Determination of labile trace metals with screen-printed electrode modified by a crown-ether based membrane

Analytica Chimica Acta 573–574 (2006) 14–19

Determination of labile trace metals with screen-printed electrode modified by a crown-ether based membrane Corinne Parat ∗ , St´ephanie Betelu, Laurent Authier, Martine Potin-Gautier Laboratoire de Chimie Analytique Bioinorganique et Environnement, U.F.R. Sciences, Avenue de l’Universit´e, 64000 Pau, France Received 2 December 2005; received in revised form 28 March 2006; accepted 27 April 2006 Available online 4 May 2006

Abstract In this work, we have undertaken the construction of a screen-printed electrode modified by a specific membrane to protect the working surface from interferences during the analysis of trace metals by anodic stripping voltammetry. Different crown-ethers selected for their metals affinity have been incorporated into a membrane then deposed on the working surface of the electrode. Each modified electrode has been first tested in an acidified KNO3 10−1 mol L−1 solution (pH 2) doped by free Cd2+ and Pb2+ ions. The response and selectivity of the modified electrodes have been investigated according to different parameters: (i) the substrates (commercial ink or carbon based homemade ink), (ii) the electrode support (polystyrene or transparency film) and (iii) crown-ethers nature (dibenzo-24-crown-8 and tetrathiacyclododecane 12-crown-4). The influence of the substrate on the response of the electrode is clearly demonstrated. Homemade ink appears as the most appropriate substrate to modify the working surface of the screen-printed electrode by a crown-ether based membrane. The effect of the composition of the membrane has been shown too. The best membrane developed showed a detection limit of 0.6 × 10−8 mol L−1 for Cd and 0.8 × 10−8 mol L−1 for Pb and a quantification limit of 10−8 mol L−1 for Cd and 2 × 10−8 mol L−1 for Pb. This method, which integrates the extraction, preconcentration and measurement, was successfully applied to environmental samples without pretreatment. © 2006 Elsevier B.V. All rights reserved. Keywords: Screen-printed electrode; Mercury film; Crown-ether; Anodic stripping voltammetry

1. Introduction In risk assessment, the free metal ion concentration appeared as a key parameter for bioavailability and toxicity of heavy metals to plants and living organisms [1]. In natural systems, exposure to metal ions not only depends on the total metal concentration but also on the speciation and on a number of environmental conditions (e.g. pH, concentration of other ions and concentrations of ligands in solution). Consequently, many analytical techniques have been developed in order to determine the free metal ion concentrations in the raw samples such as diffusive gradients in thin films (DGT) [2] or Donnan Membrane Technique (DMT) [3]. These methods allowed to determine the free metal ion in aqueous system in agreement with speciation models (WHAM or MINTEQ). However, they required a long time of equilibrium, which is not suitable for routine analysis and need of complementary equipments to measure lower concentrations.

∗

Corresponding author. Tel.: +33 3 559 40 76 70; fax: +33 3 559 40 76 74. E-mail address: [email protected] (C. Parat).

0003-2670/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2006.04.081

In summary, a technique is required that can preconcentrate and measure the labile fraction of trace metals without disturb the chemical equilibrium in the solution, the lability concept being defined as the ability of a complex to contribute to the metal flux to plants and living organisms. Crown-ethers have proved to be an excellent choice in preconcentration techniques because of their ability to complex selectively ions. Crown-ethers can form complexes by fitting of the metal cation into a cage formed by crown structure. Since the cage can be designed to accommodate any ion of a certain size, selective extractions may be possible [4]. Researches have been performed by using this kind of compounds in solidphase micro-extractions [5,6] or as carrier in supported liquid membrane extractions [7–10] but also in the fabrication of ion selective sensors [8,11–13]. Most of these ion selective sensors offer a good selectivity, a large pH and a wide concentration range but suffer from a lack of sensitivity with a detection limit around 10−5 mol L−1 for the majority of ions. Because of its high sensitivity, stripping voltammetry was widely used to determine trace levels of metal ions. Moreover, this technique allowed to determine the different forms

