Unprecedented stable aqueous semiquinone methide radical formation interferes with adsorptive cathodic stripping voltammetry of cobalt methyl thymol blue

A putatively highly sensitive and selective method for the determination of cobalt in aqueous samples by catalytic adsorptive cathodic stripping voltammetry using methyl thymol blue (MTB) as the ligand has been documented [A. Safavi, E. Shams, Talanta 51 (2000) 1117] and its underlying mechanism has been briefly explored [A. Safavi, E. Shams, Electroanalysis 14 (2002) 708]. In an attempt to adapt the method for application in metalloprotein analysis we obtained erratic results, which were traced down to the redox non-innocence of the free ligand in the potential range prescribed for the metal analysis. On the hanging mercury drop electrode free methyl thymol blue is reversibly one-electron reduced to the semiquinone form with E_m,7.0 = −482 mV versus NHE at 22 °C, and the radical is subsequently quasi-reversibly one-electron reduced to the quinol form with E_m,7 ≈ −0.9 V. This observation invalidates the use of MTB in electrochemical analysis of metal ions. This is also the first observation ever of a stable quinone methide radical in aqueous solution.     

High sensitive determination of trace amount of cobalt by catalytic adsorptive stripping voltammetry

A very sensitive and selective catalytic adsorptive cathodic stripping procedure for trace measurements of cobalt is presented. The method is based on adsorptive accumulation of cobalt-CCA (calcon carboxylic acid) complex onto a hanging mercury drop electrode followed by reduction of the adsorbed species by voltammetric scan using differential pulse modulation. The reduction current is enhanced catalytically by nitrite. The effect of various parameters such as pH, concentration of CCA, concentration of nitrite, accumulation potential and accumulation time on the selectivity and sensitivity were studied. The optimum condition for the analysis of cobalt, include pH 5.2 (Acetate buffer), 2.1 μM clacon carboxylic acid, 0.032 M sodium nitrite and an accumulation potential of 0.05 V (versus Ag/AgCl). Under these optimum conditions and for an accumulation time of 60 s, the measured peak current at −0.480 V is proportional to the concentration of cobalt over the entire concentration range tested 0.003–2.0 ng ml⁻¹ with a detection limit of 1 pg ml⁻¹ for an accumulation time of 60 s and 2.0–10.0 ng ml⁻¹ for an accumulation time of 40 s. The relative standard deviations for ten replicate measurement of 0.5 ng ml⁻¹ of cobalt were 3.1%. The main advantage of this new system is the microtrace Co(II) determination by ASV. The method was applied to determination of cobalt in a water sample and some analytical grade salts with satisfactory results. Published in Elektrokhimiya in Russian, 2009, Vol. 45, No. 2, pp. 221–228. The article is published in the original.     

Chromium speciation study in polluted waters using catalytic adsorptive stripping voltammetry and tangential flow filtration

The catalytic adsorptive stripping voltammetry (CAdSV) has been applied to physico-chemical chromium speciation study in the upper Dunajec catchment, severely polluted by the tannery wastewater. The method is based on the adsorptive preconcentration of the Cr(III)-diethylenetriammine-N,N,N′,N″,N″-pentaacetic acid (DTPA) complex and the utilization of the catalytic reaction in the presence of nitrate. Under optimized conditions the CAdSV enables the oxidation state speciation study of Cr content by direct determination of Cr(VI) in the presence of the predominant Cr(III) concentration with the detection limit for chromium(VI) of 0.08 nM and the linearity range from 0.1 to 80 nM obtained for 20 s of accumulation, as well as the determination of total Cr after UV oxidation of Cr(III) to Cr(VI). Due to the difference in the chemical properties of different chromium species the CAdSV method makes possible a speciation study of Cr(III) and Cr(VI) oxidation state. The RSD of the determination of Cr(VI) and Cr(III) varies from 0.5 to 5%. It has been proved that in natural water in which strong complexants of Cr(III) such a humid acids are presented, Cr(VI) can be determined accurately in the presence of high excess of Cr(III). Fractionation of selected water samples with tangential flow filtration (TFF, cut-off 10 and/or 1 kDa) provides insight into physical Cr speciation, i.e. partitioning of the Cr(VI) and Cr(III) between the colloidal and the dissolved fractions. It has been shown that the content of the Cr species in the Dunajec river depends on the season, and is significantly higher in autumn and winter during the most intensive tanneries production processes. The concentration of total Cr exceeds occasionally the legally admissible level. A large fraction of total Cr(III) concentration is associated with the colloidal material, while Cr(VI) occurs solely in the truly dissolved form.     

