Contribution of migration current to the voltammetric deposition and stripping of lead with and without added supporting electrolyte at a mercury-free carbon fibre microdisc electrode

In order to assess the contribution and analytical significance of migration, electrochemical studies on the deposition and stripping of lead at a carbon fibre microelectrode (diameter of 10 μm) have been undertaken in aqueous solutions containing 1 mM lead ions with variable KNO₃ supporting electrolyte concentrations (10⁻¹ to 10⁻⁵ M), as well as in the total absence of deliberately added supporting electrolyte. The methodology involved the application of cyclic voltammetry to characterise the Pb²⁺ (solution)+2e⁻Pb (metal) process in both the reduction (Pb deposition) and stripping (Pb dissolution) directions. The use of a mercury-free carbon surfaces means that the lead stripping does not occur from the amalgam state, as is commonly the case in anodic stripping voltammetry. In the deposition step, the current rises sharply with potential in response to a lead nucleation-growth process and then reaches an almost potential independent limiting value. The stripping step, obtained on the reverse scan, exhibited oxidation peak currents resulting from the redissolution or stripping of the metal from the electrode surface. The influence of the electrolyte concentration and hence migration current at −0.8 V versus Ag/AgCl for the deposition process, as well on the redissolution peak current and the dependence of the voltammograms on scan rate (10–1000 mV) are discussed. Interestingly, neither deposition limiting nor stripping peak currents vary in a simple manner with added supporting electrolyte concentrations, with maximum values being observed at 10⁻⁵ M rather than zero concentration of added KNO₃. An important implication for the voltammetric determination of lead in low ionic strength media by the very sensitive technique of anodic stripping voltammetry is that use of the method of standard additions commonly employed to minimise unknown matrix problems, is prone to error when contributions to the process from migration current are important.     

Development of an anodic stripping voltammetric assay, using a disposable mercury-free screen-printed carbon electrode, for the determination of zinc in human sweat

This paper reports on the development of a novel electrochemical assay for Zn²⁺ in human sweat, which involves the use of disposable screen-printed carbon electrodes (SPCEs). Initially, SPCEs were used in conjunction with cyclic voltammetry to study the redox characteristics of Zn²⁺ in a selection of supporting electrolytes. The best defined cathodic and anodic peaks were obtained with 0.1 M NaCl/0.1 M acetate buffer pH 6.0. The anodic peak was sharp and symmetrical which is typical for the oxidation of a thin metal film on the electrode surface. This behaviour was exploited in the development of a differential pulse anodic stripping voltammetric (DPASV) assay for zinc. It was shown that a deposition potential of −1.6 V versus Ag/AgCl and deposition time of 60 s with stirring (10 s equilibration) produced a well-defined stripping peak with E_pa = −1.2 V versus Ag/AgCl. Using these conditions, the calibration plot was linear over the range 1 × 10⁻⁸ to 5 × 10⁻⁶ M Zn²⁺. The precision was examined by carrying out six replicate measurements at a concentration of 2 × 10⁻⁶ M; the coefficient of variation was calculated to be 5.6%. The method was applied to the determination of the analyte in sweat from 10 human volunteers. The concentrations were between 0.39 and 1.56 μg/mL, which agrees well with previously reported values. This simple, low-cost sensitive assay should have application in biomedical studies and for stress and fatigue in sports studies.     

Determination of naringin in grapefruit juice by cathodic stripping differential pulse voltammetry at the hanging mercury drop electrode

A highly sensitive cathodic stripping voltammetric method for the determination of naringin is presented. It is based on the formation and accumulation of two naringin–mercury complexes at the electrode surface, followed by reduction of the surface species during a differential pulse voltammetric scan. The cathodic stripping responses at −0.25 V and −0.42 V, are evaluated with respect to various experimental conditions, such as composition and pH of the supporting electrolyte, naringin concentration, accumulation potential and preconcentration time. The new method is suitable for the determination of naringin concentrations between 0.1 mg l⁻¹ (1.72×10⁻⁷ mol l⁻¹) and 40 mg l⁻¹ (6.88×10⁻⁵ mol l⁻¹). A 3σ limit of detection of 32 μg l⁻¹ (55 nmol l⁻¹) can be reached. The relative standard deviation (r.s.d.) is <1.5%. Recovery experiments yielded a mean recovery of 97% (r.s.d.=4.1%). The application of the procedure to the selective determination of naringin in grapefruit juice is demonstrated.     

