Stripping voltammetry for the determination of trace metal speciation and in-situ measurements of trace metal distributions in marine waters

Progress in marine chemistry has been driven by improved sampling and sample handling techniques, and developments in analytical chemistry. Consequently, during the last 20 years our understanding of marine trace metal biogeochemistry has improved a great deal. Stripping voltammetric techniques (anodic stripping voltammetry and adsorptive cathodic stripping voltammetry) have made an important contribution to this understanding. The selectivity and extremely low detection limits have made stripping voltammetry a widely used technique for trace metal speciation and trace metal distribution measurements in seawater. Stripping voltammetry is very suitable for ship-board and in-situ applications because of the portability, low cost and capability for automation of the voltammetric instrumentation. Future developments in stripping voltammetry can be expected in the field of stand-alone submersible voltammetric analysers, capable of continuous trace metal measurements. Future applications of stripping voltammetry can be found in the interactions between trace metal speciation and growth and the functioning of organisms in pristine and metal polluted marine waters.     

Trace metal speciation by adsorptive stripping voltammetry of metal chelates of solochrome violet RS

Adsorptive stripping voltammetry has become one of the most sensitive methods for trace metal determinations. The growing application of the method to natural water systems prompted an investigation into the fraction of the metal concentration that contributes to the adsorptive stripping response. Recent procedures for trace measurement of iron, titanium and gallium, based on chelation with solochrome violet RS (SVRS) are coupled to systematic ligand competition experiments. Tannic acid, EDTA, NTA, glycine, cysteine, carbonate and chloride ions are used as model natural ligands. It is shown that adsorptive stripping voltammetry measures the free ion and metal displaced from complexes by the “analytical” ligand. The exact fraction of the metal measured thus depends on the thermodynamic stability of the metal-SVRS chelate (compared to that of natural complexes), and on the relative concentrations of the competing ligands. The method offers possible distinction between metal complexes, based on their thermodynamic stabilities. The use ofa large excess of the “analytical” ligand can lead to measurement of the total metal content. Implications of these results relative to the use of this procedure for studying the speciation of trace elements in natural waters are discussed.     

Electrochemical methods for speciation of trace elements in marine waters. Dynamic aspects

The contribution of electrochemical methods to the knowledge of dynamic speciation of toxic trace elements in marine waters is critically reviewed. Due to the importance of dynamic considerations in the interpretation of the electrochemical signal, the principles and recent developments of kinetic features in the interconversion of metal complex species will be presented. As dynamic electrochemical methods, only stripping techniques (anodic stripping voltammetry and stripping chronopotentiometry) will be used because they are the most important for the determination of trace elements. Competitive ligand exchange-adsorptive cathodic stripping voltammetry, which should be considered an equilibrium technique rather than a dynamic method, will be also discussed because the complexing parameters may be affected by some kinetic limitations if equilibrium before analysis is not attained and/or the flux of the adsorbed complex is influenced by the lability of the natural complexes in the water sample. For a correct data interpretation and system characterization the comparison of results obtained from different techniques seems essential in the articulation of a serious discussion of their meaning.     

Research on Electroanalytical Chemical Methods for Trace Metal Speciation in Sea Water Samples

Since bioavalability of a trace metal in natural waters is affected by its chemical forms, the speciation of a trace metal becomes more and more important in environmental chemistry. Differential pulse anodic stripping voltammetry (DPASV) titration procedures are developed and applied to identify organic trace metal species of copper (Cu), lead (Pb) and zinc (Zn) in sea water and to determine their complexing capacity.     

Analytical methods for determination of free metal ion concentration, labile species fraction and metal complexation capacity of environmental waters: A review

Different experimental approaches have been suggested in the last few decades to determine metal species in complex matrices of unknown composition as environmental waters. The methods are mainly focused on the determination of single species or groups of species.The more recent developments in trace elements speciation are reviewed focusing on methods for labile and free metal determination.Electrochemical procedures with low detection limit as anodic stripping voltammetry (ASV) and the competing ligand exchange with adsorption cathodic stripping voltammetry (CLE-AdCSV) have been widely employed in metal distribution studies in natural waters. Other electrochemical methods such as stripping chronopotentiometry and AGNES seem to be promising to evaluate the free metal concentration at the low levels of environmental samples. Separation techniques based on ion exchange (IE) and complexing resins (CR), and micro separation methods as the Donnan membrane technique (DMT), diffusive gradients in thin-film gels (DGT) and the permeation liquid membrane (PLM), are among the non-electrochemical methods largely used in this field and reviewed in the text. Under appropriate conditions such techniques make possible the evaluation of free metal ion concentration.     

