Juan M. Marioli

Electrochemical characterization of carbohydrate oxidation at copper electrodes

Abstract The electrochemical oxidation of carbohydrates at copper electrodes in alkaline solution is studied by cyclic voltammetric and rotating ring-disk electrode experiments. It is demonstrated herein that an important step in the oxidation of carbohydrates is their interaction with the oxide/hydroxide layer covering the electrode. This interaction is believed to involve the Cu(I) oxidation state although interaction with Cu(II) may also occur once it is formed. The extent of this interaction is measured by the decrease in peak height and electrochemical charge under the Cu(II) formation wave, and is sugar dependent. It is interesting that there is a correspondence between the extent of the interaction and the oxidative charge measured between 0.4 and 0.6 V (Ag/AgCl reference electrode) in the presence of carbohydrates. Structural effects may be influencing the interaction between carbohydrates and the electrode surface. The oxidation of many carbohydrates occurs in the potential region corresponding to the formation of Cu(III). Rotating ring disk electrode experiments strongly suggest that Cu(III) plays an important role in carbohydrate oxidation after the adsorption step. The electrooxidation of glucose at copper electrodes in alkaline solutions goes beyond the formation of gluconic acid (two electron transfer). Further oxidation may continue through CC bond cleavage. A mechanism for the oxidation of carbohydrates at copper electrodes is proposed. The mechanism includes the formation of complex species between the carbohydrates and the copper.

Nickel—chromium alloy electrode as a carbohydrate detector for liquid chromatography

Abstract A nickel—chromium alloy electrode was applied as an electrochemical detector for the analysis of carbohydrates by high performance anion-exchange liquid chromatography. This alloy electrode exhibited excellent sensitivity, reproductivity and stability. The cabohydrates are oxidatively detected by a surface catalyzed process, proposed to involve nickel(III) oxyhydroxides, which are formed in the range of ca. 0.45 to 0.5 V vs a Ag/AgCl reference electrode. The cyclic voltammograms of the NiCr electrode are compared to pure Ni in order to understand the basic mechanism of the oxidation. The effect of the temperature, the sodium hydroxide concentration, the mobile phase flow-rate, and the working potential on the electrode response were analyzed to optimize the detection conditions. The reproducibility of the electrode response to the injection of 100 pmol of glucose was studied continuously for a period of 21 days. A limit-of-detection (LOD) of approximately 500 fmol of glucose (signal-to-noise ratio of 3) makes this alloy electrode well suited for the high sensitive detection of carbohydrates.

HPLC Analysis of Carbohydrates with Electrochemical Detection at a Poly-1-naphthylamine/Copper Modified Electrode

Carbohydrates are detected by constant potential amperometric detection at a modified electrode consisting of poly-1-naphthylamine coated glassy carbon with copper ions incorporated into the polymeric matrix. The polymer is electrodeposited onto the glassy carbon surface by cyclic voltammetric experiments, and copper ions incorporated by immersion of the modified electrode in a 0.10 M cupric ions solution. The amount of copper incorporated into the polymeric matrix is regulated by controlling the immersion time. Cyclic voltammetric experiments showed that glucose was oxidized at the modified electrode surface. The catalytic current for glucose oxidation was dependent on the amount of copper incorporated. Flow injection experiments allowed the determination of the working electrode potential at which maximum signal-to-noise ratios were obtained. The electrode was successfully used in the high performance liquid chromatographic analysis of various carbohydrates.

Optimization of the electrodeposition of copper on poly-1-naphthylamine for the amperometric detection of carbohydrates in HPLC

A modified electrode consisting of copper dispersed in a poly-1-naphthylamine (p-1-NAP/Cu) film on a glassy carbon electrode was used as an amperometric detector for the on-line analysis of various carbohydrates separated by high performance liquid chromatography. The results obtained with this new sensor were compared to those obtained with a modified electrode based on the same polymer but with copper ions incorporated at open circuit, as described in a previous paper. In this new modified electrode the copper microparticles were electrochemically deposited into the polymeric matrix by single potential step chronoamperometry. A nucleation and growth mechanism was proposed to explain the current transients of copper electrodeposition. The experimental results were fitted to the proposed mechanism by using a mathematical equation that considers three-dimensional growth and progressive nucleation, assuming a no overlap and no diffusion mechanism. Cyclic voltammetric experiments showed that the electrodeposited copper microparticles provided a catalytic surface suited for the oxidation of glucose and several carbohydrates. The sensitivity of the electrode was influenced by the amount of copper electrodeposited, which in turn depended on the applied overpotential used for the deposition of copper. Liquid chromatographic experiments were carried out to test the analytical performance of these electrodes for the determination of various carbohydrates.

