Amperometric Detection In Capillary Electrophoresis Research Articles

Study on production of free hydroxyl radical and its reaction with salicylic acid at lead dioxide electrode

The formation of free hydroxyl radical ( OH) in oxygen transfer reactions at PbO 2 electrodes and the reaction between the electrochemically produced OH and salicylic acid (SA) were studied by cyclic voltammetry and capillary electrophoresis-amperometric detection (CZE-AD) in this paper. Experiments showed that OH was formed in the potential range of 1.0–1.4 V corresponding to the anodic discharge of H 2O at the PbO 2 electrode, while this phenomenon was not obvious at the Pt electrode. Optimum CZE-AD conditions for indirect determination of OH were explored. It was also found that when salicylic acid was used to trap OH produced on the PbO 2 electrode surface, the amount of 2,5-dihydroxybenzoic acid produced is much more than that of 2,3-dihydroxybenzoic acid. This phenomenon is explained by the proposal that SA first forms an active aromatic radical on the PbO 2 electrode surface before trapping OH, where OH performs a stronger preference in attacking at the 5-position than for attacking at the 3-position of the aromatic radical.

Disposable twin gold electrodes for amperometric detection in capillary electrophoresis.

We describe a simple and easy way to construct gold microelectrodes for amperometric detection in capillary electrophoresis (CE). The gold microelectrodes, in single or twin sets, were obtained from recordable compact discs (gold-sputtered type), which present highly reproducible surface characteristics. The performance of these electrodes was evaluated by using a home-made CE equipment. The basic steps for the electrode construction are: drawing on a microcomputer; laser printing of the design on wax paper; heat-transfer of the toner onto the gold surface of a peeled recordable compact disc (CD-R); etching of the gold layer from unprinted regions; removal of the toner with a solvent; sealing of unused electrode areas with varnish. One electrode at a time was connected to a potentiostat (or two, to a bipotentiostat) and operated in a wall jet configuration relative to the CE capillary outlet. The amperometric signals were integrated for quantification purposes. Repetitive injections (n = 10) of a mixture containing iodide, ascorbic acid, dipyrone, and acetaminophen (20, 200, 500, and 100 microM), presented relative standard deviations of 2.9, 4.5, 6.1, and 4.0%, respectively. For these analytes, the detection limits (S/N = 3, 30 s of 100 mm hydrodynamic injection) were 0.1, 0.5, 3.1, and 1.1 microM, respectively.

Capillary electrophoresis microchip with a carbon nanotube-modified electrochemical detector.

Significant improvements in the performance of a capillary electrophoresis (CE) microchip with an electrochemical detector are observed using a carbon nanotube (CNT)-modified working electrode. The CNT-modified electrode allows CE amperometric detection at significantly lower operating potentials and yields substantially enhanced signal-to-noise characteristics. The electrocatalytic detection is coupled to resistance to surface fouling and hence enhanced stability. Such advantages are illustrated in connection with several classes of hydrazine, phenol, purine, and amino acid compounds. Substantial minimization of surface fouling effects has been demonstrated in connection with the monitoring of phenol and tyrosine. Factors affecting the performance of the new CNT detector were assessed and optimized. CNTs from different sources are evaluated, and the effect of an anodic pretreatment is explored. The broad and significant catalytic activity exhibited by CNT-based CE detectors indicates great promise for a wide range of bioanalytical and environmental applications.

Separation high voltage field driven on-chip amperometric detection in capillary electrophoresis.

