Voltammetric/amperometric detection for liquid chromatography

A voltammetric/amperometric detector based on a dual-electrode electrochemical detector is described for liquid chromatography. The detector combines the advantages of both voltammetric and amperometric detection. A three-dimensional data array of current response as a function of both time (chromatographic domain) and potential (electrochemical domain) is obtained. From the chromatographic point of view, this allows post-experimental choice of the optimal detection potential. Different detection potentials can even be chosen for each chromatographic peak. Having the voltammetric data as well as the chromatographic data provides ready identification of chromatographically unresolved compounds and the ability to resolve such co-eluting compounds voltammetrically. The voltammetric data also provide a second method of peak identification for greater certainty in peak assignments. Voltammetric detection limits of less than 10 pmol of material injected on the column were achieved with this detection method. From the electrochemical perspective, voltammetric/amperometric detection provides a technique for obtaining hydrodynamic voltammograms with small amounts or small volumes of sample. Voltammograms can also be obtained for the individual components of complex mixtures without the need for isolation steps.     

Electrochemical behavior of 2'-halogenated derivatives of N,N-dimethyl-4-aminoazobenzene at mercury electrode.

The electrochemical behavior of 2'-halogeno-N,N-dimethyl-4-aminoazobenzene derivatives was investigated using various polarographic and voltammetric methods. The peak potentials of these derivatives were observed to shift towards negative values along with an increase in the pH. A reduction of the azo linkage took place via two electrons at pH > 4, but four electrons at pH < 4 in aqueous-ethanol mixtures. The standard rate constants were determined with (Laviron technique) or without (Nicholson technique) taking the adsorption phenomena into account. The diffusion coefficients were calculated from the cyclic voltammetric data using a method developed by Garrido. The amount of adsorbed substances and transfer coefficients for the electron transfer were also determined. A mechanism for the electrode reaction is proposed. These compounds can be quantitatively determined between 1 x 10(-5) M and 1 x 10(-7) with DPP and CV.     

Quantification of refractory organic substances in freshwaters: further insight into the response of the voltammetric method

A recently published method for quantifying refractory organic matter (often referred to as humic substances) in freshwaters was applied to a wide range of International Humic Substance Society (IHSS) humic compounds in order to (i) gain a better understanding of the mechanism of the voltammetric response which is the basis of the analytical method and (ii) provide guidance on choosing the optimal standard to be used. At the same time, the sensitivity of the technique has been increased by switching from the pulse mode initially proposed to the square-wave mode. The results obtained show that (i) differences in adsorption onto the electrode rather than differences in complexation strength are responsible for the differences in the intensity of the signal obtained for the different humic compounds, (ii) carboxylate, N- and S-containing groups do not play a role in the voltammetric signal.     

Voltammetric determination of diethylstilbestrol at carbon paste electrode using cetylpyridine bromide as medium

In this paper, the voltammetric properties of diethylstilbestrol (DES) at the carbon paste electrode were described. The oxidation peak currents of DES increase significantly in the presence of surfactant cetylpyridine bromide (CPB), compared with that in the absence of CPB. Based on this fact, a voltammetric technique for determining DES is proposed. The accumulation potential has no effects on the peak current of the DES. An open-circuit accumulation is carried out. The experimental parameters, such as pH value of buffer, scan rate, and accumulation time were optimized. The interferences of some metal ions and organic compounds have also been studied, and some metal ions almost do not interfere with the determination of DES. Using this voltammetric method, DES in the injection sample was measured. The results show that this voltammetric method is reliable for the practice determination of DES.     

In vivo Electrochemical Monitoring of Signaling Transduction of Plant Defense Against Stress in Leaves of Aloe vera L.

In situ recording of the voltammetric response of Aloe vera L. leaves using graphite and platinum microelectrodes inserted in the subcuticular region is described. The plant response against the mechanical stress results in changes of the voltammetric signals associated to signal transduction compounds salicylic and jasmonic acids and hydrogen peroxide, which were separately monitored. A kinetic model is proposed to describe the time variation of the respective voltammetric signatures, denoting the possibility of in vivo electrochemical monitoring of biochemical processes in plants. The obtained results suggest that two defense pathways with different participation of ROS are operative.     