The potentiodynamic behavior of nickel-chromium (80:20) alloy electrodes in 0.10 N sodium hydroxide

Abstract Cyclic voltammetry is used to study the current-potential characteristics of a nickel-chromium alloy in alkaline solutions. These results are compared to those obtained with pure nickel anodes in the same background electrolyte. The electrochemical behavior of the alloy is qualitatively similar to that of the pure nickel electrode. However, a “higher degree of disorder” of the Ni(II)-Ni(III) oxide layer structure is proposed to be present on top of the alloy. This structure accounts for the higher reactivity to carbohydrate oxidation and the higher electrochemical charge under the Ni(II) oxidative wave observed with the alloy electrode. Waves assigned to chromium oxidation are only observed during the first potential scan. The surface final distribution of Ni(II) oxidation products at nickel-chromium alloys is different from that of pure nickel. We consider that this is due to the influence of chromium on the electrochemistry of the alloy.

Multi-wavelength spectrophotometric determination of propofol acidity constant in different acetonitrile-water mixtures

The acid-base constant of an anaesthetic drug, propofol, was determined in different acetonitrile-water mixtures by using multiwavelength spectrophotometric titration data. The pH values measured in acetonitrile-water mixtures with a glass electrode calibrated with an aqueous buffer, swpH, were corrected to obtain sspH, that is, the pH of the acetonitrile-water mixture as measured with a glass electrode calibrated with a buffer in the same solvent mixture. This last pH scale is directly related to the thermodynamic acid-base constant. The pKa value of propofol in pure water was found to be 11.67. Linear relationships were observed when the calculated pKa values of propofol in different solvent mixtures were plotted against the acetonitrile molar fraction. Also, a linear relationship was observed between pKa values and the reciprocal of the relative permittivity of the solvent mixture, 1/e. This behavior was taken as indicative of the electrostatic nature of the ionization process.

Square wave voltammetry with multivariate calibration tools for determination of eugenol, carvacrol and thymol in honey.

The simultaneous determination of eugenol (EU), thymol (Ty) and carvacrol (CA) in honey samples, employing square wave voltammetry (SWV) and chemometrics tools, is informed for the first time. For this purpose, a glassy carbon electrode (GCE) was used as working electrode. The operating conditions and influencing parameters (involving several chemical and instrumental parameters) were first optimized by cyclic voltammetry (CV). Thus, the effects of the scan rate, pH and analyte concentration on the electrochemical response of the above mentioned molecules were studied. The results show that the electrochemical responses of the three compounds are very similar and that the voltammetric traces present a high degree of overlap under all the experimental conditions used in this study. Therefore, two chemometric tools were tested to obtain the multivariate calibration model. One method was the partial least squares regression (PLS-1), which assumes a linear behaviour. The other nonlinear method was an artificial neural network (ANN). In this last case we used a supervised, feed-forward network with Levenberg-Marquardt back propagation training. From the accuracies and precisions analysis between nominal and estimated concentrations calculated by using both methods, it was inferred that the ANN method was a good model to quantify EU, Ty and CA in honey samples. Recovery percentages were between 87% and 104%, except for two samples whose values were 136% and 72%. The analytical methodology was simple, fast and accurate.

Electrochemical Detection of 2,6‐Diisopropylphenol (Propofol) in Reversed Phase HPLC at High pH

Abstract The electrochemical response of propofol (2,6‐diisopropylphenol) at glassy carbon electrodes was studied by cyclic voltammetric experiments under different experimental conditions. The voltammetric peak current and potential for the oxidation of propofol were analyzed at different scan rates, pHs, propofol concentrations, and organic modifier amounts. The results obtained helped to optimize the setup parameters for the amperometric detection of propofol in HPLC. The use of mobile phases at high pH significantly lowered the detection potential and chromatographic capacity factors. The effect of the organic modifier amount on the chromatographic capacity factor of propofol was also evaluated.

Effect of Different Experimental Variables on the Separation of Catecholamine Metabolites by Ioninteraction RP-HPLC

Abstract The liquid chromatographic separation of catecholamine acidic metabolites using tetrabutylammonium bromide as ion interaction agent is investigated. The effect of several experimental variables, such as mobile phase pH, organic modifier concentration, ion interaction agent concentration, and temperature is analyzed. The mobile phase composition that produces the best chromatographic resolution for the analysis of the compounds of interest is determined. The hydrodynamic voltammogram of the analytes showed that good selectivity can be achieved with electrochemical detection, allowing for the analysis of different compounds at different applied potentials. The Signal-to-Noise ratio is studied as a function of the applied potential.