Liquid chromatography amperometric detection of nitrite using a polypyrrole modified glassy carbon electrode doped with tungstodiphosphate anion

Chromatography-amperometric detection of nitrite with a polypyrrole modified glassy carbon electrode doped with tungstodiphosphate anion (Dawson-type P2W18O626-/PPy/GC electrode) based on its electrocatalytic reduction of nitrite is described. The cyclic and hydrodynamic voltammetry of nitrite at the P2W18O626-/PPy/GC electrode was studied. The factors affecting the detection of nitrite and the analytical performance of the modified electrode in flowing stream were investigated. The results show that the modified electrode has a good sensitivity (the limit of detection is 1 mu mol dm(-3)) and a satisfactory reproducibility (RSD = 3.78%, N = 21). The modified electrode was used in the chromatographic detection of nitrite spiked in the liquid from a tin of mushrooms and the mineralized spring water. It was found that the modified electrode exhibited good selectivity for nitrite.

A novel method of electrodepositing highly dispersed nano palladium particles on glassy carbon electrode

The volumetric behavior of a chloride complex of palladium was studied at a glassy carbon electrode (GCE). The Pd-IV complex existing on the GCE surface was found, which was proposed to form an octahedral surface complex through coordination to the oxygen atom of an oxygen functional group on the pretreated GCE surface. The ferri/ferrocyanide redox couple was used as a probe to examine the activity of the GCE. X-ray photoelectron spectroscopy provided the evidence of the surface complex existing on the GCE. Highly dispersed Pd particles can be obtained when the surface complexes were reduced electrochemically to Pd atoms. The Pd particles obtained in this way were in nanometer scale and exhibit high catalytic activity towards the oxidation of hydrazine. (C) 1997 Elsevier Science Ltd.

Electrocatalytic reduction of nitrite using Dawson-type tungstodiphosphate anions in aqueous solutions, adsorbed on a glassy carbon electrode and doped in polypyrrole film

The electrochemical behavior of Dawson-type P2W18O626- adsorbed on a glassy carbon electrode and doped in a polypyrrole film electrode was described. These modified electrodes all display catalytic activity for nitrite reduction, either in acid solutions or in pH > 4.0 solutions.

The direct electrochemistry of cryo-hydrogel immobilized myoglobin at a glassy carbon electrode

A cryo-hydrogel membrane (CHM) immobilized at a glassy carbon (GC) electrode is reported for the direct electron transfer of redox proteins. The most attractive characteristics of this CHM were its hydrophilic micro-environment for incorporated proteins to retain their activities, its high ability for protection against interference of denatured and adsorbed proteins at the electrode, its potential applications for various proteins or enzymes, as well as its high mechanical strength and thermal stability. A clear well developed and stable redox wave was obtained for commercially available horse heart myoglobin without further purification, giving a peak to peak separation Delta E(p) = 93 mV at 5 mV s(-1) and the formal electrode potential E(0)' = -0.158 V (vs. Ag/AgCl). The formal heterogeneous electron transfer rate constant was calculated as k(0)' = 5.7 X 10(-4) cm s(-1) at pH 6.5, showing rapid electron transfer was achieved. The pH controlled conformational equilibria, acid state --> natural state --> basic I state --> basic II state, of myoglobin at the CHM GC electrode in the pH range 0-13.8 were also observed and are discussed in detail.

ELECTROCATALYTIC OXIDATION AND FLOW DETECTION OF CYSTEINE AT AN AQUOCOBALAMIN ADSORBED GLASSY-CARBON ELECTRODE

A chemically modified electrode (CME) constructed by adsorption of aquocobalamin (VB12a) onto a glassy carbon electrode surface was demonstrated to catalyze the electro-oxidation of cysteine, a sulfhydryl-containing compound. The sulfhydryl oxidation occured at 0.54-0.88 V vs. Ag/AgCl depending on pH value (3.0-10.0). The electrocatalytic behavior of cysteine is elucidated with respect to solution pH, operating potential and other variables as well as the CME preparation conditions. When used as the sensing electrode in flow injection amperometric detection, the CME permitted detection of the compound at 0.8 V. The detection limit was 1.7 pmol. The linear response range went up to 1.16 nmol. The stability of the CME was shown by RSD (4.2%) over 10 repeated injections.

