Sensitive and selective determination of mercury by differential pulse stripping voltammetry after accumulation of mercury vapour on a gold plated graphite electrode

A selective and sensitive method is proposed for the determination of mercury by anodic stripping voltammetry after its preconcentration from the gas phase. Mercury from the sample solution is reduced to elemental Hg by SnCl₂ and volatilized by the bubbles of a carrier gas. The gas containing mercury vapour is dried and passed through a capillary onto a gold coated graphite electrode. An anodic stripping voltam-mogram is recorded from 0.1 mol/1 HClO₄ + 3 × 10⁻³ mol/1 HCl solution. The calibration curve is linear from 1 × 10⁻⁹ to 4 × 10⁻⁸ mol/1 Hg(NO₃)₂. The absolute detection limit is 0.46 ng Hg. The relative standard deviations for 4 × 10⁻⁹ mol/1 and 2 × 10⁻⁸ mol/1 Hg(NO₃)₂ are 9.8% and 6.1%, respectively n = 5).     

Gas-permeation continuous flow coulometric analysis: determination of sulphur dioxide

 A gas permeation system using two gaseous streams flowing on both sides of a membrane is developed. This gas permeation device and a coulometric detector are adapted for the continuous measurement of relatively high concentrations of sulphur dioxide. The interferences of other gases (NO₂, NO and CO₂) can be eliminated by using a scrubber behind the gas permeation device in the acceptor stream. The effects of the donor flow rates and gas pressure as well as the membrane thickness on the signal are discussed. The relative standard deviation is 1.3% (n=7) for 2.002×10^-3 mol/mol certified sulphur dioxide. Received: 19 July 1996/Revised: 22 October 1996/Accepted: 29 October 1996     

Determination of mercury(II) traces in drinking water by inhibition of an urease reactor in a flow injection analysis (FIA) system

 The integration of an urease reactor into a gas diffusion flow injection system was investigated for the determination of urease inhibitors. The enzyme was immobilized by entrapping in polyacrylamide gel. Besides copper and silver ions mercury ions inhibit the conversion of urea to carbon dioxide and ammonia catalysed by urease. The pH change of the carrier solution caused by the ammonia released was measured potentiometrically with a pH electrode. The inhibition behaviour of Hg(II) ions was investigated. A linear range from 2 to 20 μg L^-1 Hg(II) was obtained after a 90 s inhibition, with a correlation coefficient of r=0.9997. The relative standard deviation was 1.4% for five measurements of 2 μg L^-1Hg(II). A sample frequency of 7 h^-1 was achieved. The inhibited enzyme can be reactivated. The method was applied to the determination of Hg(II) in two drinking water samples. Received: 16 April 1996/Revised: 3 June 1996/Accepted: 11 June 1996     

Chelating resin preconcentration and Hg(II) assisted elution followed by differential-pulse anodic stripping voltammetry for the determination of Cd, Cu, and Pb in seawater

A new and efficient Hg(II) back-elution method for the desorption of Cd, Cu, and Pb from Chelex-100 chelating resin was developed. A smaller eluent volume and shorter elution time can be achieved using an Hg(II) containing eluent rather than pure nitric acid. Owing to the remaining Hg(II) ion in the effluent, a mercury thin-film electrode is formed in-situ during the anodic stripping voltammetric determination without any further addition of Hg(II). The results indicate that all the analytes in seawater matrix can be completely adsorbed on Chelex-100 resin from the sample at pH 6.5, and subsequently eluted from the resin with an acid solution of 5 × 10^–4 mol/L Hg²⁺ + 1 mol/L HClO₄. The detection limits obtained from the differential-pulse anodic (μg L^–1 to ng L^–1) stripping voltammetry are at sub-ppb to ppt (μg L^–1 to ng L^–1) levels permitting to determine Cd, Cu and Pb traces in seawater. The analytical reliability was confirmed by the analysis of the certified reference material CASS-II (open ocean seawater). Received: 22 April 1997 / Revised: 5 August 1997 / Accepted: 7 August 1997     