The voltammetric approach in trace metal chemistry of natural waters and atmospheric precipitation

Advanced voltammetry, predominantly in the differential-pulse mode, has become one of the most significant and suitable methods for investigations of the level, fate and transfer of heavy metals with toxicological significance in the ecology of aquatic systems and atmospheric precipitations. In addition, voltammetry offers particular potentialities for studies of heavy metal speciation in natural waters. The salient aspects of the overall analytical procedure are discussed. The substantial possibilities for application are emphasized by a number of examples taken from recent work.     

Speciation analysis — why and how?

Summary The different aspects of speciation analysis are reviewed. Species-specific instrumental techniques as well as various speciation schemes are considered for the determination of species of metals and metalloids, including organometallic compounds. The application of the methods are discussed in some detail for the analysis of natural waters, air, soil, sediment and biological samples. The relationship between metal species and bioavailability is also briefly dealt with.     

Evaluation of micromolar compleximetric titrations for the determination of the complexing capacity of natural waters

The titration of micromolar levels of complexing agents with metal ion titrants, and voltammetric methods to locate the equivalence point, has been evaluated experimentally and theoretically. Both anodic stripping voltammetry and differential pulse polarography give systematically low results if labile metal ions are used as titrants. Low-temperature (0 °C) dual-cell anodic stripping voltammetry greatly minimizes the effects of metal complex lability but the mercury film electrodes deteriorate rapidly because of temperature cycling. A micromolar compleximetric titration with a voltammetric end-point is not a practical method for determining the complexing capacity of natural waters.     

Cathodic pseudopolarography: A new tool for the identification and quantification of cysteine,cystine and other low molecular weight thiols in seawater

Thiols are compounds of paramount importance in the cellular metabolism due to their double detoxifying role as radical scavengers and trace metal ligands. However, we have scarce information about their extracellular cycling as limited data are available about their concentration, stability and speciation in the aquatic medium. In natural waters, they form part of the pool of reduced sulfur substance (RSS) whose presence has been documented by voltammetric and chromatographic methods. Traditional use of cathodic stripping voltammetry (CSV) for the analysis of RSS could only give an overall concentration due to the coalescence of their CSV peaks. Recently, it has been shown that the use of multiple deposition potentials could take voltammetry of RSS to a higher level, permitting the identification and quantification of the mixtures of RSS despite showing as a single coalescent peak. Here, due to its similarity with classical pseudopolarography, we propose to rename this analytical strategy as cathodic pseudopolarography (CP) and we present for the first time its use for the analysis of mixes of low molecular weight thiols (LMWT) at the nanomolar level. Despite limitations caused by the identical behavior of some LMWT, the CP allowed to isolate the contribution of cysteine and cystine from a coalescent signal in LMWT mixtures. Sample handling with clean protocols allowed the direct determination of the cystine:cysteine ratio without sample modification. Finally, we show the application of CP to identify LMWT in seawater samples extracted from benthic chambers and suggest future applications in other areas of environmental electroanalysis.     

Novel disposable bismuth-sputtered electrodes for the determination of trace metals by stripping voltammetry

This work describes a novel type of bismuth electrode for stripping voltammetry based on coating a silicon substrate with a thin bismuth film by means of sputtering. The bismuth-based sensors were characterized by optical methods (scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD)) and as well as by linear sweep voltammetry. Subsequently, the electrodes were tested for the detection of low concentrations of trace metals (Cd(II), Pb(II) and Ni(II)) by stripping voltammetry. Well-formed stripping peaks were observed for trace concentrations of the target analytes demonstrating “proof-of-principle” for these sensors. This type of electrochemical device, utilizing thin-film technology for the formation of the bismuth film, holds promise for future applications in trace metal analysis.     

Modulated polarographic and voltammetric techniques in the study of natural water chemistry

Modulated polarographic and voltammetric techniques are of particular importance in natural water chemistry because of their sensitivity not only to very low concentrations of electroactive components but also to their chemical form. Direct polarographic techniques are most useful for the analysis of non-metallic components at low concentrations since metal levels are only rarely high enough for the analysis of untreated samples. Preconcentration by chemical or electrochemical techniques have both been employed. Potentially the most productive field of application of polarographic and voltammetric methods is in determining the chemical speciation of electroactive components in natural waters. Some clarification is required of the chemical and biological significance of operational classifications currently employed. Intermetallic interferences and the influence of surface films on electrode behaviour need to be more thoroughly investigated before analyses or speciation studies on untreated samples can be routinely undertaken. Chemical and electrochemical understanding rather than increased sophistication in the instrumentation is required at this stage if full advantage is to be taken of the capabilities of modulated polarographic and voltammetric methods in natural water chemistry.     