DETERMINATION OF CYSTEINE AND REDUCED GLUTATHIONE IN HUMAN PLASMA BY LIQUID-CHROMATOGRAPHY WITH PULSED AMPEROMETRIC ELECTROCHEMICAL DETECTION USING A PLATINUM-PARTICLES MODIFIED GLASSY-CARBON ELECTRODE

An assay procedure utilizing pulsed amperometric detection at a platinum-particles modified electrode has been developed for the determination of cysteine and glutathione in blood samples following preliminary separation by reversed-phase liquid chromatography. A chemically modified electrode (CME) constructed by unique electroreduction from a platinum-salt solution to produce dispersed Pt particles on a glassy carbon surface was demonstrated to catalyze the electo-oxidation of sulfhydryl-containing compounds: DL-cysteine (CYS), reduced glutathione (GSH). When used as the sensing electrode in flow-system pulsed-amperometric detection (PAD), electrode fouling could be avoided using a waveform in which the cathodic reactivation process occurred at a potential of - 1.0 V vs. Ag/AgCl to achieve a cathodic desorption of atomic sulfur. A superior detection limit for these free thiols was obtained at a Pt particle-based GC electrode compared with other methods; this novel dispersed Pt particles CME exhibited high electrocatalytic stability and activity when it was employed as an electrochemical detector in FIA and HPLC for the determination of those organo-sulfur compounds.

KINETIC-STUDY OF ELECTROCATALYTIC REDUCTION OF HYDROGEN-PEROXIDE AT IRON(III) PORPHYRIN MODIFIED GLASSY-CARBON ELECTRODE IN ALKALINE-MEDIUM USING THE ROTATION-SCAN METHOD

Reduction of hydrogen peroxide at a glassy carbon (GC) electrode modified with sigma-bonded pyrrole iron(III) octaethylporphyrin complex, (OEP)Fe(Pyr), was studied by cyclic voltammetry and a rotating disk electrode. In 0.1N NaOH solution, it is shown that such an (OEP)Fe(Pyr)/GC electrode has a significant catalytic activity towards hydrogen peroxide reduction (E(D) = -0.80 V, k = 0.066 cm s(-1)); however, the electrode stability is low. The deactivation is observed when the reaction charge (Q) is passing through the (OEP)Fe(Pyr)/GC disk electrode. A linear rotation scan method is applied to study the kinetic process by determining the disk electrochemical response (i(D)) to rotation rate (omega) at a definite disk potential (E(D)). Considering that the number of adsorbed electroreduced catalyst molecules (Red) varies according to the disk potential, a factor theta(= Gamma(Red)/(Gamma(Red) + Gamma(Ox))) is introduced to describe the electrode surface area fraction for electroreduced species. The obtained Koutecky-Levich equation is applicable whatever the potential is.

ELECTROCATALYTIC OXIDATION AND ION-CHROMATOGRAPHIC DETECTION OF BR-, I-, SO32-, S2O32- AND SCN- AT A PLATINUM PARTICLE-BASED GLASSY-CARBON MODIFIED ELECTRODE

A modified method for dispersing platinum particles on a glassy carbon (GC) electrode was investigated. The ultramicro Pt particle-modified electrode obtained exhibited high catalytic stability and activity towards the oxidation of some halide ions (Br-, I-) and inorganic sulfur species (S2O32-, SO32- and SCN-). These anions were separated and detected by using ion chromatography and electrochemical detection via this novel dispersed Pt particles-GC working electrode. The detection limits were 20 ng/ml for Br-, 1.0 ng/ml for I-, 10 ng/ml for SO32- and 4.0 ng/ml for SCN-. This method was employed for the analysis of industrial and environmental waste waters.

DETERMINATION OF AMINOPYRINE AND ITS METABOLITE IN BIOLOGICAL FLUID BY LIQUID-CHROMATOGRAPHY ELECTROCHEMISTRY WITH A GLASSY-CARBON ELECTRODE DISPERSED WITH ALPHA-ALUMINA PARTICLES

A sensitive high-performance liquid chromatographic method has been developed for the quantitative determination of aminopyrine (AM) and its metabolite 4-aminoantipyrine (AAN). The method utilizes reverse-phase chromatography/amperometric detection with a glassy carbon electrode dispersed with alpha-arumina particles as the working electrode, on which the oxidation of AM and AAN was greatly improved compared with that on a bare glassy carbon electrode. As a result, the detection limit was as low as 1.4 ng for AM and 0.8 ng for AAN, and the calibration plots for the above compounds have wide linear ranges from 100 ng/mL to 100 mu g/mL and 60 ng/mL to 80 mu g/mL (for AM and AAN, respectively). The above method was applied for the detection of these materials in human urine with satisfactory results.

