Anodic stripping voltammetry enhancement by redox magnetohydrodynamics

The effect of an external magnetic field on linear scan anodic stripping voltammetry (ASV) in solutions of 10(-6)-10(-7) M concentrations of lead, cadmium, and copper at mercury films on glassy carbon electrodes has been investigated. A high concentration of Hg(2+) was added to the analyte solution to induce a large cathodic current during the deposition step. Therefore, a large Lorentz force from the net flux of charge through the magnetic field resulted in convection due to magnetohydrodynamics. The faster delivery of analytes to the mercury film electrode during deposition caused an increase in the anodic stripping peaks. The effect of varying Hg(2+) concentrations (0-60 mM) and magnetic field strengths (0-1.77 T) on the enhancement of the stripping peaks was investigated. Enhancements as large as 129% for peak currents and 167% for peak areas were observed. An enhancement of approximately 100% was observed when 60 mM Fe(3+) replaced high concentrations of Hg(2+). This method of convection exhibits promise for small-volume ASV analysis with possible improved limits of detection and decreased preconcentration times.     

Electrochemical detection of arsenic(III) using iridium-implanted boron-doped diamond electrodes

Iridium-modified, boron-doped diamond electrodes fabricated by an ion implantation method have been developed for electrochemical detection of arsenite (As(III)). Ir+ ions were implanted with an energy of 800 keV and a dose of 10(15) ion cm(-2). An annealing treatment at 850 degrees C for 45 min in H2 plasma (80 Torr) was required to rearrange metastable diamond produced by an implantation process. Characterization was investigated by SEM, AFM, Raman, and X-ray photoelectron spectroscopy. Cyclic voltammetry and flow injection analysis with amperometric detection were used to study the electrochemical reaction. The electrodes exhibited high catalytic activity toward As(III) oxidation with the detection limit (S/N = 3), sensitivity, and linearity of 20 nM (1.5 ppb), 93 nA microM(-1) cm(-2), and 0.999, respectively. The precision for 10 replicate determinations of 50 microM As(III) was 4.56% relative standard deviation. The advantageous properties of the electrodes were its inherent stability with a very low background current. The electrode was applicable for analysis of spiked arsenic in tap water containing a significant amount of various ion elements. The results indicate that the metal-implanted method could be promising for controlling the electrochemical properties of diamond electrodes.     

Bismuth-coated carbon electrodes for anodic stripping voltammetry

Bismuth-coated carbon electrodes display an attractive stripping voltammetric performance which compares favorably with that of common mercury-film electrodes. These bismuth-film electrodes are prepared by adding 400 microg/L (ppb) bismuth(III) directly to the sample solution and simultanously depositing the bismuth and target metals on the glassy-carbon or carbon-fiber substrate. Stripping voltammetric measurements of microgram per liter levels of cadmium, lead, thallium, and zinc in nondeaerated solutions yielded well-defined peaks, along with a low background, following short deposition periods. Detection limit of 1.1 and 0.3 ppb lead are obtained following 2- and 10-min deposition, respectively. Changes in the peak potentials (compared to those observed at mercury electrodes) offer new selectivity dimensions. Scanning electron microscopy sheds useful insights into the different morphologies of the bismuth deposits on the carbon substrates. The in situ bismuth-plated electrodes exhibit a wide accessible potential window (-1.2 to -0.2 V) that permits quantitation of most metals measured at mercury electrodes (except of copper, antimony, and bismuth itself). Numerous key experimental variables have been characterized and optimized. High reproducibility was indicated from the relative standard deviations (2.4 and 4.4%) for 22 repetitive measurements of 80 microg/L cadmium and lead, respectively. Such an attractive use of "mercury-free", environmetally friendly electrodes (with a performance equivalent to that of mercury ones) offers great promise to centralized and decentralized testing of trace metals.     

Measurement of ultrasmall volumes using anodic stripping voltammetry

This paper discusses a new and effective method for small volume determination. The benefits of working with small volumes are numerous and include using less material, generating less waste, increased functionality in less space, portability, and high throughput. As sample size decreases, measuring its volume becomes more challenging. The most prevalent method for small volume determination is visual inspection. The method presented in this paper is precise, more quantitative, and adaptive to many solution geometries, because it is based on a coulometric approach. Demonstration of this method is performed on volumes of approximately 1 nL containing silver and using a self-contained microcavity device that contains multiple electrodes. The silver is "exhaustively" deposited during a cathodic potential step, followed by anodic stripping voltammetry. It is expected that an even faster and more accurate analysis of volumes much smaller than those evaluated here is easily possible using this approach.     