Determination of trace metoclopramide by anodic stripping voltammetry with nafion modified glassy carbon electrode

A novel method is described for determination of metoclopramide (MCP) by second-derivative adsorptive anodic stripping voltammetry with a nafion-modified electrode. The stripping peak current is proportional to the concentration of MCP over the range 1.2×10⁻⁹–4.6×10⁻⁷ M. The detection limit is 8.0×10⁻¹¹ M with 4-min accumulation. The method has been successfully applied to the determination of MCP in human serum.     

Bismuth film electrode for anodic stripping voltammetric determination of tin

The bismuth film electrode (BiFE), in combination with anodic stripping voltammetry, offers convenient measurement of low concentrations of tin. The procedure involves simultaneous in situ formation of the bismuth film electrode on a glassy carbon substrate electrode, together with electrochemical deposition of tin, in a non-deaerated model solution containing bismuth ions, catechol as complexing agent and the metal analyte, followed by an anodic stripping scan. The BiFE is characterized by an attractive electroanalytical performance, with two distinct voltammetric stripping signals corresponding to tin, accompanied with low background contributions. Several experimental parameters were optimized, such as concentration of bismuth ions and catechol, deposition potential, deposition time and pH of the model solution. In addition, a critical comparison is given with bare glassy carbon and mercury film electrodes, revealing the superior characteristics of BiFE for measurement of tin. BiFE exhibited highly linear behavior in the examined concentration range from 1 to 100 μg L⁻¹ of tin (R² = 0.997), an LoD of 0.26 μg L⁻¹ tin, and good reproducibility with a calculated R.S.D. of 7.3% for 10 μg L⁻¹ tin (n = 10). As an example, the practical applicability of BiFE was tested with the measurement of tin in a real sample of seawater.     

Electrochemistry at carbon fibers : Part 1. Characteristics of the Mercury Film Carbon Fiber Electrode in Differential Pulse Anodic Stripping Voltammetry

Carbon fibers are proposed as a support electrode for a mercury film electrode. The response of these electrodes is evaluated for use in differential pulse anodic stripping voltammetry. The mercury film is deposited in situ in aqueous solution and used to quantify cadmium in solutions of cadmium salts and organo cadmium compounds in the 1–10 μg l^-1 (ppb) concentration range. The good resolution and extremely low background current obtained allow a limit of detection at 0.04 μg Cd l^-1.     

Simultaneous derivative spectrophotometric determination of thorium and uranium in a micellar medium

Derivative photometric methods for trace analysis of Th(IV) and UO₂(II), and their simultaneous determination in mixtures using 5,8-dihydroxy-1,4-naphthoquinone in a micellar medium are reported. Molar absorptivity and Sandell's sensitivity of 1:2 Th(IV) and 1:1 UO₂(II) complexes at their λ_max, 614.5 nm and 637.0 nm are, 1.19 × 10⁴ 1/mol/cm and 1.12 × 10⁴ 1/mol/cm and 1.95 × 10⁻² μg/cm² and 2.13 × 10⁻² μg/cm² μg/cm², respectively. Calibration graph is linear over the range 9.28 × 10⁻²−18.56 μg/ml of Th(IV) and 9.52 × 10⁻²−19.04 μg/ml of UO₂(II). Though presence of Th(IV) and UO₂(II) causes interference in each others determination, 9.28 × 10⁻¹−9.28 μg/ml Th(IV) and 9.52 × 10⁻¹−9.52 μg/ml UO₂(II) when present together, can be simultaneously determined using derivative spectra.     