A new potentiostatless detection scheme for amperometric detection in capillary electrophoresis is presented based on the use of microband array electrodes positioned in the capillary electrophoresis electric field. In the present study, the spatial potential difference in the CE separation high-voltage field was measured using two gold microband electrodes positioned in the proximity of the capillary outlet. The induced potential difference between the two electrodes was recorded as a function of the applied separation high voltage and the dependence of the electrochemically generated current on the high-voltage field, and the concentration of a redox couple (Fe(CN)6(4-)/Fe(CN)6(3-)) was investigated. The results show that plots of the generated current versus the CE separation voltage have the same shape as cyclic voltammograms obtained with the same electrodes in a traditional potentiostatic setup and that the current is proportional to the concentration of the redox couple. As a decoupling device is not needed, the described potentiostatless approach significantly simplifies the instrumental setup for amperometric detection. This approach consequently holds great promise for application in inexpensive portable chip-based CE devices.

Etched electrochemical detection for electrophoresis in nanometer inner diameter capillaries.

Capillary electrophoresis in nanometer inner diameter capillaries allows for the analysis of extremely small volume samples, such as the contents of single cells. However, the utilization of these ultrasmall capillaries requires a very sensitive and low volume detector. An improved method for end-column amperometric detection for capillary electrophoresis in nanometer inner diameter (i.d.) capillaries is presented. This new method involves etching both the electrode and the detection end of the capillary. These design improvements allow for better alignment between the capillary bore and the electrode. As a result, dead volume in the detector is minimized. The etched method for electrochemical detection in ultrasmall capillaries provides average coulometric efficiencies of 70 +/- 10% for dopamine and 40 +/- 20% for catechol in 770 nm i.d. capillaries. Furthermore, this technique provides peak efficiencies as high as 100,000 theoretical plates and detection limits as low as 340 zmol for both dopamine and catechol.

Determination of rutin and forsythin in fruit ofForsythia suspensa(Thunb.) Vahl by capillary electrophoresis-electrochemical detection

Capillary electrophoresis-amperometric detection is evaluated for simultaneous determination of rutin and forsythin. The cyclic voltammogram, hydrodynamic voltammogram, effect of pH, buffer concentration and SDS, and percent organic modifier on separation and detection were studied. Conditions were optimized as follows: 1.2 V detection potential; separation at 12 kV; 5 s at 15 kV for sample injection time and sample injection voltage; mobile phase 20 mM boric acid buffer; pH 8.4, containing 40 mM SDS and 10% (v/v) acetontrile. The method gave low detection limit as 0.001 mg mL−1 and 0.0005 mg mL−1 (S/N=3), wide linear range 0.005–0.5 mg mL−1 for rutin and forsythin, respectively. The relative standard deviations of peak current and migration time for 8 consecutive injections of the standard solution containing 0.1 mg mL−1 each compound were 4.78%, 3.63% and 6.40%, 2.95% for rutin and forsythin, respectively. In addition, levels of the two compounds in traditional Chinese herbal drugs were easily determined.

Capillary-electrode alignment by an optical-fiber connector for amperometric detection in capillary electrophoresis.

We describe here an electrochemical cell ideal for routine analysis of CE-EC experiments (capillary electrophoresis coupled with electrochemical detection). The cell was modified from a fiber-optic connector, named MT, which allowed frequent change and fast alignment between a pair of 4-strand fiber ribbons. The relative standard deviations of the current response and migration time for 100 microM dopamine were, respectively, 3.7 and 0.5% in five repetitive routines of disconnecting, polishing, and assembling the CE cell. The time required for alignment of the separation capillary and the working electrode was < 10 s, once all components were assembled in an MT fiber-optic plug. These features enabled CE-EC users to polish the working electrode and reassemble the EC cell as in HPLC-EC. However, to accommodate the channel dimensions of the commercially available MT, a special order capillary with outer diameter of 125 microm is necessary at this stage.

Applications of a versatile sol-gel derived renewable electrode for capillary electrophoresis.