Determination of chromium (VI) using cathodic voltammetry technique in gold microelectrodes

Electrochemical reduction of chromium (VI) is studied on a gold microelectrode assembly (Au-MEA) using the cathodic voltammetry (VA) technique. The optimum conditions for obtaining the analytical signal of Cr (VI) are chosen. The range of the analyzed concentrations of Cr (VI) are 0.005?C0.2 mg/L, and the detection limit is 5 × 10^?5 mg/L. Dissolved oxygen, manifold excesses of alkali and alkaline-earth ions, 200-fold excesses of Cr (III), and 10-fold excesses of Fe (III) and Cu (II) do not interfere with determination of 0.005 mg/L Cr (VI). In the presence of anionic surfactants (DDCNa), the signal of Cr (VI) decreases, but the linear dependence on the concentration remains. A rapid technique for determination of Cr (VI) in purified technical water without sample preparation is suggested. The advantage of Au-MEA consists in the simplicity of synthesis, running time, high sensitivity.     

Anodic stripping voltammetry combined on-line with inductively coupled plasma-MS via a direct-injection high-efficiency nebulizer

A direct-injection high-efficiency nebulizer (DIHEN) is used to couple a thin-layer electrochemical flow cell on-line with an ICP-mass spectrometer to perform anodic stripping voltammetry (ASV) at a thin mercury film followed by subsequent ICPMS measurements for the stripped metal analytes. The resultant hyphenated technique (ASV-DIHEN-ICPMS) is capable of analyzing select heavy metals present at ultratrace levels (down to low-ppt to sub-ppt levels) that are lower than the detection limits obtained by conventional ICPMS. In addition to its good analytical performance, the technique offers other attractive features such as the ability to eliminate detrimental matrix effects that can compromise ICPMS analyses and the possibility of probing electrode reactions involving trace amounts metal species with ICPMS. For conducting ASV on-line with ICPMS, the DIHEN was found to be more advantageous than the microconcentric nebulizer in terms of minimizing memory effects and potential artifacts caused by the erosion of the Hg film into the flowing solution stream. Compared to a direct injection nebulizer (DIN), the DIHEN was easier to operate. Moreover, its simpler design and the lack of back pressure from the DIHEN capillary made it more compatible with coupling to the thin-layer electrochemical cell than a DIN system.     

Development of a method for total inorganic arsenic analysis using anodic stripping voltammetry and a Au-coated, diamond thin-film electrode

We demonstrate that a Au-coated, boron-doped, diamond thin-film electrode provides a sensitive, reproducible, and stable response for total inorganic arsenic (As(III) and As(V)) using differential pulse anodic stripping voltammetry (DPASV). As is preconcentrated with Au on the diamond surface during the deposition step and detected oxidatively during the stripping step. Au deposition was uniform over the electrode surface with a nominal particle size of 23 +/- 5 nm and a particle density of 109 cm-2. The electrode and method were used to measure the As(III) concentration in standard and river water samples. The detection figures of merit were compared with those obtained using conventional Au-coated glassy carbon and Au foil electrodes. The method was also used to determine the As(V) concentration in standard solutions after first being chemically reduced to As(III) with Na2SO3, followed by the normal DPASV determination of As(III). Sharp and symmetric stripping peaks were generally observed for the Au-coated diamond electrode. LODs were 0.005 ppb (S/N = 3) for As(III) and 0.08 ppb (S/N = 3) for As(V) in standard solutions. An As(III) concentration of 0.6 ppb was found in local river water. The relative standard deviation of the As stripping peak current for river water was 1.5% for 10 consecutive measurements and was less than 9.1% over a 10-h period. Excellent electrode response stability was observed even in the presence of up to 5 ppm of added humic acid. In summary, the Au-coated diamond electrode exhibited better performance for total inorganic As analysis than did Au-coated glassy carbon or Au foil electrodes. Clearly, the substrate on which the Au is supported influences the detection figures of merit.     

General theory of cathodic and anodic stripping voltammetry at solid electrodes: mathematical modeling and numerical simulations

Theory is presented to describe the voltammetric signals associated with the stripping phase of stripping voltammetry at solid electrodes. Three mathematical models are considered, and the importance of the hemispherical diffusion associated with electrochemical dissolution of particles in the micrometer range is investigated. Model A considers a "monolayer" system where the coverage at a specific point cannot exceed a maximum value. Model B considers a thin layer of metal or metal oxide, but in contrast to model A, the maximum surface coverage is not restricted. Model C represents the stripping of a "thick layer" where the deposition is also unrestricted.     