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of trace metals: free ions, labile complexes characterized by high rates of association/dissociation and inert complexes. The working electrodes used were mainly mercury electrodes or various kinds of mercury-film electrodes. However, the organic compounds lead to adsorb on the mercury-film coated electrode thus inhibiting the metal deposition or stripping processes [14]. Strategies have been developed for eliminating these interferences including filtering, UV digestion, acidification or mineralization. Chemically modified electrodes (CME) have also attracted considerable interest for a direct analysis of natural water samples, by deliberate modification of the surface or bulk matrix material of the electrode with a selected reagent (monomeric or polymeric). Such manipulation aims at improving sensitivity, selectivity and/or stability requiring for analytical needs. In recent years, determinations of metal ions by various chemically modified electrodes have been reported: overoxidized polypyrrole films [15], graphite working electrode modified by a cellulose-derivative mercury coating [16], Nafion® -coated glassy carbon electrode [17–21] and 1-(2pyridylazo)-2-naphthol (PAN) [22]. Another approach consists to modify the working surface of a screen-printed electrode. This technology has been already developed in other fields such as electrochemical immunoassays or immunosensors [23,24]. This kind of sensors has the advantages to definitely solve problems related to contamination between samples. Moreover, their surface can be easily modified and therefore different improvements of the final sensor, such as enhanced selectivity and/or sensitivity, can be achieved. Finally, screen-printing is a simple and fast method for large-scale production of reproducible disposable low-cost electrochemical sensors. However, few works have exploited these sensors in the trace metal analysis. The objective of this study was therefore to modify a thin-film mercury-coated screen-printed carbon electrode by a crownether based membrane in order to quantify the different forms of Cd and Pb (free ions, labile complexes or inert complexes) in environmental samples such as soil solutions. Two crownethers have been chosen, the dibenzo-24-crown-8 (DB24C8) and a thiacrown-ether, the tetrathiacyclododecane 12-crown-4 (TT12C4). Optimum experimental parameters have been examined and application to trace metal determinations in environmental samples is described. 2. Experimental 2.1. Reagents Dibenzo-24-crown-8 (DB24C8) and tetrathiacyclododecane 12-crown-4 (TT12C4) were obtained from Aldrich Chemicals, USA. Tetrahydrofuran (THF), Mesitylene, potassium tetrakis(4chlorophenyl borate) (KTCPB), all from Aldrich, were of the highest purity available and used without any further purification. Potassium nitrate (99.997% metal basis) was obtained from Alfa Aesar. Nitric acid of the Suprapur grade and Mercury(II) nitrate for atomic absorption standard was obtained from J.T. Baker. Polystyrene support was obtained from Sericol. Commercial ink (electrodag PF-407A) was purchased from Acheson

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Colloids. The carbon powder used in the homemade ink is a porous graphitic carbon provided by Thermo Electron Corporation. Polystyrene used in the membrane conception was taken from packing. The divalent metal ion solutions were prepared from cadmium nitrate (Normapur) and lead nitrate (Normapur) obtained from VWR International. Working solutions were prepared by the dilution of a 10−2 mol L−1 solution in milliQ water (resistivity of 18 M
cm) and stored in polyethylene containers. 2.2. Sensors 2.2.1. Electrode preparation The electrodes were screen-printed on two different supports: flexible polyester films (transparency films for copiers) and 1 mm-thick polystyrene plates. Two different inks were used to screen-print electrodes. The first is a commercial ink used as received. A manual screen-printer was used to produce an array of six electrodes, by forcing the conductive ink to penetrate through the mesh of a screen stencil [23]. After a drying step (1 h at room temperature) and a curing step (1 h in an oven at 60 ◦ C), an insulator layer was spread manually over the conductive track, leaving a working disk area of 9 mm2 and an electrical contact. The second ink is a homemade ink based on a porous graphitic carbon powder. The conductive carbon ink was prepared by thoroughly hand-mixing the carbon mixture and a polystyrene/mesitylene mixture as previously described [25]. The same screen-printing process was used except the curing step. 2.2.2. Membrane preparation The ionophores (DB24C8 or TT12C4) were dissolved in a mixture of THF (1.5 mL) and mesitylene (1 mL) and then mixed with KTCPB and polystyrene according to the proportion given in Table 1. The mixture was vigorously shaken. A drop of the mixture (3 ␮L) was deposited on the electrode working surface and dried at 30 ◦ C during one night. 2.2.3. Mercury film deposition Thin-film mercury-coated screen-printed carbon electrodes consist of a very thin film layer of Hg atoms adsorbed onto the electrode surface. The Hg film was deposited at −1 V for 300 s from a stirred solution of KNO3 0.1 mol L−1 –HNO3 0.2 mol L−1 containing 2 mL of a 1000 mg L−1 Hg(NO3 )2 solution. Before using, each mercury-coated electrode is pretreated by applying −1 V for 300 s with stirring, then, differential pulse voltammetric scans were carried out three times in order to stabilize the background current. 2.3. Analytical parameters All the experiments were carried out using Autolab PGSTAT12 potentiostat (Eco Chemie, The Netherlands). A volume of 10 mL of the sample solution was transferred into the voltammetric cell. Cd2+ and Pb2+ concentrations were determined by linear scan anodic stripping voltammetry (LSASV).