Simultaneous determination of trace uranium(VI) and zinc(II) by adsorptive cathodic stripping voltammetry with aluminon ligand

Uranium(VI) complexed with aluminon (3-[bis(3-carboxy-4-hydroxy-phenyl)methylene]-6-oxo-1,4-cyclohexadiene-1-carboxylic acid triammonium salt) was determined by adsorptive cathodic stripping voltammetry (ACSV) using a hanging mercury drop electrode. Trace uranium(VI) and zinc(II) can be simultaneously determined in a single scan in the presence of aluminon and urea. Optimal conditions were found to be: accumulation time; 180–200 s, accumulation potential; 50 mV versus Ag/AgCl, scan rate; 40 mV s⁻¹, supporting electrolyte; 0.1 M sodium acetate buffer at pH 6.5–7.0, and concentration of aluminon; 1×10⁻⁶ M. The linear range of uranium(VI) and zinc(II) were observed over the concentration range 2–33 and 30–120 ng ml⁻¹, respectively. The detection limit (S/N=3) are 0.2 ng ml⁻¹ (uranium) and 30 ng ml⁻¹ (zinc). A good reproducibility shows RSDs of 2.5–4.0% (n=10). The procedure offers high selectivity, with the presence of urea masking some metal ions.     

Micellar effect on the sensitivity of spectrophotometric Mo(VI) determination based on the formation of gallic acid complex providing evidence for the polyoxoanion structure of molybdate ions

In this study effects of anionic (sodium dodecyl sulfate, SDS), cationic (cetyltrimethylammonium bromide, CTAB) and nonionic (Triton X-100, TX100) micelles on the sensitivity of spectrophotometric molybdenum(VI) (Mo) determination based on the formation of a binary complex with gallic acid (GA) were investigated.

Micellar CTAB was found to enhance the formation of Mo–GA complex. SDS micelles exerted an inhibitory effect while TX100 micelles had no effect on the complex formation. By the optimization of experimental conditions, the determination limit of the method suggested in the literature was lowered from 5.2 × 10⁻⁵ to 4.6 × 10⁻⁶ and to 5.7 × 10⁻⁷ M, in the absence and presence of CTAB, respectively.

The mechanism of the effect of CTAB was investigated by spectrophotometric titrations and it was concluded that CTAB did not form a ternary complex with Mo and GA. The stoichiometry of the complex, deduced from the results of spectrophotometric titrations, provided evidence for the formation of para-Mo₇O₄⁶⁻ polyanions at pH 4.5, indicating to the formation of a charge transfer complex between these ions and GA in micellar medium.     

Exploiting flow injection and sequential injection anodic stripping voltammetric systems for simultaneous determination of some metals

Flow injection (FI) and sequential injection (SI) systems with anodic stripping voltammetric detection have been exploited for simultaneous determination of some metals. A pre-plated mercury film on a glassy carbon disc electrode was used as a working electrode in both systems. The same film can be repeatedly applied for at least 50 analysis cycles, thus reducing the mercury consumption and waste. A single line FI voltammetric system using an acetate buffer as a carrier and an electrolyte solution was employed. An injected standard/sample zone was mixed with the buffer in a mixing coil before entering a flow cell. Metal ions were deposited on the working electrode by applying a potential of −1.1 V vs Ag/AgCl reference electrode. The stripping was performed by anodically scanning potential of working electrode to +0.25 V, resulting a voltammogram. Effects of acetate buffer concentration, flow rate and sample volume were investigated. Under the selected condition, detection limits of 1 μg l⁻¹ for Cd(II), 18 μg l⁻¹ for Cu(II), 2 μg l⁻¹ for Pb(II) and 17 μg l⁻¹ for Zn(II) with precisions of 2–5% (n=11) were obtained. The SI voltammetric system was similar to the FI system and using an acetate buffer as a carrier solution. The SI system was operated by a PC via in-house written software and employing an autotitrator as a syringe pump. Standard/sample was aspirated and the zone was then sent to a flow cell for measurement. Detection limits for Cd(II), Cu(II), Pb(II) and Zn(II) were 6, 3, 10 and 470 μg l⁻¹, respectively. Applications to water samples were demonstrated. A homemade UV-digester was used for removing organic matters in the wastewater samples prior to analysis by the proposed voltammetric systems.     