Depletive Stripping Chronopotentiometry: A Major Step Forward in Electrochemical Stripping Techniques for Metal Ion Speciation Analysis

A comparative evaluation of the utility of the various modes of stripping chronopotentiometry (SCP) for trace metal speciation analysis is presented in the broad context of stripping voltammetric techniques. The remarkable fundamental advantages of depletive SCP at scanned deposition potential (SSCP) are highlighted, and the rigorous underpinning theory is described. The distinctive features of SSCP include: i) an effective getting around part of the Nernstian extension of the reoxidation process, leading to ii) greater resolution than conventional stripping voltammetries; iii) a certain insensitivity to electrochemical irreversibility, especially at a microelectrode; iv) in principle, freedom from induced metal ion adsorption interferences; v) no requirement for excess ligand during stripping; and vi) ability to provide a certain measure of any chemical heterogeneity in the metal speciation that is easily distinguishable from effects of electrochemical irreversibility.     

Characteristics of Voltammetric Determination and Speciation of Chromium – A Review

This article reviews the voltammetric methods of chromium determination, including adsorptive and catalytic adsorptive stripping voltammetry at liquid mercury, metallic films, and modified carbon paste electrodes. The principle applications of the catalytic adsorptive stripping voltammetric method of chromium(VI) determination in the presence of DTPA and nitrate, most useful in the analysis of chromium traces and its speciation, is presented in detail. Special emphasis is put on the presentation and characterization of the voltammetric procedures which make it possible to conduction speciation studies of chromium(VI) in the presence of a great excess of chromium(III) and surfactants. This survey is based on 173 articles.     

Speciation analysis of heavy metals in natural waters: a review.

Metal speciation in natural waters is of increasing interest and importance because toxicity, bioavailability, environmental mobility, biogeochemical behavior, and potential risk in general are strongly dependent on the chemical species of metals. This paper provides an overview of the need for speciation of heavy metals in natural waters, the chemical and toxicological aspects of speciation, and the analytical procedures for separation and the different techniques for final determination that are used today. The trends and developments of speciation are also discussed. Finally, the case of chromium (Cr) was selected for a detailed presentation because the speciation of this metal has attracted a great deal of interest in view of the toxic properties of Cr(VI).     

Evaluation of metal fractions in river sediments and waters: application of chelation chromatography-differential pulse anodic stripping voltammetry

The ability of chelation chromatography in combination with differential pulse anodic stripping voltammetry (DPASV) to provide a simple, fast and reliable way of dealing with interionic interferences, competitive complexations, re-adsorption of released metal ions and sorption of spiking metal ions by organic/inorganic materials in the complex matrixes of real natural samples has been critically examined. The technique is based on the selective complexation of target metal fractions on some novel sorbents which are polymeric chelating resins doped on stationary supports (Whatman No. 1 paper and silica gel). The usual complications of leaching of the resin and/or the chelating ligand and colloid retention on the sorption bed at any stage of separation were largely obviated with these sorbents under the operational conditions of metal sorption. A detailed study on the application of such sorbents to the differentiation of ionic (free), labile (ionic plus weakly complexed) and bound (strongly complexed) metal fractions present in local river-sediment and water samples was carried out. Chelating resin-impregnated paper (CRIP) and chelating resin-immobilized silica gel column (CRISC) methods of chromatographic separation of analyte trace metals in combination with the follow-up 'standard addition' procedure of the DPASV technique were employed. A modest attempt has been made to formulate a speciation (fractionation) scheme for metal contents present in river-sediments and waters on the basis of selective retention of ionic and labile fractions on complexing resins.     

Stripping-voltammetric signals on carbon electrodes modified with mercury

Conditions for the formation of a stable and reproducible thin-film mercury microdrop cover on electrodes from carbon glass ceramics, carbon glass, and graphite are proposed. The influence of various factors on the parameters of signals from cadmium (II), lead (II), and copper (II) is studied by stripping voltammetry with linear, differential-pulse, sinusoidal, and square-wave potential sweep. The parameters of the stripping voltammetric experiment are optimized. The value of RSD in determining the specified modeling trace components did not exceed 15%. Approaches to the multicomponent stripping voltammetric analysis of waters are proposed.     