Determination of water in room temperature ionic liquids by cathodic stripping voltammetry at a gold electrode

An electrochemical method based on cathodic stripping voltammetry at a gold electrode has been developed for the determination of water in ionic liquids. The technique has been applied to two aprotic ionic liquids, (1-butyl-3-ethylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate), and two protic ionic liquids, (bis(2-hydroxyethyl)ammonium acetate and triethylammonium acetate). When water is present in an ionic liquid, electrooxidation of a gold electrode forms gold oxides. Thus, application of an anodic potential scan or holding the potential of the electrode at a very positive value leads to accumulation of an oxide film. On applying a cathodic potential scan, a sensitive stripping peak is produced as a result of the reduction of gold oxide back to gold. The magnitude of the peak current generated from the stripping process is a function of the water concentration in an ionic liquid. The method requires no addition of reagents and can be used for the sensitive and in situ determination of water present in small volumes of ionic liquids. Importantly, the method allows the determination of water in the carboxylic acid-based ionic liquids, such as acetate-based protic ionic liquids, where the widely used Karl Fischer titration method suffering from an esterification side reaction which generates water as a side product.     

Determination of picomolar levels of iron in seawater using catalytic cathodic stripping voltammetry

A new procedure for the direct determination of picomolar levels of iron in seawater is presented. Cathodic stripping voltammetry (CSV) is preceded by adsorptive accumulation of the iron(III)-2,3-dihydroxynaphthalene (DHN) complex from seawater, containing 20 microM DHN at pH 8.0, onto a static mercury drop electrode, followed by reduction of the adsorbed species. The reduction current is catalytically enhanced by the presence of 20 mM bromate. Optimized conditions include a 60-s adsorption period at -0.1 V and a voltammetric scan using sampled dc modulation at 10 Hz. In these conditions, a detection limit of 13 pM iron in seawater was achieved which can be lowered further by extending the adsorption time to 300 s. The new catalytic CSV method is approximately 5 times more sensitive than existing CSV methods and was tested on samples from the Atlantic Ocean.     

Direct evidence of arsenic(III)-carbonate complexes obtained using electrochemical scanning tunneling microscopy

Electrochemical scanning tunneling microscopy (ECSTM), ion chromatography (IC), and electrospray ionization-mass spectrometry/mass spectrometry were applied to investigate the interactions between arsenite [As(III)] and carbonate and arsenate [As(V)] and carbonate. The chemical species in the single and binary component solutions of As(III), As(V), and carbonate were attached to a Au(111) surface and then imaged in a 0.1 M NaClO4 solution at the molecular level by ECSTM. The molecules formed highly ordered adlayers on the Au(111) surface. High-resolution STM images revealed the orientation and packing arrangement of the molecular adlayers. Matching the STM images with the molecular models constructed using the Hyperchem software package indicated that As(III) formed two types of complexes with carbonate, including As(OH)2CO3- and As(OH)3(HCO3-)2. No complexes were formed between As(V) and carbonate. IC chromatograms of the solutions revealed the emergence of the new peak only in the aged As(III)-carbonate solution. MS spectra showed the presence of a new peak at m/z 187 in the aged As(III)-carbonate solution. The results obtained with the three independent methods confirmed the formation of As(OH)2CO3-. The results also indicated that As(OH)3 could be associated with HCO3- through a hydrogen bond. The knowledge of the formation of the As(III) and carbonate complexes will improve the understanding of As(III) mobility in the environment and removal of As(III) in water treatment systems.     

Diagnostics of anodic stripping mechanisms under square-wave voltammetry conditions using bismuth film substrates

A mechanistic study to provide diagnostics of anodic stripping electrode processes at bismuth-film electrodes is presented from both theoretical and experimental points of view. Theoretical models for three types of electrode mechanisms are developed under conditions of square-wave voltammetry, combining rigorous modeling based on integral equations and the step function method, resulting in derivation of a single numerical recurrent formula to predict the outcome of the voltammetric experiment. In the course of the deposition step, it has been assumed that a uniform film of the metal analyte is formed on the bismuth substrate, in situ deposited onto a glassy carbon electrode surface, without considering mass transfer within either the bismuth or the metal analyte film. Theoretical data are analyzed in terms of dimensionless critical parameters related with electrode kinetics, mass transfer, adsorption equilibria, and possible lateral interactions within the deposited metal particles. Theoretical analysis enables definition of simple criteria for differentiation and characterization of electrode processes. Comparing theoretical and experimental data, anodic stripping processes of zinc(II), cadmium(II), and lead(II) are successfully characterized, revealing significant differences in their reaction pathways. The proposed easy-to-perform diagnostic route is considered to be of a general use while the bismuth film exploited in this study served as a convenient nonmercury model substrate surface.     