Simultaneous determination of arsenic, antimony and selenium by gas-phase diode array molecular absorption spectrometry, after preconcentration in a cryogenic trap

This paper describes a method for the simultaneous determination of As(III), Sb(III) and Se(IV) by combining hydride generation and gas phase molecular absorption spectrometry. A system for continuous hydride generation has been designed and developed, based on the use of a double process of gas-liquid separation, and optimal compromise operation conditions for the three compounds have been found. After generation, the hydrides are collected in a liquid nitrogen cryogenic trap, and then evaporated and driven to the flow cell of a diode array spectrophotometer, in which the transient signals over the 190–250 nm wavelength interval are measured. Under the recommended conditions (sample flow: 35 ml min⁻¹, 0.5 M HCl; reductor flow: 4 ml min⁻¹ of 4% NaBH₄, solution) linear response ranges above 50 μg 1⁻¹ for As(III), 30 μg 1⁻¹ for Sb(III) and 200 μg 1⁻¹ for Se(IV) are obtained with detection limits of 22 μg 1⁻¹, 15 μg 1⁻¹ and 65 μg 1⁻¹, respectively. Multiwavelength linear regression equations were used for the simultaneous determination of the three elements in different synthetic samples, with good precision and accuracy and to study simultaneously the interference from different chemical species for the three compounds. Results were similar to those obtained by other techniques using hydride generation.     

Differential pulse anodic stripping voltammetry detection of metallothionein at bismuth film electrodes

As a representation of metalloproteins, metallothionein (MT), which plays important biological and environmental roles such as in the metabolism and detoxification of some metals, was detected at bismuth film electrode (BiFE) by differential pulse anodic stripping voltammetry (DPASV). In pH 2–5.5, two well-defined anodic peaks were produced and attributed to the Zn²⁺ and Cd²⁺ inherent to MT. The calibration plot of DPASV peak currents for Cd²⁺ inherent to MT versus MT concentrations showed a good linearity with a detection limit of 3.86 × 10⁻⁸ mol/L for MT. As a non-toxic excellent electrode material, BiFE shows good performance for detecting MT, and is expected to find further applications in the studies of many other metalloproteins.     

Spectrophotometric determination of molybdenum in steels with o-nitrophenylfluorone and cetyltrimethylammonium bromide

A method has been developed for determining microamounts of molybdenum(VI) in aqueous solution by means of the Mo-o-nitrophenylfluorone—cetyltrimethylammonium bromide system, in which micellar solubilization is applied. A red complex is formed in 0.2–0.6M hydrochloric acid medium. The sensitivity of the method is high, and the apparent molar absorptivity is 1.55 × 10⁵ l.mole⁻¹. cm⁻¹. The absorption peak of the complex appears at 530 nm. The colour of the complex develops quickly and is stable for more than 24 hr. The composition of the complex is Mo: o-NPF = 1:1, and the system obeys Beer's law in the range 0–10 μg of Mo per 25 ml. The method has been used for the rapid determination of molybdenum in alloy steels with satisfactory results.     