A new electrode has been developed and applied for amperometric detection in capillary electrophoresis (CE), comprised of carbon sol-gel composite material. The versatility of the sol-gel technique permits the flexible configuration of the electrode. The performance of such a sol-gel carbon composite electrode (CCE) is first evaluated in a typical CE application for the detection of purine-based compounds. Application of the CCE is also demonstrated for the detection of phenolic compounds in a micellar system. Separation resolution for non-ionic phenolic compounds can significantly be enhanced by introducing sodium dodecyl sulfate (SDS) at a concentration above its critical micelle concentration (cmc) to the buffer. Another design of the CCE incorporating the electrocatalyst Cu2O is employed for the analysis of sugars and organic acids based on dynamic modification with cetyltrimethylammonium bromide (CTAB). It has been found that the presence of surfactant in the separation buffer does not adversely influence the electrochemical detection using a sol-gel derived carbon electrode.

Determination of Propranolol by Capillary Electrophoresis with End-Column Amperometric Detection

A capillary electrophoresis-amperometric detection system was developed for the determination of propranolol (PRO) at a 33 µm carbon fiber microdisk electrode (CFE). The cyclic voltammogram, the hydrodynamic voltammograms and the effect of pH were studied. Under the optimum conditions: separation voltage 15 kV; injection 3 s at 15 kV; 10 mM pH 7.5 phosphate buffer, 1.15 V (vs. Ag/AgCl) detection potential, the detection limit (LOD) for PRO was 0.05 µM (S/N=3). The response for PRO was linear over two orders of magnitude with a linear correlation coefficient of 0.994. The feasibility of this method was demonstrated by the detection of PRO in urine sample.

On-column amperometric detection in capillary electrophoresis with an improved high-voltage electric field decoupler

An improved fabrication method for a decoupler for on-column amperometric detection in capillary electrophoresis (CE) is described. The decoupler is fabricated by etching one side-wall of the capillary with hydrofluoric acid after the polymer coating had been etched by laser, then the etched hole is sealed with adhesive. The steady time, electric conductivity efficiency and performance are investigated. On-column amperometric detection by CE of para-substituted phenols was carried out by coupling with a carbon-fiber microelectrode (10-μm diameter) and a practical small electrochemical detection cell.

Amperometric detection for capillary electrophoresis at a sol–gel carbon composite electrode

The application of a newly developed sol–gel carbon composite electrode (CCE) was evaluated for the amperometric detection in capillary electrophoresis (CE). A sol–gel CCE of ca. 150 μm diameter was employed as a detector in a wall-jet arrangement. The use of simple anodic electrochemical pre-treatment at +1.80 V versus SCE for 2 min was shown to produce an improved and reasonably stable electrode response compared to that of the untreated electrode. Although, the enhancement resulting from electrochemical pre-treatment was not permanent, the results obtained under CE conditions demonstrated that the sol–gel CCE with a single electrochemical pre-treatment was suitable for CE applications. Following the electrochemical pre-treatment, a sample mixture containing dopamine, catechol, epinephrine and norepinephrine was separated in a CE system. The calibration graphs for all these neurotransmitters were found to be linear in the range 1–400 μM and the detection limit (signal/noise = 3) for each neurotransmitter examined was <350 amol.

Parallel-Opposed Dual-Electrode Detector with Recycling Amperometric Enhancement for Capillary Electrophoresis

The assembly and characterization of dual-electrode amperometric detection for capillary electrophoresis are described. The detector consists of a disk electrode and an integrated on-capillary electrode fabricated by depositing a gold film onto the end of the separation capillary. The two electrodes are brought together, aligned, and fixed in position using a pair of acrylic plates with a straight groove on one of the plates, the same design as that of a conventional end-column detector. A portion of the on-capillary electrode is parallel-opposed to the disk electrode in a thin-layer geometry. In this region, the redox cycling established between these two electrodes significantly enhances the amperometric signals of electrochemically reversible analytes. For measurements of dopamine in pH 6.9 phosphate electrolyte with a 12.5-μm-i.d. capillary, such a configuration is 10-fold more sensitive than conventional end-column detection. The linear range exceeds 4 orders of magnitude (1.2 mM-50 nM) and the detection limit is 12 nM (4.2 amol, S/N = 3). Various modes of potential settings for the dual-electrode detection are also discussed.