Selective method for the determination of gold by anodic stripping voltammetry

A highly selective method for the deter- mination of gold by anodic stripping voltammetry is described. For preconcentration a glassy carbon electrode, activated by deposition of small amounts of gold before the measurement, is proposed. The Au³⁺ reduction process at such an electrode is effective starting with the potential +0.4 V vs. Ag/AgCl electrode. A linear dependence of the current of the gold stripping peak on the gold concentration was obtained in the range from 5×10⁻⁸ to 1×10⁻⁶ mol/l. The relative standard deviation for 2×10^-7 mol/l HAuCl₄ was 4.2% (n=5). The detection limit was 4×10^-9 mol/l. The accuracy of the method was verified by the determination of gold in reference materials.     

Novel surface acoustic wave-interdigitated array electrode gas sensor for dissolved ammonia

 A novel surface acoustic wave-interdigitated array electrode (SAW-IDA) ammonia gas sensor is proposed. A gas-permeable membrane is employed to separate the buffer solution in the inner cell of the gas-sensing probe from the sample solution in the detection cell. The response of the IDA conductive electrodes is based on the impedance change of the buffer solution during ammonia adsorption. Therefore, this gas sensor overcomes the influence of water vapour in the conventional film-coated SAW gas sensor and can be used for the detection of gases in aqueous solutions. The ammonia sensor exhibits a favourable frequency response to 5×10^-7–1×10^-3 mol/l ammonia. The optimal buffer composition and probe parameters have been determined. Dynamic range, response time, selectivity, and temperature drift are discussed. The ammonia sensor was also applied to the determination of serum ammonia. Results were in good agreement with those from the conventional enzymatic-spectrophotometric method. Received: 1 December 1995/Revised: 9 April 1996/Accepted: 14 April 1996     

Sensitive determination of nucleotides and polynucleotides based on the fluorescence quenching of the Tb3+-Tiron complex

 A sensitive method using fluorescence quenching for the determination of nucleotides (ATP, ADP, AMP, CTP, UTP) and polynucleotides[poly(A), poly(I), poly(U)] is proposed. It is based on the ability of nucleotides and polynucleotides to inhibit the formation of a strongly fluorescent complex of Tb³⁺ ion with Tiron. The possibilities of spectrofluorimetric measurements of these systems were studied under optimal conditions (pH 6.9 in hexamethylene tetramine-HCl buffer, 1.2×10^-6 mol/L of Tb³⁺, 4.0×10^-6 mol/L of Tiron, λ_ex=317 nm, λ_em=546 nm). The results showed that the Tb³⁺-Tiron complex could be used as a fluorescence test for the phosphate moieties of nucleotides and polynucleotides. The detection limits are 0.3, 1.2, 3.7, 0.2, 0.3, 1.1, 0.6 and 0.9 ng/mL for ATP, ADP, AMP, CTP, UTP, poly(A), poly(I), and poly(U), respectively. The relative standard deviations (6 replicates) are within 4.0% in the middle of the linear range. The fluorescence quenching mechanism of these systems is also discussed. Received: 16 July 1996 / Revised: 13 November 1996 / Accepted: 13 November 1996     

Vermiculite clay mineral as an effective carbon paste electrode modifier for the preconcentration and voltammetric determination of Hg(II) and Ag(I) ions

A vermiculite modified carbon paste electrode (VMCPE) was employed for the in situ preconcentration of traces of Hg(II) and Ag(I) via an ion-exchange route. Heavy metal ions were accumulated in Britton-Robinson (BR) buffer pH 7 for Hg(II) and pH 6 for Ag(I), and afterwards reduced at –0.7 V vs. Ag/AgCl in the separate measurement solution (BR buffer pH 5 + 0.05 mol/L NaNO₃) prior to the anodic stripping square-wave voltammetric (ASSWV) detection. For Hg(II) ions, at 15 min accumulation, a linear range from 1.0 × 10^–7 to 8.0 × 10^–6 mol/L was obtained, with a 5.7 × 10^–8 mol/L limit of detection. The VMCPE response was linear for Ag(I) ions in the concentration range from 2.0 × 10^–7 to 8.0 × 10^–6 mol/L, at 10 min accumulation with a corresponding limit of detection of 6.3 × 10^–8 mol/L. The relative standard deviation of the analytical procedure including accumulation from a 5 × 10^–7 mol/L solution of Hg (15 min) or Ag(I) (10 min), electrolysis, ASSWV detection, regeneration and activation of the VMCPE, was 4% (n = 6). The optimisation of the parameters for the application of the VMCPE in combination with ASSWV detection is presented and discussed. Received: 10 July 1997 / Revised: 31 October 1997 / Accepted: 3 November 1997     