Simultaneous determination of chromium(III) and cadmium(II) by differential pulse anodic stripping voltammetry on a stannum film electrode

A stannum film electrode has been developed for the simultaneous determination of trace levels of chromium(III) and cadmium(II) by differential pulse anodic stripping voltammetry (DPASV). The stannum film electrode was generated in situ by depositing simultaneously the stannum film and the metals obtained by reduction of Cd(II) and Cr(III) at −1.4 V on a glassy carbon electrode. Then, the reduced products were oxidized by scanning the potential of the electrode from −1.4 to −0.4 V using DPASV. The electrode exhibited well-defined and separated stripping signals for both metals accompanied with a low background contribution. The possible mechanism of this design was proposed. Under the optimized working conditions, the detection limit was 2.0 and 1.1 μg l⁻¹ for Cr(III) and Cd(II) at a deposition time of 3 min. Finally, the stannum film electrode was successfully applied to the determination of Cd(II) in tap water with satisfactory results.     

Electrocatalytic response of norepinephrine at a thiolactic acid self-assembled gold electrode

The thiolactic acid (TLA) self-assembled monolayer modified gold electrode (TLA/Au) is demonstrated to catalyze the electrochemical response of norepinephrine (NE) by cyclic voltammetry. A pair of well-defined redox waves were obtained and the calculated standard rate constant (k_s) is 5.11×10⁻³ cm s⁻¹ at the self-assembled electrode. The electrode reaction is a pseudo-reversible process. The peak current and the concentration of NE are a linear relationship in the range of 4.0×10⁻⁵–2.0×10⁻³ mol l⁻¹. The detection limit is 2.0×10⁻⁶ mol l⁻¹. By ac impedance spectroscopy the apparent electron transfer rate constant (k_app) of Fe(CN)³⁻/Fe(CN)⁴⁻ at the TLA/Au electrode was obtained as 2.5×10⁻⁵ cm s⁻¹.     

Diagnosis of copper ions in vascular tracts using a fluorine-doped carbon nanotube sensor

The voltammetric assay of Cu(II) was investigated using a carbon nanotube electrode (CNE) and fluorine immobilized onto a carbon nanotube electrode (FCNE) in cyclic voltammetry (CV), square-wave (SW) stripping voltammetry, and chronoamperometry. Optimum SW conditions were attained at working ranges of 0.01–0.11 ng L⁻¹ Cu(II) (11 points), and a relative standard deviation of 1.68% (RSD, n = 15) was observed at 10.0 μg L⁻¹ Cu(II). Within a 200 s accumulation time, detection limit of 0.006 μg L⁻¹ was attained. The life span of each electrode was more than 1 month. The sensor was applied to tap water, blood, and rat tail vascular (in vivo). It was found that the sensor could be used with an interface system in the assay of live cells and non-treated blood.     

End-column electrochemical detection for inorganic and organic species in high-voltage capillary electrophoresis

The electronics and construction for an end-column ultramicroelectrode (3–10 μm) detection system that permits the use of medium-sized capillaries (25 μm I.D.) without appreciable effects from the potential field at the end of the capillary. Normal peak-to-peak noise over 10 s was 0.01–0.1 pA. The background noise observed for a 200 × μm carbon-fiber electrode placed either 180 μm within a 25-μm capillary or at a point 500 μm away from the capillary was essentially the same. A study of detector response as a function of the position of the electrode has shown that accurate location of the electrode is important for sensitive and reproducible detection. These studies also showed that differences between the density of the electrolyte existing the capillary and the electrolyte in the detection cell could cause anomalous electrode response depending on the location of the electrode relative to the end of the capillary. Application of a carbon fiber or an Hg film electrode gave detection limits (twice the peak-to-peak noise over 10 s) of 2 · 10⁻⁸ mol/l for Pb²⁺, 1 · 10^{− 5} mol/l for NO₂⁻ and 5 · 10⁻¹⁰ mol/l for catechol.     