Voltammetric detection of metals and metal compounds on surfaces following a new sampling technique using alkali amalgam droplets

We propose a new method for the voltammetric detection of metals and metal compounds on surfaces of sold materials. The method is based on the reduction of oxidized metal compounds using an alkali amalgam droplet, the dissolution of the metals in the amalgam droplet, finally followed by recording the voltammetric anodic dissolution of the metal. The sample collection and all the following steps, including the voltammetric measurement, are performed at the same site of the sample and without any temporal interruption as one and the same electrode system is used for all steps. The proposed method is as an easy to handle, non-destructive express analysis for a wide range of samples.     

Simultaneous determination of three 5-nitroimidazoles in foodstuffs by differential pulse voltammetry and chemometrics

The voltammetric behaviour of three 5-nitroimidazoles,metronidazole,tinidazole and ornidazole,was investigated,and a method was developed for the simultaneous determination of these compounds,based on their reduction at a hanging mercury drop electrode(HMDE) in pH 8.95 buffer with differential pulse voltammetric(DPV) approach.Well defined voltammetric waves with peak potentials of -692,-640 and -652 mV were observed for these compounds,respectively.It is difficult to determine them individually from their mixtures without preseparation,for their voltammetric peaks overlapped seriously,so the chemometrics were used to resolve the overlapped voltammogram and quantify the mixtures.The proposed method was successfully applied to the determination of three 5-nitroimidazoles in milk and honey samples.     

Differential pulse and square-wave cathodic stripping voltammetry of xanthine and xanthosine at a mercury electrode.

The surface activity of xanthine (Xan) and xanthosine (Xano) at a hanging mercury drop electrode (HMDE) was studied using out-of-phase ac and cyclic dc voltammetry. The results show that Xan and Xano were strongly adsorbed and chemically interacted with the charged mercury surface, which is the prerequisite step for applying the cathodic adsorptive stripping voltammetric determination of such biologically important compounds. Differential pulse cathodic adsorptive stripping voltammetry (DPCASV) and square-wave cathodic adsorptive stripping voltammetry (SWCASV) were applied for the ultratrace determination of Xan and Xano compounds. Moreover, a rapid and sensitive controlled adsorptive accumulation of Cu(II) complexes of both compounds provided the basis of a direct stripping voltammetric determination of such compounds to submicromolar and nanomolar levels. Operational and solution conditions for the quantitative ultratrace determination of Xan and Xano were optimized in absence and presence of Cu(II). The calibration curve data were subjected to least-squares refinements. The effects of several types of inorganic and organic interfering species on the determination of Xan or Xano were considered.     

A non-conventional way to perform voltammetry

In a conventional voltammetric experiment, the electroactive species is dissolved in solution, and then diffuses from the solution phase to the electrode phase. In our proposed non-conventional voltammetric experiment, the electroactive species is trapped in the electrode phase instead of being dissolved in solution. A non-aqueous solvent was first used to trap the organic species in a porous surface layer and the modified electrode then transferred to an aqueous buffer to conduct voltammetry measurements.We tested the non-conventional voltammetric mode using a modified multi-walled carbon nanotube electrode containing mono-, di- and tri-nitroaromatic compounds trapped in the porous three-dimensional network of the CNTs. From these experiments, we conclude that the non-conventional mode produces higher peak currents and displacement of the peak potentials, yielding lower overpotentials. Furthermore, it is possible to obtain more selective voltammograms in the non-conventional mode, showing peaks that could not be resolved in the conventional mode.These results are due to a change in the mass transport regime, with thin layer diffusion being the main transport method in the non-conventional mode, compared to semi-infinite diffusion in the conventional mode.The proposed approach is an excellent alternative for performing voltammetric studies on insoluble or slightly soluble organic compounds.     

Analysis of metal alloys by second-harmonic phase-selective a.c. voltammetry

Second-harmonic phase-selective a.c. voltammetry and second-harmonic a.c. anodic stripping voltammetry are shown to be particularly suitable for simultaneous determinations of elements having very close half-wave potentials (differences of <50 mV). The technique are applied to real matrices. Samples of standard materials BCS 207/2 Gunmetal, SRM 631 SPectrographic Zinc Spelter and SRM 899 (Nickel-base High-temperature Alloy) were digested with a sulphuric acid/nitric acid mixture; after dissolution, the samples were taken up in hydrochloric acid. After adjustment to 1 mol l^?1 HCl, the solutions were used directly for voltammetric measurements of Pb(II)/Sn (II), Sb(III)/Bi(III), Pb(II)/Tl(I) and In(III)/Cd(II) as appropriate. The confidence intervals of the experimental data were in agreement with the certified values for each element. Both the accuracy, expressed as percentage error, and precision, expressed as relative standard deviation, were better than 5%. The standard addition technique was found to improve the resolution of the a.c. voltammetric peaks even in the case of severe overlapping.     