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Table 1 Composition of crown-ether based membranesa Crown-ether

No.

Crown-ether (mg, %w/w)

KTCPB (mg, %w/w)

Polystyrene (mg, %w/w)

DB24C8

1 2 3 4

5, 1.9 10, 3.8 30, 10.4 15, 10.1

6, 2.3 6, 2.2 10, 3.4 9, 6.0

250, 95.8 250, 94.0 250, 86.2 125, 83.9

TT12C4

1 2 3 4 5

5, 3.2 15, 14.9 15, 10.3 15, 10.7 15, 10.1

2, 1.3 6, 5.9 6, 4.1 0, 0 9, 6.0

150, 95.5 80, 79.2 125, 85.6 125, 89.3 125, 83.9

a

The components were dissolved in mesitylene (1 mL) and tetrahydrofuran (1.5 mL).

The voltammetric parameters for the optimisation experiments were: electrodeposition, −1 V for 120 s; equilibration time, 30 s; scan rate, 0.5 V s−1 . All measurements were performed in a quiescent solution, without deoxygenation. Between each analysis, an electrochemical cleaning method was adopted: after stripping, the working electrode was set to 0 V, the final potential of stripping step, for 120 s with stirring. 2.4. Extraction and analysis of soil solutions Soil solutions were obtained by extraction with a salt solution, centrifugation and filtration. Specifically, 5 g of soil and 50 mL of 1 mol L−1 KNO3 solution (ultrapure grade) were mixed in a centrifuge tube. The centrifuge tubes were shaken 24 h and then centrifuged at 4000 × g during 10 min. The solutions were then passed through a 0.45 ␮m cellulosic membrane and then analyzed. Total dissolved Cd and Pb concentrations were determined by graphic furnace atomic absorption spectrometry (GFAAS) with a Varian Spectra instrument equipped with Zeeman background correction. The pH was measured using a Metrohm pH meter. Total organic carbon (TOC) and inorganic carbon (IC) were measured with a TOC Shimadzu. Dissolved organic carbon was determined by difference.

3.2. Membrane characteristics Preliminary experiments were carried out to optimise the sensor coating composition. Thus, two crown-ethers were tested with different amounts of membrane components (crownether–KTCPB–polystyrene). In these optimisation experiments, Cd2+ and Pb2+ at 10−7 mol L−1 were used as solution with a KNO3 0.1 mol L−1 –HNO3 0.2 mol L−1 solution as electrolyte support. The analytical performances of electrode obtained for both Cd and Pb, allowed to compare the different membranes. The linearity and sensitivity, based on the slope of the calibration curve, were obtained from a calibration graph with five concentration levels, with triplicate analysis. The detection limit (LOD = (b + kSb )/m) was calculated statistically where k = 3, Sb is the intercept standard deviation of the regression line and m is the slope of the calibration graph [27]. The quantification limit (LOQ) was calculated through the same equation as for the detection limit, however the value k was taken as 10. 3.2.1. Membranes based on DB24C8 Among the different compositions tested (Table 1), only the DB24C8-based membrane 4 allowed to obtain well-shaped

3. Results and discussion 3.1. Support and ink characteristics It was found that the electrode screen-printed on a polystyrene support as previously described [26] and modified by a crownether based membrane does not allow to record a signal, whatever the ink used. The THF used in the membrane conception really dissolves the polystyrene support and thus changes the proportion of crown-ether/polystyrene. Transparency film [23,24] was therefore retained as support of screen-printed electrode modified by a crown-ether based membrane. Also, commercial and homemade inks were tested with crown-ether based membrane. As shown on Fig. 1, homemade ink allowed to have screen-printed electrodes clearly more sensitive than those obtained with the commercial ink. Consequently, the transparency film as support and the homemade ink were chosen for the further experiments.