End-column amperometric detection in capillary electrophoresis: influence of separation-related parameters on the observed half-wave potential for dopamine and catechol.

Capillary electrophoresis (CE) was coupled to a micro-electrode-based end-column amperometric detector. The influences of separation voltage, CE buffer concentration, and capillary-to-electrode distance on the observed hydrodynamic voltammetry of dopamine and catechol were studied using a separation capillary with an i.d. of 25 microns. It was found that an increased CE voltage, increased buffer concentration, or decreased capillary-to-electrode distance resulted in a positive shift of the observed half-wave potentials for both dopamine and catechol. At a constant separation current of 1.6 microA, the observed half-wave potential was found to increase with applied separation voltage. Furthermore, when experiments were carried out with a platinum quasi-reference electrode instead of a Ag/AgCl reference electrode, similar shifts in half-wave potential were observed. These results indicate that the observed shifts are an effect of the separation voltage rather than the separation current or a change in the reference potential. The characteristics of end-column detection with and without a fracture decoupler were compared. It was found that the effects of separation voltage, CE buffer concentration, and capillary-to-electrode distance were minimized by the use of a decoupling device. The observed half-wave potentials for dopamine and catechol were more positive when a CE capillary without a decoupler was employed compared to when a decoupler was used. Additionally, using the fracture decoupler, the observed half-wave potentials for both dopamine and catechol were approximately the same as when no CE voltage was applied (i.e., when the hydrodynamic voltammograms were recorded under flow injection conditions).

Preliminary Study on Reverse Pulse Amperometric Detection in Capillary Electrophoresis for Simultaneous Determination of Cysteine and Cystine

AbstractSimultaneous determination of cysteine (RSH) and cystine (RSSR), two important sulfur‐containing amino acids, in capillary electrophoresis (CE) has been an analytical task. Dual‐microelectrode amperometric detection seems to be a good scheme, but significant difficulty in electrode construction and poor detection limit for RSSR determination remain major short‐comings. In reverse pulse amperometric (RPA) detection, the applied potentials are repeatedly pulsed back and forth between the reducing initial potential (e.g., Ei = −1.4 V) and the oxidizing final potential (e.g., Ef = 0.0 V) at a single, gold‐mercury amalgam (Au/Hg) microelectrode. At Ei, RSSR is reduced to RSH which causes catalytical oxidation of the Au/Hg amalgam microelectrode when the potential is pulsed to Ef. The resulting anodic current is then recorded. Therefore, by using RPA detection after CE separation, RSH and RSSR can be simultaneously determined.

Electrocatalytic oxidation of hydrazines at a 4-pyridyl hydroquinone self-assembled platinum electrode and its application to amperometric detection in capillary electrophoresis

4-Pyridyl hydroquinone on a platinum electrode adsorbs through the pyridine nitrogen forming stable self-assembled layers. The electrocatalytical oxidation of hydrazines was performed by the modified electrode. The overpotential of hydrazines was decreased markedly at the self-assembled monolayer (SAM) electrode. The mechanism of hydrazine oxidation was also investigated. Amperometric detection of hydrazine under zero potential (vs Ag|AgCl|sat. KCl) was exhibited by the SAM electrode used as an electrochemical detector in a flow system.

Voltammetric detection for capillary electrophoresis.

Several approaches to implementing amperometric detection for capillary electrophoresis have been reported. This report describes the development of a voltammetric detector for CE. The detector is designed to minimize distortion of the voltammetry due to ohmic potential drop. This was accomplished by using a cast Nafion detection cell at the end of the separation capillary. The cast Nafion detection cell provided a low-dead-volume, low-resistance cell that minimized ohmic potential drop and peak band broadening. The ability to detect the current due to oxidation of analytes superimposed on a large background current was also improved. A dynamic background subtraction scheme was used in which a second working electrode, positioned in the electrochemical cell but outside of the detection cell, was used to compensate for the background current in real time. The output of the compensating working electrode was subtracted from the output of the detecting working electrode prior to analog-to-digital conversion. Postexperimental digital background subtraction was also implemented. This approach provided optimal elimination of the background current with maximal detection of the analytical signal. The voltammetric detector developed produced high-quality voltammetric response of analytes with injected concentrations as low as 0.20 microM. The system was evaluated by obtaining CE voltammograms of a mixture of eight test phenolic acids.