Characterization of arsenic (V) and arsenic (III) in water samples using ammonium molybdate and estimation by graphite furnace atomic absorption spectroscopy

Arsenic (V) is known to form heteropolyacid with ammonium molybdate in acidic aqueous solutions, which can be quantitatively extracted into certain organic solvents. In the present work, 12-molybdoarsenic acid extracted in butan-1-ol is used for quantification of As (V). Total arsenic is estimated by converting arsenic (III) to arsenic (V) by digesting samples with concentrated nitric acid before extraction. Concentration of As (III) in the sample solutions could be calculated by the difference in total arsenic and arsenic (V). The characterization of arsenic was carried out by GFAAS using Pd as modifier. Optimization of the experimental conditions and instrumental parameters was investigated in detail. Recoveries of (90-110%) were obtained in the spiked samples. The detection limit was 0.2 microg l(-1). The proposed method was successfully applied for the determination of trace amount of arsenic (III) and arsenic (V) in process water samples.     

Factors controlling stripping voltammetry of lead at polycrystalline boron doped diamond electrodes: new insights from high-resolution microscopy

We report wide-ranging studies to elucidate the factors and issues controlling stripping voltammetry of metal ions on solid electrodes using the well-known Pb/Pb(2+) couple on polycrystalline boron doped diamond (pBDD) as an exemplar system. Notably, high-resolution microscopy techniques have revealed new insights into the features observed in differential pulse anodic stripping voltammetry (DPV-ASV) which provide a deeper understanding of how best to utilize this technique. DPV-ASV was employed in an impinging wall-jet configuration to detect Pb(2+) in the nanomolar to micromolar concentration range at a pBDD macrodisk electrode. The deposition process was driven to produce a grain-independent homogeneous distribution of Pb nanoparticles (NPs) on the electrode surface; this resulted in the observation of narrow stripping peaks. Lower calibration gradients of current or charge versus concentration were found for the low concentrations, correlating with a lower than expected (from consideration of the simple convective-diffusive nature of the deposition process) amount of Pb deposited on the surface. This was attributed to the complex nature of nucleation and growth at solid surfaces in this concentration regime, complicating mass transport. Furthermore, a clear shift negative in the stripping peak potential with decreasing concentration was seen correlating with a change in the size of the deposited NP, suggesting an NP size-dependent redox potential for the Pb/Pb(2+) couple. At high concentrations a nonlinear response was observed, with less Pb detected than expected, in addition to the observation of a second stripping peak. Atomic force microscopy (AFM) and field emission scanning electron microscopy revealed the second peak to be due to a change in deposition morphology from isolated NPs to grain-independent heterogeneous structures comprising both thin films and NPs; the second peak is associated with stripping from the thin-film structures. AFM also revealed a substantial amount of Pb remaining on the surface after stripping at high concentration, explaining the nonlinear relationship between stripping peak current (or charge) and concentration. Finally, the use of an in situ cleaning procedure between each measurement was advocated to ensure a clean Pb-free surface (verified by AFM and X-ray photoelectron spectroscopy analysis) between each run. The studies herein highlight important and complex physicochemical processes involved in the electroanalysis of heavy metals at solid electrodes, such as pBDD, that need to be accounted for when using stripping voltammetry methods.     

On-chip electric field driven electrochemical detection using a poly(dimethylsiloxane) microchannel with gold microband electrodes

An external electric field driven in-channel detection technique for on-chip electrochemical detection in micro fabricated devices is described based on a microfluidic system containing an array of 20 microband electrodes. It is shown that an external electric field induces a potential difference between two gold microband electrodes in a poly(dimethylsiloxane) (PDMS) microchannel, and that this enables the electrochemical detection of electroactive species such as ascorbic acid and Fe(CN) 6 (4-). The results, which are supported by simulations of the behavior of the microband electrodes in the microfluidic system, show that the induced potential difference between the electrodes can be controlled by altering the external electric field or by using different microbands in the microband array. As the obtained currents depend on the concentrations of electroactive species in the flowing solution and the detection can be carried out anywhere within the channel without interference of the external electric field, the present approach significantly facilitates electrochemical detection in capillary electrophoresis. This approach consequently holds great promise for application in inexpensive portable chip-based capillary electrophoresis (CE) devices.     