A study of HPLC separation and spectrophotometric and voltammetric detection of 4′-substituted derivatives of 3-carboxy-4-hydroxy-6-acetylaminoazobenzene

 Optimised conditions have been found for the separation of 3-carboxy-4-hydroxy-6-acetylaminoazo-benzene derivatives substituted in the position 4′ (4′-R-CHAAB, where R=–H, –CH₃, –OCH₃, –Cl, –COCH₃, –NO₂ and –NHCOCH₃) using reversed phase HPLC with a C18 chemically bonded stationary phase. Suitable mobile phases are mixtures of 0.01 mol/L NaH₂PO₄ at pH 4 with methanol (1+1), 0.01 mol/L NaH₂PO₄ at pH 2 with acetonitrile (1+1) or 1% aqueous acetic acid with methanol (4+6). UV photometry is the most universal detection technique and yields limits of detection around 10^-6 mol/L. Direct anodic voltammetry on a glassy carbon fibre array detector yields lower limits of detection for –COCH₃ derivatives and higher limits of detection for –NO₂ and –NHCOCH₃ derivatives. When the analytes are chemically reduced using zinc powder in acetic acid, the voltammetric detection has limits of detection one order of magnitude lower than those obtained UV photometrically. Received: 27 June 1996/Revised: 25 October 1996/Accepted: 3 November 1996     

Adsorptive stripping voltammetric analysis of 2,4,6-tri(2′pyridyl)-1,3,5-triazine at a mercury electrode

The redox behaviour of adsorbed species of 2,4,6-tri(2′-pyridyl)-1,3-5-triazine (TPTZ) at the surface of a mercury electrode was examined using cycling voltammetry. Based on the adsorption and accumulation of TPTZ at the charged mercury surface cathodic adsorptive stripping voltammetry (CSV) is applied for the trace determination of TPTZ using a differential pulse (DP) technique. Experimental and operational parameters for the quantitative analysis of TPTZ are optimized and the detection limit was found to be 3×10⁻⁹ mol/l. The effect of some interferences, e.g. organic compounds, cations and anions, are investigated.     

Determination of methylmercury in the presence of inorganic mercury by anodic stripping voltammetry

Methylmercury is determined in a non-complexing nitrate medium by differential pulse anodic stripping voltammetry at a gold film electrode, with a detection limit of 2 × 10^-8 mol l^-1 for 5-min plating times. A method of double standard additions is proposed for determining methylmercury in the presence of mercury(II) ions.     

Improvement of an old spectrophotometric method for the microdetermination of ascorbic acid by the use of a micellar medium

₃]²⁺. Whereas the initial method is carried out in an aqueous solution, the improved one employs an aqueous micellar medium formed by the cationic surfactant cetylpyridinium bromide (CPBR). The λmax in both methods is the same, i.e. 510 nm. The mean apparent molar absorptivity (ɛ) and Sandell’s sensitivity (Ss) were calculated as ɛ=2.10×10⁴ Lmol^-1 cm^-1 and Ss=8.37 ng cm^-2 for the initial procedure, and ɛ=2.62×10⁴ Lmol^-1 cm^-1 and Ss=6.72 ng cm^-2 for the improved one. The regression line equation for the improved method was: A=1.487×10^-1C −1.415×10^-2 (r=0.9998). The accuracy and the precision of the improved method were investigated and the conclusions were satisfactory. The results obtained for ASCA by both the described method and an official one, were statistically compared by means of the Student’s t-test and by the variance ratio F-test; and no significant difference was observed. Received: 15 January 1996/Revised: 7 March 1996/Accepted: 13 March 1996     