Determinationation of kanamycin by square-wave cathodic adsorptive stripping voltammetry

A square-wave cathodic adsorptive stripping voltametric (SWCASV) method for the determination of kanamycin was developed on a thin-film mercury electrode (TFME). The optimal working conditions for the application of the method were found to be pH 8.0, provided by a Britton-Robinson (B.R.) buffer, and an adsorption potential of +0.30 V during 300 s. The equilibration time was applied during 10 s, and potential scans were performed at a scan rate of 40 mV/s, with a square-wave frequency of 100 Hz. The measuring-system response was linear over the kanamycin concentration range from 1.2 × 10⁻⁹ to 5.0 × 10⁻⁸ M and the detection limit achieved was 4.8 × 10⁻¹⁰ M. The relative error and relative standard deviation obtained were 1.20 and 4.67%, respectively. The voltammetric procedure was applied successfully to give a rapid and precise assay of kanamycin in kanamycin sulfate injection form. Published in Elektrokhimiya in Russian, 2008, Vol. 44, No. 12, pp. 1433–1437. The text was submitted by author in English     

Determination of traces of cobalt in the presence of nioxime and cetyltrimethylammonium bromide by adsorptive stripping voltammetry

A sensitive method of Co(II) determination by adsorptive stripping voltammetry is presented. The method exploits the enhancement of cobalt peak current observed in the system Co(II)-nioxime-cetyltrimethylammonium bromide-piperazine-N,N′-bis(2-ethanesulfonic acid). The calibration plot for an accumulation time of 60 s is linear from 5 × 10⁻¹¹ to 3 × 10⁻⁹ mol L⁻¹. The relative standard deviation is 3.8% for Co(II) determination at concentration 1 × 10⁻⁹ mol L⁻¹. The detection limit is 1.7 × 10⁻¹¹ mol L⁻¹. The validation of the method is performed by the analyses of certified reference materials and comparing the result of Co(II) determination in river water sample by the proposed method with those obtained by ET AAS. The main advantage of this new system is the micro-trace Co(II) determination by adsorptive stripping voltammetry, as compared to those described before, a low concentration of the supporting electrolyte used, and so commercially available reagents without additional purification can be used.     

Catalytic adsorptive stripping voltammetry determination of ultra trace amount of molybdenum using factorial design for optimization

A highly sensitive procedure is presented for the determination of ultra-trace concentration of molybdenum by catalytic adsorptive stripping voltammetry. The method is based on adsorptive accumulation of the molybdenum (Mo)-pyrocatechol violet (PCV) complex on to a hanging mercury drop electrode, followed by reduction of the adsorbed species by voltammetric scan using differential pulse modulation. The reduction current is enhanced catalytically by chlorate. The influence of variables was completely studied by factorial design analysis. Optimum analytical conditions for the determination of molybdenum were established. Molybdenum can be determined in the range 1.0 × 10⁻³-100.0 ng ml⁻¹ with a limit of detection of 0.2 pg ml⁻¹. The influence of potential interfering ions on the determination of molybdenum was studied. The procedure was applied to the determination of molybdenum in mineral water and some analytical grade substances with satisfactory results.     

Simultaneous determination of nickel(II) and cobalt(II) by square wave adsorptive stripping voltammetry on a rotating-disc bismuth-film electrode

This works reports the use of square-wave adsorptive stripping voltammetry (SWAdSV) for the simultaneous determination of Ni(II) and Co(II) on a rotating-disc bismuth-film electrode (BFE). The metal ions in the non-deoxygenated sample were complexed with dimethylglyoxime (DMG) and the complexes were accumulated by adsorption on the surface of the BFE. The stripping step was carried out by using a square-wave potential-time voltammetric excitation signal. Electrochemical cleaning of the bismuth film was employed, enabling the same bismuth film to be used for a series of measurements. The experimental variables (choice of the working electrode substrate, the presence of oxygen, the DMG concentration, the buffer concentration, the preconcentration potential, the accumulation time, the rotation speed and the SW parameters) as well as potential interferences were investigated and the figures of merit of the methods were established. Using the selected conditions, the 3σ limits of detection were 70 ng l⁻¹ for Co(II) and 100 ng l⁻¹ for Ni(II) (for 300 s of preconcentration) and the relative standard deviations were 2.3% for Co(II) and 3.9% for Ni(II) at the 2 μg l⁻¹ level (n = 8). Finally, the method was applied to the determination of nickel and cobalt in real samples with satisfactory results.     