Integrated Microanalytical System for Simultaneous Voltammetric Measurements of Free Metal Ion Concentrations in Natural Waters

A complexing gel integrated microelectrode (CGIME) for direct measurements of free metal ion concentrations in natural waters has been developed. It is prepared by the successive deposition of microlayers of a chelating resin, an antifouling agarose gel and Hg on a 100‐interconnected Ir‐based microelectrode array. The trace metals of interest are in a first step accumulated on the chelating resin in proportion to their free ion concentration in solution, then released in acidic solution and detected simultaneously by using square wave anodic stripping voltammetry (SWASV). The reliability of this sensor for the simultaneous measurement of copper, lead and cadmium has been studied by a series of replicate laboratory tests. The proportionality between the voltammetric peak current intensity and the free metal ion concentrations in solution has been demonstrated by using malonate as a model ligand. Finally, the CGIME sensor was applied to the Cu and Pb free concentration measurement in sea water samples and the results compared to the free metal ion concentrations measured using hollow fiber based permeation liquid membrane (HF‐PLM) coupled to inductively coupled plasma mass spectrophotometer (ICP‐MS). Comparable concentration values were found for both metals with both techniques allowing to validate the CGIME measurements in complex media.     

Development of physico-chemical speciation procedures to investigate the toxicity of copper,lead, cadmium and zinc towards aquatic biota

A range of model compounds was studied to test the effect of complexing agents on the adsorption of copper, lead, cadmium and zinc by Chelex-100 resin, oxine porous glass, thiol porous glass and thiol resin, from seawater and distilled water. The thiol materials, while showing behaviour similar to Chelex-100 resin and oxine porous glass for zinc, cadmium and lead, retained copper much more strongly. Methods for measuring lipid-soluble metal complexes in waters were also studied. Bio-Rad SM2 resin was the most suitable extractant, although a citrate buffer of pH 5.7 was needed to prevent the co-adsorption of free metal ions. Seawater and various fresh waters were analyzed for trace metal speciation by using Chelex-100 resin, thiol resin and anodic stripping voltammetry to determine labile metal. Bio-Rad SM2 resin and hexane—butanol extraction were used for the estimation of lipid-soluble metal. The polluted water samples had higher fractions of labile and organic-soluble metal, but it was concluded that some of the waters contained unidentified ligands which caused speciation behaviour different from that of the synthetic solutions with model ligands. The factors involved in the choice of speciation procedures for the measurement of the toxic fraction of a metal in a water sample are discussed in detail.     

Ion-exchange and permselectivity properties of poly(sodium 4-styrenesulfonate) coatings on glassy carbon: application in the modification of mercury film electrodes for the direct voltammetric analysis of trace metals in estuarine waters

The present work describes the optimisation and characterization of poly(sodium 4-styrenesulfonate)-coated thin mercury film electrodes (PSS-TMFE) for the direct analysis of trace metals in estuarine waters by square-wave anodic stripping voltammetry (SW-ASV). The morphology, thickness and ion exchange ability of the poly(sodium 4-styrenesulfonate) coatings onto glassy carbon were evaluated and these features particularly favoured the incorporation of cationic species, such as dopamine or lead cation. For the case of the heavy metal cations, a simple, sensitive and very reproducible methodology for their SW-ASV analysis could be developed. In fact, with the PSS-TMFE, a significant increase in the sensitivity of the ASV determination of lead was obtained compared both to the uncoated TMFE (ca. 82%) as well as to Nafion-coated electrodes of similar thickness (ca. 43-49%). Furthermore, the permselectivity of the poly(sodium 4-styrenesulfonate) coatings, based both on electrostatic interaction and molecular size, leads to an improved anti-fouling ability against surfactant species. The analytical usefulness of the poly(sodium 4-styrenesulfonate)-coated thin mercury film electrodes is demonstrated by application to the direct ASV determination of trace heavy metals at the low nanomolar level, in estuarine waters with moderate contents of dissolved organic matter, where the uncoated TMFE failed due to fouling.     

A Comparison of Methods for the Determination of Available Trace Metals in Sea Water

Abstract

Two methods for the determination of available trace metals CCd, Pb, and Cu) in sea water are compared. One method employs anodic stripping voltammetry at controlled pH (8.1, 5.3, and 2); the other method involves sample pretreatment with Chelex-100 resin before ASV analysis. Differences in the results are discussed in terms of the definition of available metal and differences in the analytical methods.