Microwave-radiated synthesis of gold nanoparticles/carbon nanotubes composites and its application to voltammetric detection of trace mercury(II)

Gold nanoparticles/carbon nanotubes (Au-NPs/CNTs) composites were rapidly synthesized by microwave radiation, and firstly applied for the determination of trace mercury(II) by anodic stripping voltammetry (ASV). The structure and composition of the synthesized Au-NPs/CNTs nanocomposites were characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), UV–vis absorption spectroscopy and cyclic voltammetry. Au-NPs/CNTs nanocomposites modified glassy carbon electrode (Au-NPs/CNTs/GCE) exhibited excellent performance for Hg(II) analysis. A wide linear range (5 × 10⁻¹⁰–1.25 × 10⁻⁶ mol/L) and good repeatability (relative standard deviation of 1.84%) were obtained for Hg(II) detection. The limit of detection was found to be 3 × 10⁻¹⁰ mol/L (0.06 μg/L) at 2 min accumulation, while the World Health Organization’s guideline value of mercury for drinking water is 1 μg/L, suggesting the proposed method may have practical utility.     

Simultaneous separation and microdetermination of cobalt(II), nickel(II) and copper(II)

 A sensitive and selective flotation procedure for the separation of microamounts of Co(II), Ni(II) and Cu(II) separately or in admixture is described. The maximum separation rate (∼1) for 0.1 mmol/L of each analyte is achieved using 1 mmol/L of both oleic acid (HOL) surfactant and 4-phenylthiosemicarbazide (HPTS) as a collector in the pH range 6–7. A method for the simultaneous separation and microdetermination of the analytes is elaborated, based on adding excess HPTS and floating the species with HOL at pH ∼6. The filtrate (which is clear brownish-yellow) obtained from the scum is used for the spectrophotometric determination of Co(II) at 350 nm. The formation constants of 1:1 and 1:2 [Co(II):HPTS] species are 6.9×10⁵ and 1.22×10¹⁰ L mol^-1, respectively. Beer’s law is obeyed up to 9 μg/mL of Co(II) with a molar absorptivity of 1.15×10⁴ L mol^-1 cm^-1. The precipitate in the scum layer is quantitatively collected, dissolved in aqua regia and aspirated directly into the flame for the (AAS) determination of Ni and Cu. The procedure is successfully applied to some natural water samples. A mechanism for the separation of the analytes is proposed. Received: 23 January 1996/Revised: 1 April 1996/Accepted: 9 April 1996     

Investigations on Bioanalytical Chemistry Part I. On Differential Pulse Voltammetry of Bilirubin

Abstract

The electrochemical behaviour of the bilirubin in many kinds of supporting electrolytes on a glassy carbon electrode (GCE) and a hanging mercury drop electrode (HMDE) was investigated by means of anodic or cathodic differential pulse voltammetry. The influences of different methods of pre-treatment of the glassy carbon electrode was also discussed. In Na₂B₄.O₇-KH₂PO₄ buffer solution, the linear range was 2×10^?9-1×10^?9 mol/l and the detection limit was 3.3×10^?9 mol/l by anodic differential pulse voltammetry at GCE. A linear relationship holds between the peak current and the concentration of bilirubin in a concentration range of 1×10^?9-4×10^?7 mol/l with good precision and accuracy, and the limit of detection was 2×10^?10 mol/l, when cathodic differential pulse adsorption voltammetry at HMDE was used.     

Direct determination of traces of silver in mercury by voltammetry at the hanging mercury drop electrode in acetonitrile medium

A direct method for the determination of silver in mercury is described. The sample of mercury is introduced into the container of the hanging mercury drop electrode and the anodic voltammograms are recorded in a 0.1 M lithium perchlorate solution in acetonitrile. The anodic peak of silver obtained under these conditions is well separated from the mercury dissolution current. The peak height is proportional to silver concentration over the wide range 2 × 10^?6 mol dm^?3 (1.6 × 10^?6%) to at least 2.0 × 10^?2 mol dm^?3. No prior separation is needed; the procedure requires less than 20 min. The diffusion coefficient of silver in mercury was determined at several temperatures. It was found that silver in mercury does not form intermetallic compounds with copper, lead, thallium, cadmium, tin and bismuth.     