Amperometric Detection in Capillary Electrophoresis with an Etched Joint

A system for coupling amperometric detection with capillary electrophoresis (CE) is demonstrated. In CE amperometric detection, the separation and detection capillaries are usually coupled with a porous joint to isolate the detector from the high applied voltage when the capillary inner diameter is larger than 25 μm. In this report a simple method is suggested. The joint was prepared by etching the capillary wall with HF after the polyimide coating of the capillary had been removed. This etched joint can efficiently isolate the detector from the high applied voltage. The performance of the CE-amperometric detection with an etched joint was evaluated with hydroquinone. The successful separation and amperometric detection of catecholic compounds, diphenols, and phenol have been carried out by this system.

An Electrochemical Cell for End-Column Amperometric Detection in Capillary Electrophoresis

With end-column amperometric detection in capillary electrophoresis, precise positioning and stabilization of the working electrode are highly important. Noises from vibration and breakage of the microelectrode (e.g., carbon fiber electrode) are typical problems. To overcome these drawbacks, a new electrochemical cell assembly was designed. In this assembly, alignment of the working electrode with the capillary outlet can be achieved precisely and easily. Coupling with a disk-type Pt electrode (50 μm diameter), detection limits of 3.0 and 5.2 amol and separation efficiencies of about 70 000 and 150 000 theoretical plates for the determination of dopamine and catechol, respectively, as test compounds can be obtained. The relative standard deviations (n = 21) of migration time and peak current obtained are 3.5, 5.1% and 2.2, 2.7%, respectively, for these two compounds. Applicability of this assembly as an electrochemical detector for capillary electrophoresis was demonstrated by running a synthetic sample containing dopamine, serotonin, norepinephrine, epinephrine, isoproterenol, and catechol. Results obtained are comparable with those from other end-column or off-column determinations.

Pharmaceutical and biomedical applications of capillary electrophoresis/electrochemistry

The use of capillary electrophoresis/electrochemistry (CEEC) for the analysis of microdialysis samples obtained for pharmacokinetic and neurochemical studies is described, as well as the development of new types of electrodes and waveforms which increase the selectivity of this technique for specific classes of analytes. CEEC with a carbon fiber electrode was employed for the analysis of microdialysis samples. Microdialysis is an in vivo sampling technique that yields very small samples for analysis (less than 1 microL). Therefore, capillary electrophoresis, with its small volume requirements, is an excellent choice for the analytical method. CEEC was used to study the pharmacokinetics of L-dopa and the release of aspartate and glutamate following a high K+ infusion in the brain. Several modified electrodes which increase the applicability of CEEC in pharmaceutical and biomedical analysis are described. One of these is a gold/mercury electrode which is highly selective for thiols and was used for the determination of glutathione in a rat brain. An alternative method for the detection of thiols employed a chemically modified electrode containing cobalt phthalocyanine. In this case, an electrocatalyst reduces the overpotential of thiols at the carbon electrode and makes it possible to detect them at a much lower and more selective oxidation potential. This electrode was used for the detection of cysteine in urine. The development of pulsed amperometric detection for capillary electrophoresis is also described and is demonstrated by the detection of glucose in blood. Lastly, a method for the detection of peptides based on the formation of a copper complex and detection at a carbon fiber electrode is discussed.

Amperometric detection in capillary electrophoresis with normal size electrodes

Amperometric detection in capillary electrophoresis with normal size electrodes