Batch-injection stripping voltammetry (tube-less flow-injection analysis) of trace metals with on-line sample pretreatment

The most essential limitation of batch-injection analysis (BIA) methodology compared to other flow methods (CFA, FIA, SIA) is the lack of possibility of on-line sample processing in the measuring system. Some procedures of on-line sample pretreatment in BIA are possible by changing the plastic tip of the automatic micropipette used for sample injection into a flow-through reactor, e.g. by packing it with a bed of a solid sorbent. This concept is employed in the voltammetric stripping determinations of trace metals using a bed of commercial chelating resin Chelex-100. It was found that, besides the electrochemical preconcentration of analytes in the form of amalgams on the surface of mercury thin film electrodes, an approximately 10-fold additional preconcentration can be achieved on the packed sorbent bed by using different volumes of aspirated sample solution and eluent. This procedure allows also efficient elimination of some matrix effects.     

Determination of Cu, Pb, Cd, and Zn in river sediment extracts by sequential injection anodic stripping voltammetry with thin mercury film electrode

Determination of Cu, Pb, Cd and Zn was performed in sediment extracts obtained according to the three steps sequential extraction procedure proposed by the European Community Standards, Measurements and Testing Program. The metal content was determined by anodic stripping voltammetry with a thin mercury film electrode controlled by a sequential injection (SIA) system. The proposed method improved the reproducibility of conventional anodic stripping voltammetry, as well as the sample throughput, allowing analysis of 30 to 45 samples per hour. The influence of flow rate and sample volume was studied to achieve an adequate sensitivity for the leachate studied. No interferences due to adsorption of organic matter, colloids, or complexes with slow rate of dissociation were observed. The intermetallic formation of Cu-Zn was avoided by forming the mercury film in presence of Ga(III) ions in the SIA system, resulting in low consumption of reagent in comparison to flow injection or continuous flow systems. Results were in good agreement with those obtained by Induced Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES).     

Determination of Cu, Pb, Cd, and Zn in river sediment extracts by sequential injection anodic stripping voltammetry with thin mercury film electrode

Determination of Cu, Pb, Cd and Zn was performed in sediment extracts obtained according to the three steps sequential extraction procedure proposed by the European Community Standards, Measurements and Testing Program. The metal content was determined by anodic stripping voltammetry with a thin mercury film electrode controlled by a sequential injection (SIA) system. The proposed method improved the reproducibility of conventional anodic stripping voltammetry, as well as the sample throughput, allowing analysis of 30 to 45 samples per hour. The influence of flow rate and sample volume was studied to achieve an adequate sensitivity for the leachate studied. No interferences due to adsorption of organic matter, colloids, or complexes with slow rate of dissociation were observed. The intermetallic formation of Cu-Zn was avoided by forming the mercury film in presence of Ga(III) ions in the SIA system, resulting in low consumption of reagent in comparison to flow injection or continuous flow systems. Results were in good agreement with those obtained by Induced Coupled Plasma – Atomic Emission Spectroscopy (ICP- AES). Received: 18 October 1999 / Revised: 14 December 1999 / Accepted: 19 December 1999     

On-line coupling of sequential injection lab-on-valve to differential pulse anodic stripping voltammetry for determination of Pb in water samples

Sequential injection lab-on-valve (LOV) was first proposed for analyzing ultra-trace amounts of Pb using differential pulse anodic stripping voltammetry (DPASV) with a miniaturized electrochemical flow cell fabricated in the LOV unit. Deposition and stripping processes took place between the renewable mercury film carbon paste electrode and sample solution, the peak current was employed as the basis of quantification. The mercury film displayed a long-term stability and reproducibility for at least 50 cycles before next renewal, the properties of integrated miniature LOV unit not only enhanced the automation of the analysis procedure but also declined sample/reagent consumption. Potential factors that affect the present procedure were investigated in detail, i.e., deposition potential, deposition time, electrode renewable procedure and the volume of sample solution. The practical applicability of the present procedure was demonstrated by determination of Pb in environmental water samples.     

Determination of mercury by total-reflection X-ray fluorescence using amalgamation with gold

Analysis by total-reflection X-ray fluorescence (TXRF) is unsuitable for determining mercury concentrations because the usual sample preparation produces evaporation and loss of this element as a consequence of its high vapour pressure and low boiling point.A method that has been developed to achieve this determination involves forming an amalgam while a thin layer of silver (obtained by sputtering or evaporation) is in contact with an ionic solution of Hg; subsequently, a traditional TXRF analysis is performed. This was the first method reported in the literature to apply the TXRF technique for reliably determining mercury concentrations with high sensitivity.This work shows how a similar procedure may be employed to measure mercury concentrations. This second method involves forming an amalgam of gold using microlitre quantities of the solution to be analysed. As gold is a highly malleable material, it allows the production of very thin films, the weight of which is a few orders of magnitude higher than the mass of mercury present in the amalgam. The determination is performed in the usual way using the TXRF technique. The sensitivity of this method (≈ 5 ppm) is inferior to that of the former method, and data processing is quite difficult because the peaks for mercury and gold overlap, but the experiment is simple to execute and improved sensitivity is expected to be attained by forming the amalgam with larger volumes of sample and with a more responsive data processing scheme.     

