Determination of mercury in process and lagoon waters by inductively coupled plasma-mass spectrometric analysis after electrochemical preconcentration: comparison with anodic stripping at gold and polymer coated electrodes

doi:10.1016/S0003-2670(01)00838-8

Analytica Chimica Acta

Volume 434, Issue 2, 11 May 2001, Pages 291-300

https://doi.org/10.1016/S0003-2670(01)00838-8 Get rights and content

Abstract

A combined electrochemical-inductively coupled plasma-mass spectrometry (EC-ICP-MS) method for the determination of trace mercury in water samples is presented. It takes advantage of the electrochemical preconcentration of mercury onto a gold spiral electrode followed by ICP-MS analysis after the electrochemical reoxidation of deposited mercury in pure supporting electrolyte. The advantages of the EC-ICP-MS approach with respect to conventional ICP-MS, are the increased sensitivity and the elimination of the effect of interfering substances eventually present in the sample.

EC-ICP-MS is applied to the determination of nanomolar and subnanomolar concentrations of mercury(II) ions in real samples such as process waters (from a chlor-alkali plant) and lagoon waters (from Venice channels). Analytical performances obtained by EC-ICP-MS are discussed and compared with those obtained by anodic stripping voltammetry at gold and at Tosflex-coated glassy carbon electrodes.

Introduction

Anodic stripping voltammetric methods for the determination of trace mercury have been introduced successfully some years ago [1], [2], [3]. The best electrodic material to this aim is gold, which allows the efficient Faradaic preconcentration of metallic mercury as an amalgam. Sensitive methods have been developed also by using chronopotentiometric stripping on the same electrode material [4], [5], [6], [7]. In spite of the high sensitivity and relatively short preconcentration time, the use of gold electrodes suffers for some limitations, particularly for analysis in chloride containing samples; in fact, during the anodic stripping scan, gold itself can be oxidised to give soluble Au(III) chloride complexes [2]. Moreover, the interference coming from copper can be quite heavy. In real samples copper is usually present in large excess with respect to Hg and is codeposited with mercury. These drawbacks can be overcome by resorting to medium exchange and/or by using unconventional electrode configurations [2].

More recently, modified electrodes have been applied to trace mercury analysis; they take advantage of the preconcentration ability of the modifiers which can incorporate the analyte via ion-exchange [8], [9], [10], [11] or via complexation reactions [12], [13], [14]. Among the others, the use of glassy carbon electrodes modified with the perfluorinated anion-exchanger Tosflex IE-SA 48 [8] appeared particularly suitable for practical purposes. The application of the Tosflex coating is achieved by simple solvent evaporation of deposited microvolume of polymer solution; moreover, the modified electrodes are stable and the determination of mercury at trace levels can be performed with very good rejection of the copper interference thanks to the different ionic charges of Hg(II) and Cu(II) complexes in chloride containing media [8], [15].

Recently, for determinations in complex matrices, electrochemical preconcentration has been introduced also as an auxiliary tool for spectroscopic analysis [16], [17] giving rise to electrochemical-inductively coupled plasma-mass spectrometry (EC-ICP-MS) methods. In the case of mercury analysis, ICP-MS can be used for the direct determination of mercury at trace levels [18], [19]. However, in addition to problems related to the relatively low ionisation efficiency which characterises the ICP-MS determination of this element [20], ICP-MS analyses in matrices with high salts contents, such as seawaters, suffer for the formation of salt deposits on the small orifices which constitute the interface between the plasma torch and the mass analyser. These problems can be solved by performing the electrochemical deposition of mercury followed by stripping after transfer in a “clean” solution which is then sent to the ICP-MS instruments. The EC-ICP-MS approach has been used for a variety of analytes such as for instance Cr and V [21], As and Se [22], Cu and Cd [23], while for mercury only one example has been reported in the literature [24]. The method proposed in ref. [24] consisted of the electrodeposition of mercury, together with other trace metals, on a niobium electrode followed by a time consuming (2 h) chemical stripping step in concentrated nitric acid solution and subsequent dilution.

With the goal of developing a faster method specifically suitable for mercury trace determinations in complex samples, in the present paper we examine the use of gold spiral electrodes and the adoption of an electrochemical reoxidation procedure coupled to the ICP-MS analyses.

The EC-ICP-MS method is applied to mercury analysis in process waters (from a chlor-alkali plant) and in lagoon waters (from Venice channels). Analytical performances offered by EC-ICP-MS are compared with those obtained by anodic stripping voltammetry at gold electrodes or at Tosflex-coated glassy carbon electrodes.

Section snippets

Chemicals

All chemicals used were of analytical reagent grade or higher (HNO₃, HCl, NaCl Suprapur^® from Merck). Milli-Q water was used throughout for preparing supporting electrolyte solutions. Hg(II) solutions were prepared by proper dilution of mercury standard solution (1000 mg/l, Aldrich). Aqueous alcoholic solutions (water+methanol+2-propanol, 1:1:1) of Tosflex^® IE-SA 48 were prepared from the thick solid membrane using the method of Dunsch et al. [25]. The filtered solution had a concentration of

Synthetic samples

As mentioned in the Section 1, the adoption of an electrochemical preconcentration step coupled to an ICP-MS detection method, in principle, can eliminate some of the main drawbacks found in the ICP-MS determination of mercury in real samples. A prerequisite which should drive the choice of the electrode used for the preconcentration is that it should be a large area electrode (see below) which allows the quick and (possibly) complete stripping of preconcentrated mercury in small volume

Conclusions

The combination of an electrochemical preconcentration step at a gold macroelectrode with ICP-MS detection improves the applicability of mercury ICP-MS analysis to complex samples which contain high amounts of salts or other interfering substances. The electrochemical preconcentration step improves the sensitivity of the ICP-MS determination of mercury, however, the not perfect reproducibility of this step (related to its heterogeneous nature) limits its application to determinations of

Acknowledgements

Financial support by MURST, Rome (Cofinanziamento 1999–2000 and Piano “Servizi al cittadino ed al territorio”, Cluster C 22, Progetto 28) is gratefully acknowledged. We thank Mr. Danilo Rudello (University of Venice) and Mrs. Maristella Zago (EniChem) for skilful technical assistance.

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Determination of mercury in process and lagoon waters by inductively coupled plasma-mass spectrometric analysis after electrochemical preconcentration: comparison with anodic stripping at gold and polymer coated electrodes

Abstract

Introduction

Section snippets

Chemicals

Synthetic samples

Conclusions

Acknowledgements

Anal. Chim. Acta

Anal. Chim. Acta

Anal. Chim. Acta

Anal. Chim. Acta

Anal. Chim. Acta

J. Electroanal. Chem.

J. Electroanal. Chem.

Mar. Chem.

Mar. Chem.

Mar. Chem.

Mar. Chem.

Anal. Chim. Acta

J. Anal. Chem. Acta

Anal. Chem.

Electroanalysis

Anal. Chim. Acta

Electroanalysis

Frezenius’Z. Anal. Chem.

Anal. Chem.