Fig. 1. Voltammograms obtained with electrodes screen-printed with two different inks (commercial and homemade inks) and coated with the same crown-ether based membrane, in a solution containing 10−7 mol L−1 Cd2+ and Pb2+ ; supporting electrolyte KNO3 0.1 mol L−1 –HNO3 0.2 mol L−1 ; electrodeposition, 120 s at −1 V; equilibration, 10 s; scan rate, 0.5 V s−1 .

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Fig. 3. Relationship between the sensitivity and the TT12C4 based membrane composition.

closely related metal ions based on the relative fit of the ligand cavity size to the metal ion radius [28]. The fact that Pb ionic radius (0.132 nm) is slightly higher than that of Cd (0.109 nm) suggested that Pb ionic radius is more appropriate to the ligand cavity size. The membranes 3 and 5 showed similar sensitivities, significantly higher than that of the membrane 4 which did not contain KTCPB (Fig. 3). Table 2 shows that the membrane 3, with composition 15/6/125 (in mg) (TT12C4–KTCPB–polystyrene), exhibited the best results with a detection limit of 0.6 × 10−8 mol L−1 for Cd and 0.8 × 10−8 mol L−1 for Pb and a quantification limit of 10−8 mol L−1 for Cd and 2 × 10−8 mol L−1 for Pb. In comparison with an unmodified electrode, the electrodes modified by the membrane 3 allowed to improve the peak current of 32% for Cd and 17% for Pb. The membrane 3 was therefore selected for further experiments. Fig. 2. Analytical performances of electrodes modified with the DB24C8 based membrane: (a) Voltammograms obtained into KNO3 0.1 mol L−1 –HNO3 0.2 mol L−1 for 0, 10−7 , 2.5 × 10−7 , 5 × 10−7 and 7.5 × 10−7 mol L−1 of Cd2+ and Pb2+ and (b) linear relationship between peak currents and ion concentrations.

voltammograms of Cd and Pb (Fig. 2). The linear relationship between peak currents and ion concentrations is clearly shown up to 7.5 × 10−7 mol L−1 Pb and Cd. Considering the regression line obtained with the electodes modified by the membrane 4, the sensitivity was 13.1 nA ␮g−1 mL−1 for Cd and 12.4 nA ␮g−1 mL−1 for Pb. The detection limit was 2 × 10−8 mol L−1 for Cd and 5 × 10−8 mol L−1 for Pb. The quantification limit was 7 × 10−8 mol L−1 for Cd and 15 × 10−8 mol L−1 for Pb. 3.2.2. Membranes based on TT12C4 The TT12C4 based membranes allowed to obtain wellshaped voltammograms of Cd and Pb. However, the membranes 1 and 2, which had, respectively, the smallest amount of crownether and the smallest amount of polystyrene, exhibited the smallest peak currents for a same concentration of Cd and Pb (10−7 mol L−1 ). The membranes 3–5, which differed from the amounts of KTCPB, showed similar voltammograms, the peak current being systematically lower for Cd than for Pb. This behaviour can be attributed to the selective uptake of Pb2+ by the crown-ether. The TT12C4 has the ability to discriminate among

3.3. Analytical parameters optimisation 3.3.1. Mercury film deposition As reported previously, before use the electrode has to be pretreated by applying a negative potential for a fixed time in order to reduce the mercury(II) to metallic mercury. Two periods were tested: 120 and 300 s. The time of 300 s appeared as the optimal time to use the sensors during several days (not shown). The storage stability was studied by comparing the peak currents obtained for a solution of Cd and Pb at 10−7 mol L−1 . It was found that an electrode modified by a crown-ether based membrane allowed to perform 100 measurements, in continuous or not, with the same mercury film with a relative standard deviation smaller than 10%. Moreover, four films can be deposited on the same electrode without loss of sensitivity, which is not possible with an unmodified electrode. This means that the electrodes Table 2 LOD and LOQ (in 10−8 mol L−1 ) of different membrane-coated electrodes obtained from calibration curves from 5 × 10−8 to 3 × 10−7 mol L−1 of Cd and Pb