Fabrication of nanoscale gold disk electrodes using ultrashort pulse etching

A novel method for preparing a nanometer-sized gold disk electrode is described. The electrode was fabricated by electrochemical etching of a gold wire, insulating the etched wire with a varnish with the exception of the apex, and cutting down the apex by applying ultrashort (40 ns) etching pulses in an HCl solution. Cyclic voltammograms of a 0.10 M ferrocyanide solution recorded at the electrode showed that a series of etching pulses gradually reduced the diffusion-limited current at the electrode by etching its apex. Characterization by cyclic voltammetry and scanning electron microscopy revealed the transformation of a cone-shaped apex into a disk-shaped electrode with a radius of typically between 50 and 250 nm.     

Salicylate-selective electrodes based on AI(III) and Sn(IV) salophens

New salicylate-selective electrodes based on aluminum-(III) and tin(IV) salophens are described. The electrodes were prepared by incorporating the ionophores into plasticized poly(vinyl chloride) (PVC) membranes, which were directly coated on the surface of graphite electrodes. These novel electrodes display high selectivity for salicylate with respect to many common inorganic and organic anions. The influence of membrane composition and pH and the effect of lipophilic cationic and anionic additives on the response properties of the electrodes were investigated. The electrode based on aluminum salophen, with 32% PVC, 65.8% plasticizer, and 2.2% ionophore, shows the best potentiometric response characteristics and displays a linear log [Sal-] vs EMF response over the concentration range 1 x 10(-6) - 1 x 10(-1) M in phosphate buffer solutions of pH 7.0, with a Nernstian slope of -59.2 mV/decade of salicylate concentration. Highest selectivity was observed for the membrane incorporating 38.8% PVC, 57.3% plasticizer, 2.6% Sn(salophen), and 1.3% sodium tetraphenylborate. The electrodes exhibit fast response times and micromolar detection limits (approximately 1 x 10(-6) M salicylate) and could be used over a wide pH range of 3-8. Applications of the electrodes for determination of salicylate in pharmaceutical preparations and biological samples are reported.     

Imprinted polymer-based sensor system for herbicides using differential-pulse voltammetry on screen-printed electrodes.

A sensor system for the herbicide 2,4-dichlorophenoxy-acetic acid has been developed based on specific recognition of the analyte by a molecularly imprinted polymer and electrochemical detection using disposable screen-printed electrodes. The method involves a competitive binding step with a nonrelated electrochemically active probe. For batch binding assays, imprinted polymer particles are incubated in suspension with the analyte and the probe, followed by centrifugation and quantification of the unbound probe in the supernatant. Two different compounds, namely 2,4-dichlorophenol and homogentisic acid, were tested as potential electroactive probes. Both compounds could be conveniently detected by differential-pulse voltammetry on screen-printed, solvent-resistant three-electrode systems having carbon working electrodes. Whereas 2,4-dichlorophenol showed very high nonspecific binding to the polymer, homogentisic acid bound specifically to the imprinted sites and thus allowed calibration curves for the analyte in the micromolar range to be recorded. An integrated sensor was developed by coating the imprinted polymer particles directly onto the working electrode. Following incubation of the modified electrode in a solution containing the analyte and the probe, the bound fraction of the probe is quantified. This system provides a cheap, disposable sensor for rapid determination of environmentally relevant and other analytes.     

彼阳新盖口服液中钙锌含量测定

本文研究了乳酸钙,葡萄糖酸锌含量测定,采用同位镀汞阳极溶出法在pH=5.8的HAc—NaAc缓冲溶液中,测定补锌制剂中锌的含量.线性范围0.02～0.08μg/ml,检出限为0.08μg/ml.以EDTA掩蔽Zn~(2+),间接KMnO_4法测定钙的含量。本文提出的方法是一种不经分离而能直接测定乳酸钙和葡萄糖酸锌的简便易行的方法.     

Label-free determination of picogram quantities of DNA by stripping voltammetry with solid copper amalgam or mercury electrodes in the presence of copper

Highly sensitive label-free techniques of DNA determination are particularly interesting in relation to the present development of the DNA sensors. We show that subnanomolar concentrations (related to monomer content) of unlabeled DNA can be determined using copper solid amalgam electrodes or hanging mercury drop electrodes in the presence of copper. DNA is first treated with acid (e.g., 0.5 M perchloric acid), and the acid-released purine bases are directly determined by the cathodic stripping voltammetry. Volumes of 5-3 microL of acid-treated DNA can easily be analyzed, thus making possible the determination of picogram and subpicogram amounts of DNA corresponding to attomole and subattomole quantities of 1000-base pair DNA. Application of this determination in DNA hybridization detection is demonstrated using surface H for the hybridization (superparamagnetic beads with covalently attached DNA probe) and the mercury electrodes only for the determination of DNA selectively captured at surface H.