Analysis of amino acids in complex samples by using voltammetry and multivariate calibration methods

A voltammetric method is proposed for the simultaneous determination of tryptophan, cysteine, and tyrosine using multivariate calibration techniques. Various electrodes and voltammetric techniques were explored to ascertain the optimum measurement strategy. Among them, differential pulse voltammetry (DPV) with a Pt electrode was selected as analytical technique since it provided a suitable compromise between sensitivity and reproducibility while allowing the oxidation peaks of the three compounds to be reasonably discriminated. The sensitivity of DPV with Pt electrode for Trp standards was 8.4×10⁻² A l mol⁻¹, the repeatability 3.7% and the detection limit below 10⁻⁷ M. The lack of full selectivity of the voltammetric data was overcome using multivariate calibration methods on the basis of the differences in the voltammetric waves of each compound. The accuracy of predictions was evaluated preliminarily from the analysis of three-component synthetic mixtures. Subsequently, this method was applied to the analysis of oxidizable amino acids in feed samples. Results obtained were in good concordance with those given by the standard method using an amino acid analyzer.     

Electroanalysis of antitubercular drugs in pharmaceutical dosage forms and biological fluids: A review

Tuberculosis remains a major global public health problem. Given the need for extensive analysis of antitubercular drugs, the development of sensitive, reliable and facile analytical methods to determine these compounds becomes necessary. Electrochemical techniques have inherent advantages over other well-established analytical methods, this review aiming to provide an updated overview of the latest trends (from 2006 till date) in the voltammetric determination of antitubercular drugs. Furthermore, the advantages and limitations of these methods are critically discussed. The review reveals that in spite of using a variety of chemically modified electrodes to determine antitubercular drugs, there is still a dearth of applicability of the voltammetric methods to quantify these compounds in human body fluids, especially in blood plasma.     

Fickian diffusion constrained on spherical surfaces: voltammetry.

A model is developed for the voltammetric response due to surface charge injection at a single point on the surface of a sphere on whose surface the electroactive material is confined. Accordingly, charge diffusion is constrained to the spherical surface and thus mimicks the voltammetric response of immobilised microparticles derivatised with electroactive material. The full cyclic voltammetric response is investigated, and the peak currents, the peak-to-peak separation and the symmetry of the voltammetric wave are shown to be indicative of the heterogeneous kinetics and the geometry of the adsorbed microparticle. The results show strong deviations from the responses expected for planar diffusion.     

Derivative hydrodynamic-modulation voltammetry

A technique based on derivative hydrodynamic-modulation voltammetry has been developed for improving the resolution in sensitive analyses of mixtures, at solid electrodes. By simple treatment of the hydrodynamic-modulation data the voltammetric current-potential waves can be transformed into peak-shaped curves, which are frequently more convenient for further data processing. To obtain such curves the rate of change in hydrodynamic-modulation current with potential is plotted against potential. Single and multiple peak systems are evaluated at the micromolar concentration level, by use of stopped-rotation and stopped-flow voltammetry. Dopamine, ascorbic acid, NADH, homovanillic acid and chlorpromazine were used as test compounds. The method is simple and suitable for on-line computerization.     

An Electrochemical Sensor Based on Silver Nanoparticles‐Benzalkonium Chloride for the Voltammetric Determination of Antiviral Drug Tenofovir

A simple voltammetric nanosensor was described for the highly sensitive determination of antiviral drug Tenofovir. The benzalkonium chloride and silver nanoparticles were associated to build a nanosensor on glassy carbon electrode. Surface characterictics were achieved using scanning electron microscopic technique. The voltammetric measurements were performed in pH range between 1.0 and 10.0 using cyclic, adsorptive stripping differential pulse and adsorptive stripping square wave voltammetry. The linear dependence of the peak current on the square root of scan rates and the slope value (0.770) demonstrated that the oxidation of tenofovir is a mix diffusion‐adsorption controlled process in pH 5.70 acetate buffer. The linearity range was found to be 6.0×10⁻⁸–1.0×10⁻⁶ M, and nanosensor displayed an excellent detection limit of 2.39×10⁻⁹ M by square wave adsorptive stripping voltammetry. The developed nanosensor was successfully applied for the determination of Tenofovir in pharmaceutical dosage form. Moreover, the voltammetric oxidation pathway of tenofovir was also investigated at bare glassy carbon electrode comparing with some possible model compounds (Adenine and Adefovir).     