ELECTROOXIDATION OF A CHLORIDE COMPLEX OF PLATINUM(II) AT A GLASSY-CARBON ELECTRODE

The electro-oxidation of PtCl42- was studied on a glassy carbon (GC) electrode. A Pt(IV) complex was formed on the electrode surface through coordination to the oxygen atom of an oxide functional group on the electrode, which results in its deactivation. The ferri/ferrocyanide redox couple was used as a probe to examine the activity of the GC electrode. X-ray photoelectron spectroscopy was employed to characterize the platinum on the electrode surface, and showed that the oxidation state of the Pt element changes depending on the electrochemical treatment of GC electrode. The platinum complex on the surface of the GC electrode can be transformed to Pt-0 by cycling the electrode between -0.25 and +1.65 V/SCE in 0.1 M H2SO4 solution. The above procedure can be used to disperse platinum ultramicroparticles on the surface of a GC electrode.

ELECTROCATALYTIC OXIDATION AND FLOW-INJECTION DETERMINATION OF REDUCED NICOTINAMIDE COENZYME AT A GLASSY-CARBON ELECTRODE MODIFIED BY A POLYMER THIN-FILM

G chemically modified electrode (CME) was prepared by electrochemical copolymerization of pyrrole and Methylene Blue. The resulting CME exhibits effective electrocatalytic activity towards the oxidation of reduced nicotinamide coenzymes (NADH and NADPH),

ELECTROCATALYTIC OXIDATION AND AMPEROMETRIC DETERMINATION OF SULFHYDRYL COMPOUNDS AT A COPPER HEXACYANOFERRATE FILM GLASSY-CARBON ELECTRODE IN LIQUID-CHROMATOGRAPHY

Electrocatalytic oxidation of sulfhydryl compounds was effective on a copper hexacyanoferrate (CuHCF) film glassy carbon electrode, at a significantly reduced overpotential (0.55 to 0.65 V) and for a broader pH range (2.0 to 7.0). The electrocatalysis was

DETERMINATION OF CATECHOL DERIVATIVES ON PRETREATMENT AND COPOLYMER COATED GLASSY-CARBON ELECTRODE

A glassy carbon electrode was pretreated electrochemically and was coated with a copolymer of maleic acid anhydride attached with Eastman-AQ55D (MA/AQ). The voltammetric behavior of a series of biologically important compounds, such as dopamine, L-DOPA, D

ANODIC-OXIDATION OF HYDRAZINE ON GLASSY-CARBON ELECTRODES IN ACETONITRILE

The anodic oxidation kinetics of hydrazine on glassy carbon electrodes in acetonitrile were examined by cyclic voltammetry, a rotating ring-disc electrode technique and chronoamperometry. The experimental results of the rotating ring-disc electrode prove that hydrazine is oxidized to HN=NH, which cannot be oxidized further in acetonitrile. Hydrazine molecules are adsorbed on the electrode surface. One-third of the adsorbed hydrazine molecules are oxidized to HN=NH and the other two thirds act as proton acceptors. A possible mechanism of hydrazine oxidation is proposed.

RAPID-DETERMINATION OF KINETIC-PARAMETERS FOR THE ELECTROCATALYTIC REDUCTION OF DIOXYGEN BY VITAMIN-B12 ADSORBED ON GLASSY-CARBON ELECTRODE

In this paper, the electrochemical behavior of vitamin B-12, ie cyanocobalamin (abbr. VB12) in a weak acidic aqueous solution and adsorbed on glassy carbon (GC) surface (abbr. VB12(ad)/GC) in different pH buffer solutions have been described by using cyclic voltammetry (cv). It is found that VB12 and VB12(ad)/GC exhibit catalytic activity for the electroreduction of O2 according to two reduction peaks at -0.50 and -1.00 V vs. sce; but their electrocatalytic activity is very unstable. Based on the method of hydrodynamic amperometry [B. Miller and S. Bruckenstein, J. electrochem. Soc. 117, 1033 (1970)], some kinetic parameters for the electrocatalytic reduction of O2 by VB12(ad)/GC have been determined rapidly by using a linear rotation-scan method [Rongzhong Jiang and Shaojun Dong, Electrochim. Acta 35, 1451 (1990)]. These kinetic parameters indicate that the reduction of O2 on VB12(ad)/GC gives water predominantly in both potential ranges which correspond to those two reduction peaks. Possible reaction mechanisms have been suggested.