The determination of molybdenum in water and biological samples by graphite furnace atomic spectrometry after polyurethane foam column separation and preconcentration

A system for molybdenum separation and enrichment aiming its determination in water and biological samples by graphite furnace atomic absorption spectrometry (GFAAS) is proposed. The procedure is based on the sorption of the molybdenum (VI) thiocyanate complex onto a mini-column packed with polyurethane foam (PUF). The elution is accomplished by a 3.0 mol l⁻¹ nitric acid solution. Flow variables were optimized and an enrichment factor of 10 as well as a limit of detection (LOD) (3 s) of 0.08 μg l⁻¹ in the sample solution were achieved. The coefficient of variation showed values of 3 and 2% for molybdenum solutions of 2.0 and 10.0 μg l⁻¹, respectively. The accuracy of the method was confirmed by the good concordance between found and certified values in the analysis of certified reference materials (CRMs) (CASS-3 Nearshore Seawater, NIST 1547 Peach Leaves, NIST 1515 Apple Leaves and NIST 1572 Citrus Leaves). The procedure was also applied for the molybdenum determination in mineral waters as well as in produced water samples. The results obtained for the mineral water samples compared well with those obtained by ICP-MS. Concerning the produced water samples, in spite of their large salinity, recoveries of 90 to 120% at the 1 μg l⁻¹ were observed.     

Selective determination of airborne hexavalent chromium using inductively coupled plasma mass spectrometry

A new method for determining ultra-trace levels of hexavalent chromium in ambient air has been developed. The method involves a 24-h sampling of air into potassium hydroxide solution, followed by silica gel column separation of chromium (VI), then preconcentration by complexation and solvent extraction. The chromium (VI) complex was dissolved in nitric acid. The resultant chromium ions were determined by inductively coupled plasma mass spectrometry (ICP–MS) using a dynamic reaction cell (DRC) with ammonia as the reactive gas to reduce polyatomic interferences. The interconversion of chromium in potassium hydroxide solution and air sample matrix were investigated under ambient conditions. It was found that there was no conversion of chromium (VI) into chromium (III) species. However, it was observed that some chromium (III) species were converted into chromium (VI) species. For a KOH solution containing 100 μg l⁻¹ of chromium (III) species, the rate of conversion was found to be 3% after 24 h exposure, 8% after 48 h, 10% after 72 h and no further conversion was observed thereafter. However, in a solution containing air sample matrix, 9.3% of chromium (III) converted to chromium (VI) within 6 h, and during the course of a 11-day exposure period, 13% (range 8–17%) of chromium (III) converted to chromium (VI). The method detection limit (MDL) for chromium (VI) in potassium hydroxide solution (0.025 M) was found to be 2×10⁻² μg l⁻¹. This is equivalent to 0.2 ng m⁻³ (for 23 m³ air sampled into 200 ml of KOH solution over a 24-h period). The recovery of spiked chromium (VI) from solutions containing air sample matrix was 95±9% (n=8). Matrix related interferences were estimated to be less than 10% based on recovery studies. The concentration of airborne chromium (VI) in Sydney residential areas was found to be less than 0.2 ng m⁻³, however, in industrial areas the concentrations ranged from 0.2 to 1.3 ng m⁻³ using this analytical procedure.     

Correction of iron interface in the spectrophotometric flow injection catalytic determination of molybdenum in plants

Iron interference in the spectrophotometric catalytic determination of molybdenum based on the iodide-hydrogen peroxide reaction can be corrected by using sulphosalicylic acid as masking and color-forming reagent. The catalytic influence of iron ions is circumvented to the extent of about 90% and correction of any remaining iron ions is possible by monitoring the colored iron(III)-salicylate complex at 490 nm. In this way, iron is also determined. With the proposed system, molybdenum can be determined in plant and food digests within the 0–100 μg Mo 1⁻¹ range in the presence of up to 25 mg Fe 1⁻¹, at a sampling rate of about 50 determinations h⁻¹. The relative standard deviation of 10 consecutive measurements was estimated as < 2%. Results for samples were comparable with those obtained by graphite furnace atomic absorption spectrometry. In addition, recoveries within the range 94–100% were calculated.