Development of an Analytical Methodology for Quantification of Strong Acid in Diesel Oil

Many factors can affect the quality of diesel oil, in particular the degradation processes that are directly related to some organosulfur compounds. During the degradation process, these compounds are oxidized into their corresponding sulfonic acids, generating a strong acid content during the process. p‐Toluene sulfonic acid analysis was performed using the linear sweep voltammetry technique with a platinum ultramicroelectrode in aqueous solution containing 3 mol L^?1 potassium chloride. An extraction step was introduced prior to the voltammetric detection in order to avoid the adsorption of organic molecules, which inhibit the electrochemical response. The extraction step promoted the transference of sulfonic acid from the diesel oil to an aqueous phase. The method was accurate and reproducible, with detection and quantification limits of 5 ppm and 15 ppm, respectively. Recovery of sulfonic acid was around 90%.     

Fabrication of carbon microelectrodes with an effective radius of 1 nm

A method for producing insulated nanometer-sized carbon electrodes is presented. These electrodes are produced using electrochemical etching of carbon fibers followed by deposition of electrophoretic paint. A new deposition approach for insulating the tips, the so-called “inverted deposition” technique, is introduced. This technique allows complete insulation of the whole body of the etched carbon fiber except for the very tip, leaving an electrochemical active area with effective diameters as small as a few nanometers. The process overcomes pinhole formation that can be a problem with the normal electrophoretic paint deposition process. The fabricated electrodes show ideal steady-state voltammetric behavior. The voltammetric response corresponding to the reduction of hexacyanoferrate(III) and hexaammineruthenium(III) is investigated on these small electrodes in the absence and presence of supporting electrolyte. For these two multiple-charged ions the steady-state voltammetric behavior in the absence of supporting electrolyte is found to deviate from expected behavior, especially at very small electrodes.     

Applications of stripping voltammetric techniques in food analysis

Stripping voltammetric techniques are powerful analytical tools that are becoming widely used in various chemical analysis fields. Hence, the objective of this survey is to give a general overview on the scope of the applicability of stripping voltammetric methods in food industries. The applications discussed include recent studies on the utilization of these electroanalytical methods in determination of food contaminants (toxic metals, pesticide, fertilizers and veterinary drugs residuals), trace essential elements, food additive dyes and other organic compounds of biological significance. Tables that give method summaries referenced to the original work are provided.     

Genotoxicity screening for N-nitroso compounds. Electrochemical and electrochemiluminescent detection of human enzyme-generated DNA damage from N-nitrosopyrrolidine

We report for the first time voltammetric/electrochemiluminescent sensors applied to predict genotoxicity of N-nitroso compounds bioactivated by human cytochrome P450 enzymes.     

Rapid voltammetric determination of nitroaromatic explosives at electrochemically activated carbon-fibre electrodes

The electrochemical behaviour of some nitroaromatic explosives (2,4,6-trinitrotoluene, TNT; 2,6-dinitrotoluene, 2,6-DNT; 2-nitrotoluene, 2-NT; 2-amino-4,6-dinitrotoluene, 2-A-4,6-DNT; 3,5-dinitroaniline, 3,5-DNA; and nitrobenzene, NB) at electrochemically activated carbon-fibre microelectrodes is reported. Electrochemical activation of such electrode material by repeated square-wave (SW) voltammetric scans between 0.0 and +2.6 V versus Ag/AgCl, produced a dramatic increase in the cathodic response from these compounds. This is attributed to the increase of the carbon-fibre surface area, because of its fracture, and the appearance of deep fissures along the main fibre axis into which the nitroaromatic compounds penetrate. Based on the important contribution of adsorption and/or thin layer electrolysis to the total voltammetric response, a SW voltammetric method for rapid detection of nitroaromatic explosives was developed. No interference was found from compounds such as hydrazine, phenolic compounds, carbamates, triazines or surfactants. The limits of detection obtained are approximately 0.03 g mL^–1 for all the nitroaromatic compounds tested. The method was applied for the determination of TNT in water and soil spiked samples; recoveries were higher than 95% in all cases.