Determination of secnidazole in tablets and human serum by cathodic adsorptive stripping voltammetry

The electrochemical reduction of secnidazole was carried out in BR buffer solutions in the pH range 2.0–11.8 by dc polarography. The polarograms exhibited two irreversible reduction waves in acidic media and one wave in alkaline media, corresponding to the reduction of nitro group in the drug. The cathodic adsorptive voltammetric behavior was studied on glassy carbon electrode to optimize an analytical method for determination of secnidazole. The drug was determined in the range between 4.0 × 10^?6 and 1.2 × 10^?4 mol L^?1. The proposed method was successfully applied to the determination of the drug content in tablets with mean recovery and relative standard deviation of 100.91% and 1.82%, respectively. It was also applied to human serum with a good precision and accuracy.     

Catalytic adsorptive stripping voltammetric measurements of trace vanadium at bismuth film electrodes

Bismuth-coated glassy carbon electrodes have been successfully applied for catalytic adsorptive stripping voltammetric measurements of low levels of vanadium(V) in the presence of chloranilic acid (CAA) and bromate ion. The new protocol is based on the accumulation of the vanadium-chloranilic acid complex from an acetate buffer (pH 5.5) solution at a preplated bismuth film electrode held at −0.35 V (versus Ag/AgCl), followed by a square-wave voltammetric scan. Factors influencing the adsorptive stripping performance, including the CAA and bromate concentrations, solution pH, and accumulation potential or time have been optimized. The response compares favorably with that observed at mercury film electrodes. A linear response is observed over the 5-25 μg/L concentration range (2 min accumulation), along with a detection limit of 0.20 μg/L vanadium (10 min accumulation). High stability is indicated from the reproducible response of a 50 μg/L vanadium solution (n = 25; R.S.D. = 3.1%). Applicability to a groundwater sample is illustrated.     

Determination of trace europium by adsorptive cathodic stripping voltammetry after complexation with cupferron

This article describes the determination of Eu by adsorptive cathodic stripping voltammetry after complexation with N-nitroso-N-phenylhydroxylamine (cupferron). The accumulation of the complex at the HMDE was performed at 0.0 V and the subsequent potential scan was made in the square wave mode. The analyte signal occurred at −0.88 V. The detection limit is 0.06 nmol dm⁻³. The effect of instrumental and chemical parameters on the peak height and potential was investigated. The same technique can be used for the determination of ytterbium and of the other rare earth elements (REEs) after separation.     

Determination of gallium by adsorptive stripping voltammetry

A procedure for the determination of gallium by differential pulse adsorptive stripping voltammetry (DPADSV), using different complexing agents (ammonium pyrrolidine dithiocarbamate (APDC), pyrocatechol violet (PCV) and diethyldithiocarbamate (DDTC)), has been optimized. The selection of the experimental conditions was made using experimental design methodology. Under these conditions, the calibration was made and the detection limit was determined for each gallium-ligand complex. A robust regression method was applied which allowed the elimination of anomalous points. The detection limit, with α=β=0.05, for gallium-APDC complex was 5.0×10⁻⁸ mol dm⁻³, for gallium-PCV complex was 9.9×10⁻⁹ mol dm⁻³, and the lowest detection limit (1.3×10⁻⁹ mol dm⁻³) was obtained with DDTC. For this reason, DDTC was selected for the determination of the gallium concentration in a certificate sample and in a spiked tap water sample. The linear dynamic range for gallium-APDC complex was from 5.0×10⁻⁸ to 2.7×10⁻⁷ mol dm⁻³, for gallium-PCV complex was from 5.0×10⁻⁹ to 4.8×10⁻⁷ mol dm⁻³, and for gallium-DDTC complex was from 1.0×10⁻⁹ to 2.1×10⁻⁷ mol dm⁻³.     

Determination of tryptophan and histidine by adsorptive cathodic stripping voltammetry using H-point standard addition method

A sequential method is proposed for the determination of tryptophane and histidine by adsorptive cathodic stripping voltammetry using standard addition and H-point standard addition method (HPSAM). The complexes of copper(II) with the amino acids were accumulated onto the surface of a hanging mercury drop electrode for 60 s. Then the preconcentrated complexes were reduced by square wave voltammetry and the peak currents were measured. The effect of various parameters such as pH, concentration of copper, accumulation potential, accumulation time and scan rate on the sensitivity were studied by one-at-a time and artificial neural network. Under the optimized conditions, the peak currents at about +0.05 to −0.30 V is proportional to the concentration of tryptophan and histidine over the concentration ranges of 5–220 and 100–1200 nM, respectively. Optimization of the parameters by one-at-a time showed that at accumulation potential of 0.10 V (versus Ag/AgCl reference electrode) the peak current is proportional only to the concentration of tryptophan and histidine does not have any contribution to the current. The optimization results by artificial neural network showed that at accumulation potential of −0.06 V (versus Ag/AgCl) the peak current is proportional to the both concentrations of tryptophan and histidine. Therefore, the method of H-point standard addition has been used for resolving overlap voltamograms for determination of histidine in the present of tryptophane. The method was successfully applied to the determination of tryptophan and histidine in synthetic and real samples.     