Stripping voltammetric determination of mercury(II) at antimony-coated carbon paste electrode

A new procedure was elaborated to determine mercury(II) using an anodic stripping square-wave voltammetry at the antimony film carbon paste electrode (SbF-CPE). In highly acidic medium of 1 M hydrochloric acid, voltammetric measurements can be realized in a wide potential window. Presence of cadmium(II) allows to separate peaks of Hg(II) and Sb(III) and apparently catalyses reoxidation of electrolytically accumulated mercury, thus allowing its determination at ppb levels. Calibration dependence was linear up to 100 ppb Hg with a detection limit of 1.3 ppb. Applicability of the method was tested on the real river water sample.     

Determination of selenium by means of computerized flow constant-current stripping at carbon fibre electrodes. : Application to human whole blood and milk powder.

The main features of the flow constant-current stripping analysis for selenium(IV) are formation of a mercury film on a carbon fibre sensor in a chloride medium containing mercury(II), electrolysis in the sample at ?0.20 V vs. SCE for 15–60 s, and subsequent stripping (reduction) of the mercury(II) selenide formed on the electrode surface, by means of a constant current of 0.40 μA in an acidic magnesium chloride solution containing Triton X-100. During stripping, the potential vs. time gradient is monitored at a real-time measuring rate of 25.6 kHz. All experimental parameters are under computer control. A standard addition method is used and the results are calculated and reported, both digitally and graphically. Equations relating the magnitude of the constant current to the concentration of reducible species, and, in particular, of dissolved dioxygen, are derived. Milk powder and whole blood reference samples were analyzed by high-pressure digestion in nitric acid and dilution with hydrochloric acid, in order to reduce selenium(VI) to selenium(IV), and then constant-current stripping. The results obtained by this method were lower than those obtained by the reference technique, but the values agreed within one standard deviation of the two techniques. Ions, such as iron(III) and lead(II), known to interferè with electrochemical stripping for selenium(IV) in batch analysis did not interfere in the flow approach.     

Flow injection analysis with detection at mercury drop electrode in the presence of surfactants

The surfactant presence in a sample may cause distortion of the flow injection peak obtained with amperometric detection at a mercury drop electrode. Distortion depends on detection parameters and flow system operating parameters and in some cases it may be eliminated by their careful optimization. Introduction of fumed silica into the carrier solution allows the dynamic removal of surfactant during the sample passage through the mixing coil. In the case of higher surfactant concentration in the sample, addition of fumed silica directly to the sample may be effective. Examples with amperometric detection as well as anodic stripping and adsorptive stripping voltammetric detection are described.     

A flow-through cell for differential pulse anodic stripping voltammetry

A flow-through voltammetric cell with a hanging mercury drop electrode has been developed to fit the static mercury drop electrode (PAR 303). The design has resulted in a linear increase of sensitivity with flow rate and an enhancement of sensitivity by the wall-jet effect. The cell is used in a flow injection system in which samples are introduced with a R??i?ka—Hansen injector. The mercury drop is held at plating potentials while the sample peak passes through the cell. Stripping is done under stopped flow conditions, to reduce noise, after the sample has been washed completely from the cell. The stripping thus takes place into the carrier electrolyte which always has a constant composition independent of sample constituents. Film-forming interfering species will, however, remain on the surface of the mercury drop. The effect of medium exchange on films produced by l-cysteine is reported. The flow-through medium exchange simplifies deaeration, speeds up analysis and reduces contamination.     

Determination of trace metal speciation parameters by using screen-printed electrodes in stripping chronopotentiometry without deaerating

The objective of this study was to exploit the advantages of stripping chronopotentiometry (SCP) and stripping chronopotentiometry at scanned deposition potential (SSCP) for trace metal speciation analyses by using thin-film mercury screen-printed electrodes (TMF-SPE). At first, the SCP parameters were optimised for TMF-SPE, in order to reach the complete depletion regime. It has been shown that a stripping current higher than or equal to 10 μA allows this regime to be attained without removing oxygen from the solution.Then, these analytical conditions were used for the construction of SSCP curves for Cd-PDCA and Cd-NTA. When the concentration of free ligand in solution was known, the knowledge of the model describing the SSCP curves in absence and presence of a complex and the use of an effective fitting tool enabled estimating the stability constant and the rate constants for complexation. Further studies with complexes of restricted mobility are however necessary to assess the usefulness of this procedure to also estimate the diffusion coefficient of the complexes. Besides, this study showed that this approach was valid even when ligands were not in excess at the electrode during stripping.     

