex samples containing these biologically significant compounds.

Capillary electrophoresis with pulsed amperometric detection of carbohydrates and glycopeptides

The utility of capillary electrophoresis with pulsed amperometric detection (CE-PAD) for the analysis of carbohydrate-containing samples from a variety of biological sources is described. CE-PAD was used to separate a mixture of oligosaccharides obtained from bovine fetuin and to monitor the desialylation process used in the characterization of the oligosaccharides. Additionally, the high resolving power of the system was demonstrated using a series of glycopeptides obtained from recombinant coagulation factor VIIa, which possess the same decapeptide core but differ in the extent of sialylation. Deglycosylation of these glycopeptides for characterization purposes resulted in a mixture of carbohydrates and peptides. Unlike CE with UV detection, this system gave good responses for all analytes, demonstrating the unique ability of PAD to respond to the electrochemical features of diverse classes of biomolecules such as carbohydrates and peptides. Finally, CE-PAD was applied to the analysis of a tryptic digest of recombinant tissue plasminogen activator. The use of different detection potentials in sequential runs on a sample gave structural information about the peptides, such as glycosylation. A brief review of prior applications of CE-PAD to the analysis of standard mixtures of simple saccharides is also presented.