Signal enhancement in adsorptive stripping voltammetry of Pt by forced convection during the measurement step

Adsorptive stripping voltammetry (‘formazone-method’) is already known as one of the most sensitive methods for platinum analysis with a detection limit in the low picograms range. In this work, it is shown that the detection limit can be lowered even more by one order of magnitude to 0.2 pg (=1 fmol) Pt in 15 mL electrolyte, corresponding to 68 fmol/L, by applying forced convection during the stripping step of the voltammetric measurement. The sensitivity of the method (given in nA/pg Pt) is enhanced by a factor of 3-5 (in differential pulse mode and 15 mL vial), up to a factor of 30 (using square-wave mode and 3 mL vial). The maximum enhancement factor is limited by the maximum stirrer speed, which can be applied without negative effects on the hanging mercury drop electrode.To check for similar enhancement effects in other types of stripping methods, the behaviour of adsorptive stripping voltammetry for Pt is compared to conventional anodic stripping voltammetry (ASV) of lead, and to adsorptive stripping voltammetry of nickel and cobalt using their dimethylglyoxime (DMG) complexes. No enhancement effect is observed in ASV of lead upon stirring, and the nickel-DMG-system exhibits only a smaller enhancement factor of about 1.5. A reasonable explanation of the higher signal enhancement in the catalytic Pt-formazone-system is the mass transport of reaction products, namely hydrogen, away from the working electrode during the catalytic hydrogen evolution cycle.     

Advantages of using a mercury coated, micro-wire, electrode in adsorptive cathodic stripping voltammetry

A mercury coated, gold, micro-wire electrode is used here for the determination of iron in seawater by catalytic cathodic stripping voltammetry (CSV) with a limit of detection of 0.1 nM Fe at a 60 s adsorption time. It was found that the electrode surface is stable for extended periods of analyses (at least five days) and that it is reactivated by briefly (2 s) applying a negative potential prior to each scan. Advantages of this electrode over mercury drop electrodes are that metallic mercury use is eliminated and that it can be readily used for flow analysis. This is demonstrated here by the determination of iron in seawater by continuous flow analysis. It is likely that this method can be extended to other elements. Experiments using bismuth coated, carbon fibre, electrodes showed that the bismuth catalyses the oxidation of the important oxidants bromate and hydrogen peroxide, which makes it impossible to use bismuth based electrodes for catalytic CSV involving these oxidants. For this reason mercury coated electrodes retain a major advantage for catalytic voltammetric analyses.     

Experimental designs in the development of a new method for the sensitive determination of cadmium in seawater by adsorptive cathodic stripping voltammetry

A new differential pulse adsorptive cathodic stripping voltammetric (DPAdCSV) method for the direct determination of cadmium at subnanomolar levels in saline waters based on metal complexation with 2-acetylpyridine salicyloylhydrazone (APSH) and subsequent adsorptive deposition onto a hanging mercury drop electrode (HMDE) is presented. A study strategy based on experimental designs has been followed. Operating conditions were improved with exploratory (Plackett-Burman) and surface response (central composite) experimental designs, involving several chemical and instrumental parameters (pH, ligand concentration, pulse amplitude, time interval for voltage step, voltage step, deposition potential and deposition time). Analytical parameters as repeatability, linearity and accuracy were also investigated and a detection limit (DL) of 0.06 nM was achieved which could be lowered by extending the adsorption time. The interference of other metals and major salts present in seawater was also studied. The method was validated with reference water samples: NIST-SRM 1643d and BCR-CRM 505, showing good concordance with the certified values.     

Square wave adsorptive stripping voltammetric determination of famotidine in urine

Electrochemical studies of famotidine were carried out using voltammetric techniques: cyclic voltammetry, linear sweep and square wave adsorptive stripping voltammetry. The dependence of the current on pH, buffer concentration, nature of the buffer, and scan rate was investigated. The best results for the determination of famotidine were obtained in MOPS buffer solution at pH 6.7. This electroanalytical procedure enabled to determine famotidine in the concentration range 1 × 10⁻⁹–4 × 10⁻⁸ mol L⁻¹ by linear sweep adsorptive stripping voltammetry (LS AdSV) and 5 × 10⁻¹⁰–6 × 10⁻⁸ mol L⁻¹ by square wave adsorptive stripping voltammetry (SW AdSV). Repeatability, precision and accuracy of the developed methods were checked. The detection and quantification limits were found to be 1.8 × 10⁻¹⁰ and 6.2 × 10⁻¹⁰ mol L⁻¹ for LS AdSV and 4.9 × 10⁻¹¹ and 1.6 × 10⁻¹⁰ mol L⁻¹ for SW AdSV, respectively. The method was applied for the determination of famotidine in urine.     