Determination of mercury ions on a diphenylcarbazone bulk modified graphite electrode

A bulk modified electrode prepared by pressing a mixture of carbon powder and diphenylcarbazone at 15.2 MPa was used for the determination of mercury in aqueous solutions. Mercury(II) ions are concentrated by complexation with the modifier in acidic (HNO₃) solution. After exchange of the medium for 1 M HCl and 1 M CaCl₂, mercury is reduced at ?0.8 V vs. SCE. The signal is generated by anodic stripping in the differential-pulse mode. The calibration graph is linear in the range 5×10^?8?5× 10^?6 M with a relative standard deviation of 7%. After enrichment for 10 min the detection limit is 5×10^?8 M. Silver, chromate and strong complexing agents interfere. The use of the electrode to determine the labile fraction in mercury speciation is discussed.     

Determination of the pesticide Chlorpyrifos by cathodic adsorptive stripping voltammetry

The adsorption behavior and differential pulse cathodic adsorptive stripping voltammetry of the pesticide Chlorpyrifos (CP) were investigated at the hanging mercury drop electrode (HMDE). The pesticide was accumulated at the HMDE and a well-defined stripping peak was obtained at –1.2 V vs Ag/AgCl electrode at pH 7.50. A voltammetric procedure was developed for the trace determination of Chlorpyrifos using differential pulse cathodic adsorptive stripping voltammetry (DP-CASV). The optimum working conditions for the determination of the compound were established. The peak current was linear over the concentration range 9.90 × 10^–8– 5.96 × 10^–7 mol/L of Chlorpyrifos. The influence of diverse ions and some other pesticides was investigated. The analysis of Chlorpyrifos in commercial formulations and treated waste water was carried out satisfactorily Received: 10 July 1997 / Revised: 1 April 1998 / Accepted: 6 April 1998     

Silica-modified electrode for the selective detection of mercury

Pure silica particles were dispersed within carbon paste and the resulting modified electrode was applied to the selective voltammetric detection of mercury(II) species after their accumulation at open circuit. The remarkable selectivity observed between pH 4 and 7 was attributed to the intrinsic adsorption mechanism which involves a condensation reaction between mercury(II) hydroxide and hydroxyl groups on the silica surface, leading to the formation of an inner-sphere-type surface complex. After optimization with respect to the electrode composition, the detection medium, and the voltammetric scan mode, a linear response was obtained in the concentration range between 2 × 10⁻⁷ M to 1 × 10⁻⁵ M, by applying anodic stripping square wave voltammetry. Various silica samples were used and their sorption behavior was discussed in relation to their specific surface area and porosity. The effect of chloride and pH on the accumulation of mercury(II) on silica was also investigated. Received: 4 September 1999 / Accepted: 5 January 2000     

Evaluation of on-line coupling size exclusion chromatography electrothermal atomic absorption spectrometry for selenium speciation

 Chromatographic effluents were on-line analyzed by Zeeman-ETAAS, using a flow-through cell placed in a graphite furnace autosampler as interface. To obtain high sampling rates, the use of fast graphite furnace programmes was studied. Conventional programmes of 96 s were reduced to 18 s by using hot injection (120 °C) and reducing the charring step to 2 s. The increase of the injection volume from 20 to 60 μl lengthened the programme to 46 s. Nickel had to be added to get a comparable response for both inorganic and organic selenium species (selenite and selenomethionine) and to reduce the interferent effect produced in presence of the chromatographic eluent (TRIS 0.01 mol l^-1, NH₄NO₃ 0.1 mol l^-1, pH 7). The optimized conditions were applied to the speciation of selenium in human erythrocyte lysates by size exclusion LC-ETAAS. Using a high performance size exclusion column selenium could be assigned to proteins of 100 and 35 kDa. Detection limits in the range of 1 ng (2 μg l^-1 for 500 μl injection volume) were obtained for the combined technique. Received: 9 October 1996/Revised: 8 July 1996/Accepted: 14 July 1996