Potentiometric stripping analysis for mercury after preconcentration in a potassium permanganate trap

Water samples of total volume 0.5–5 l, containing mercury in the range 0.05–5 μg l^-1 are oxidized with excess of potassium permanganate and then reduced with tin(II) chloride. The mercury is transferred to 5–10 ml of a dilute potassium permanganate trap solution by cycling air through the sample and the trap solution. The mercury(II) content of the trap solution is determined by potentiometric stripping analysis. The accuracy, precision and detection limit of the technique are discussed.     

Galvanic Stripping - A Simple and Versatile Approach to Trace Analysis

Abstract

A rapid, simple and versatile analytical technique based on galvanio stripping has been proposed. In this technique, metal ions are preconcentrated onto a mercury film coated glassy carbon electrode (MFGCE). The amal-gamated metals are re-oxidized by galvanic stripping without the aid of any external electrical field or oxidant in solution. The development and relative advantages that can acorue by the utilization of this technique are discussed.     

Stripping analysis of mercury(II) ionic solutions under magneto-hydrodynamic convection

Inorganic mercury(II) ions are ubiquitous contaminants of world water systems and thus their determination and removal from the environment are important. The effects of magnetic field on the stripping analysis of mercury(II) ionic solutions have been experimentally investigated. During the stripping analysis, a potential difference is applied across the working and reference electrodes positioned in the working sample and a current density transmits through the electrolyte solution. When the electrochemical cell is exposed to a magnetic field, provided by a permanent magnet, the interaction between the current density and the magnetic field induces Lorentz forces, which, in turn, induce fluid motion. The induced magneto-hydrodynamic (MHD) convection enhances the ionic mass transport during the deposition and stripping steps, which leads to larger anodic current during the stripping step, thus obtaining higher detection sensitivity during the determination of the mercury(II) ions. The Hg2+ ionic solutions with concentrations ranging from 1 nM to 1 microM in the presence and absence of supporting electrolyte, 30 mM nitric acid (HNO 3) and 0.1 M potassium nitrate (KNO 3), under various magnetic flux densities (B=0,0.27,0.53, and 0.71 T) were measured with a linear sweep stripping voltammetry (LSSV) technique. The experimental results demonstrated that the stripping signals of the Hg2+ ions are enhanced, respectively, more than 10 and 30% in the absence and presence of the supporting electrolyte under a magnetic flux density B=0.71 T as compared to the cases in the absence of the magnetic field with all other identical conditions.     

Nafion-coated bismuth film and nafion-coated mercury film electrodes for anodic stripping voltammetry combined on-line with ICP-mass spectrometry

Nafion-coated bismuth film electrodes (NCBFEs) and Nafion-coated mercury film electrodes (NCMFEs) were used to electrochemically preconcentrate metal analytes for subsequent analysis by inductively coupled plasma-mass spectrometry (ICP-MS). Either type of electrodes is part of a thin-layer electrochemical flow cell that is positioned upstream of a microconcentric nebulizer for the ICP-MS. Performances of these electrodes were compared in terms of the analytical "figures of merit" (e.g., dynamic ranges, reproducibility, hydrodynamic stability, and elimination of matrix effects detrimental to ICP-MS). The coupled technique (ASV-ICP-MS) is found to possess a wide dynamic range (at least 4 to 5 orders of magnitude) and to be reproducible. Both electrodes are much more stable than the thin mercury film electrode (TMFE) traditionally used for ASV-ICP-MS, with the lifetime of the NCBFE exceeding 8 h. Adopting these electrodes for ASV-ICP-MS overcomes the problems associated with a TMFE, the erosion of which decreases the sample throughput, affects the analysis precision, and contaminates conventional glass nebulizers and spray chambers of the spectrometer. The medium exchange procedure inherent in ASV is successfully implemented with a two-valve flow injection system for the accumulation of trace Cd2+ into the electrode from a certified seawater sample, followed by stripping Cd into a solution that is compatible to the ICP-MS operation.     