Electrocatalytic adsorptive voltammetry for fludarabine determination in urine

Fludarabine (F-Ara-A, 9-β-d-arabinofuranosyl-2-fluoroadenine) is an arabinosyl nucleoside with potent cytotoxic activity against chronic lymphocytic leukemia. Herein, its electrochemical behavior is investigated on pyrolytic graphite electrodes (PGEs); and a novel catalytic adsorptive differential-pulse-voltammetric method for its determination is developed based on the strong electrocatalytic effect on nicotinamide adenine dinucleotide (NADH) oxidation exhibited by the oxidation products of F-Ara-A. This approach was applied to human urine samples, which required a previous solid-phase extraction. The F-Ara-A recovery was of 89.7%. The method proposed is fast, simple and permits detection of F-Ara-A concentrations up to 72 h after infusion.     

Determination of cobalt by adsorptive stripping voltammetry with double accumulation and stripping steps

A double accumulation and stripping steps were proposed to increase the sensitivity of Co(II) determination by catalytic adsorptive stripping voltammetry (AdSV). Electrodes with large and small surface area were used for the first and second accumulation step, respectively. As the accumulation of Co(II) complex at the first electrode was finished, the electrode was placed at a short distance opposite the second one. Then the Co(II) complex desorbed from the first electrode was accumulated at the second electrode. Taking into account the small volume of space between the electrodes, the concentration of the Co(II) complex in solution between the electrodes was drastically higher than that in the bulk solution. The accumulation step at the second electrode was performed from the solution with higher concentration of Co(II) and therefore the detection limit was lowered. The calibration graph of Co(II) determination for accumulation time of 120 s at both electrodes was linear from 1.18 to 58.9 ng L^?1. The detection limit for Co(II) was equal to 0.47 ng L^?1 and it is so far the lowest detection limit obtained for Co(II) using mercury-free electrodes. The proposed method was applied to Co(II) determination in water certified reference material.     

Electrochemical determination of rosiglitazone by square-wave adsorptive stripping voltammetry method

Square-wave adsorptive stripping voltammetry technique was used to determine rosiglitazone (ROS) on the hanging mercury dropping electrode (HMDE) surface, in Britton Robinson buffer, pH = 5. The voltammetric cathodic peak was observed at ?1520 mV vs. Ag/AgCl reference electrode. The voltammetric peak response was characterized with respect to pH, supporting electrolyte, accumulation potential, preconcentration time, scan rate, frequency, pulse amplitude, surface area of the working electrode and the convection rate. Under optimal conditions, the voltammetric current is proportional to the concentration of ROS over the concentration range of 5 × 10^?8–8 × 10^?7 mol l^?1 (r = 0.9899) with a detection limit of 3.2 × 10^?11 mol l^?1 using 120 s accumulation time. The developed SW-AdSV procedure showed a good reproducibility, the relative standard deviation RSD% (n = 10) at a concentration level of 5 × 10^?7 mol l^?1 was 0.33%, whereas the accuracy was 101% ± 1.0. The proposed method was successfully applied to assay the drug in the human urine and plasma samples with mean recoveries of 90 ± 0.71% and 86 ± 1.0%, respectively.     

Determination of traces molybdenum by catalytic adsorptive stripping voltammetry

A reliable and very sensitive procedure for the determination of ultra trace of molybdenum is proposed. Molybdenum was determined by cathodic stripping differential pulse voltammetry based on the adsorption collection of the Mo(VI)-Tiron complex on a hanging mercury drop electrode (HMDE). The variation of peak current with pH, concentration of Tiron and chlorate, plus several instrumental parameters such as accumulation time, accumulation potential and scan rate, were optimized. Under optimized condition, the relationship between the peak current and molybdenum concentration is linear in the range of 0.010-21.0 ng ml⁻¹. The limit of detection was found to be 0.006 ng ml⁻¹. The relative standard deviation for 10 replicates determination of 0.6 and 10 ng ml⁻¹ Mo(VI) is equal to 1.3 and 0.9%, respectively. The method was applied to the determination of molybdenum in river water, tap water, well water, plant foodstuff samples such as cucumber, tomato, carrot, and certified steel reference materials.