The determination of copper at thin film electrodes by constant current stripping potentiometry

A comparison of the determination of copper by constant current stripping potentiometry (CCSP) at mercury and gold films has been carried out. The preferred solution conditions for the mercury film study were determined to be 0.1M ammonium acetate at pH 4.5 and 0.1M HCl for the gold film study. The influence of chloride on the stripping signal was investigated and it was found that for the mercury film conditions, well-formed stripping signals could be obtained up to a chloride concentration of 0.5 M which permitted the ready determination of copper in seawater. With the gold film, high chloride concentrations affected both the film stability and the glassy carbon surface and repeatable results were difficult to obtain. The optimized CCSP methods were applied to various aqueous samples including tap water, seawater, TCLP (acetic acid) extracts as well as TCLP extracts using groundwater and ocean water. Based on the results obtained for these various matrices, it was concluded that there are several advantages favoring the mercury film. The interference from organic components in the sample matrix on the general applicability of CCSP for the determination of copper at either a mercury or gold film is discussed.     

Flow potentiometric stripping analysis for mercury(II)

The optimum experimental conditions, with respect to sample and stripping solution composition, in computerised flow potentiometric stripping analysis for mercury(II) with a gold working electrode are described. When pre-electrolysis -was done in a sample to which ammonia and iodide had been added and stripping was done in an acidified bromide solution containing chromium(VI), a detection limit of 2 nM (0.4 μg kg^-1) was obtained after 90 s of pre-electrolysis, the dynamic range being almost three decades. Copper(II) interfered when present in a 1000-fold excess and silver(I) when present in a 5-fold excess over mercury(II).     

Voltammetric determination of vanadium with adsorptive preconcentration of the pyrocatechol violet complex

A sensitive stripping voltammetric procedure for trace measurements of vanadium in aqueous samples is reported. The method is based on the interfacial accumulation of the vanadium-pyrocatechol complex onto the hanging mercury drop electrode, followed by reduction of the adsorbed complex. The limit of detection is 0.1 nM vanadium after a 3 min collection with a stirred solution at pH 4.7. The procedure is selective with respect to other metals and has been applied in analysing various samples.     

The determination of copper,lead and cadmium in sea water by differential pulse anodic stripping voltammetry

The effect of various instrumental parameters is investigated and optimized conditions established. The results are in accordance with the theory of differential pulse anodic stripping voltammetry. Both a hanging mercury drop electrode, and a rotating glassy carbon electrode mercury plated in situ were used. The best detection limit is obtained with the mercury film electrode, but the hanging mercury drop electrode is more reproducible. The differential pulse stripping technique is compared to linear sweep stripping, and increased sensitivity and better peak separation is demonstrated for the former technique, particularly when a hanging mercury drop electrode is used. However, the differential pulse technique will also improve the detection limit for a mercury film electrode, if the electrode has a non-ideal response with a corresponding high background current.     

Voltammetric determination of nickel and palladium in plating baths

Summary A simultaneous determination of nickel and palladium in plating baths was carried out at the hanging mercury drop electrode (HMDE), using the dc and ac single-sweep techniques, direct in the bath sample diluted with ammonia buffer. The influences of Co, Pb, Zn, Cd and the concentration of ammonia buffer on the peak shapes of Ni and Pd and on their resolution were studied. The possible determination of Cd and Pb contamination in plating baths in the same solution by anodic stripping voltammetry is also indicated. The essential advantages of the described method are: its simplicity, rapidity and very small consumption of mercury. The detailed analytical procedure is given. Relative standard deviations were found to be 3.1% for Pd and 2.6% for Ni.     

Sequential multiple analyses of atmospheric nitrous acid and nitrogen oxides

Sequential injection analysis (SIA) was applied to multi-gas monitoring for atmospheric analysis. HONO, NO₂ or NO was collected in an individual diffusion scrubber in which the channel array was filled with either HCl or triethanolamine solution. All analytes were collected in the form of nitrite ions in the scrubber, and were transferred via a 12-port selection valve into a 2.5-ml syringe. The reagent, 3-amino-1,5-naphthalenedisulfonic acid (C-acid) solution was subsequently introduced into the syringe, and inter-mixed with the nitrite sample, whereafter the mixed solution was transferred to a heated reactor and held for 3 min at 100 °C. After that, the sample/reagent solution was returned to the syringe and alkalinized. Then, the final solution was analyzed using a homemade fluorescence detector. Atmospheric HONO, NO₂ and NO were successfully monitored 3 or 4 times/h. The limits of detection were 0.22, 0.28 and 0.35 ppbv for HONO, NO₂ and NO, respectively. It was demonstrated for the first time that SIA is a good tool for multi-gas atmospheric analysis. These nitrogen-oxygen compounds are interconvertible, and the simultaneous measurement of these gases is important. Especially, HONO is a source of OH radicals which contribute greatly to atmospheric pollution, and indeed atmospheric chemistry. This method allows the three gases to be measured using one system. The NO₂ and NO data obtained by SIA was compared with those obtained using chemiluminescence instrument. SIA has been successfully applied to atmospheric measurements. Interestingly, it was observed that HONO levels rose toward the end of periods of rain.