Membrane 3 Membrane 4 Membrane 5

LODCd

LOQCd

LODPb

LOQPb

0.6 1 0.7

1 3 4

0.8 1 1

2 4 3

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Fig. 4. Effect of scan rate on the current response recorded with the TT12C4 modified electrode for 10−7 mol L−1 Cd2+ and Pb2+ into KNO3 0.1 mol L−1 –HNO3 0.2 mol L−1 ; electrodeposition, 360 s; equilibration, 30 s; scan rates, from 0.5 to 2 V s−1 ; n = 3.

modified by a crown-ether based membrane can be selected to perform continuous measurements. 3.3.2. Scan rate Current responses were recorded for Pb2+ and Cd2+ solutions at 10−7 mol L−1 , at varying scan rates (Fig. 4). The current intensity increases with the scan rate. However, a decrease of the repeatability is simultaneously observed, the error reaching 10% for the highest scan rate. Also, a shift in the peak potential was observed. The value of 0.5 V s−1 was therefore chosen. 3.3.3. Repeatability and reproducibility The repeatability was evaluated through the relative standard deviation of five replicate measurements of a solution containing 10−7 mol L−1 of Cd and Pb. The relative standard deviations were found less than 5%. The reproducibility between electrodes was evaluated through the same method by using three different electrodes. The relative standard deviation was 7% for Cd and 12% for Pb in a solution Cd and Pb 10−7 mol L−1 . 3.3.4. Electrodeposition time It is obvious that the extent of accumulation in stripping analysis is dependent on the preconcentration time. The longer the deposition step, the higher the amount of analyte is available at the electrode during the stripping analysis, as in the case in classical polarography. Fig. 5 illustrates this point with a linear increase in peak currents up to 480 s preconcentration time. Considering sensitivity and analysis frequency, a time of 120 s was chosen. 3.4. Application to natural sample analysis The electrode modified by the membrane 3 was used for the quantification of Cd and Pb in natural solutions, a river water and a soil solution extracted with KNO3 1 mol L−1 . Quantifications were carried out by the standard addition method in the raw and acidified samples and compared with the results obtained by graphic furnace AAS. Also, pH, total organic carbon (TOC), inorganic carbon (IC) and dissolved organic carbon

Fig. 5. Relationship between the preconcentration time and the peak currents for a solution of Cd2+ and Pb2+ of 5 × 10−8 mol L−1 into KNO3 0.1 mol L−1 –HNO3 0.2 mol L−1 .

(DOC) have been determined. The results are summarized in Table 3. Soil solution and river samples (10 mL) were acidified at 10−2 mol L−1 with concentrated HNO3 . In the acidified river water, the concentration value obtained by using the LSASV method was close to the GF-AAS results. Moreover, it can be noted that there is no significant difference between concentrations obtained in both acidified and raw samples, which suggested that the dissolved Cd was mainly present as labile forms. This observation is in agreement with the lower organic carbon content (1.5 mg L−1 ) dissolved in this river water sample. In the case of the soil solution extract, the three methods yielded similar results for Cd, suggesting that Cd was present as free ion and/or in labile complexes. On the contrary, for Pb, three different results were obtained according to the method used. Thus, the GF-AAS measurements showed that total dissolved Pb represents 17 ␮g L−1 . The LSASV measurements performed in the raw sample did not allow Pb detection, which indicated the presence of stable Pb-complexes. LSASV method performed on acidified sample yielded 9 ␮g L−1 , which suggested that 50% of the total dissolved Pb corresponded to slightly labile complexes. These results led to the strong affinity of Pb for the organic matter. Table 3 Results obtained on natural samples

pH TOC (mg L−1 ) IC (mg L−1 ) DOC (mg L−1 ) Cd (␮g L−1 ) LSASV-raw LSASV-acidified GFAAS Pb (␮g L−1 ) LSASV-raw LSASV-acidified GFAAS

River water

Soil solution (KNO3 extract)

8.2 20.0 18.5 1.5

6.0 3.5 0.0 3.5

14.5 (0.7) 16 (1) 16.2

34 (1) 36 (1) 31

n.d.a n.d.a 0.8

n.d.a 9 (1) 16.7

Standard deviation is given in parenthesis. a Not detected.

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4. Conclusion The proposed screen-printed electrodes modified by a crownether based membrane appear to be convenient and inexpensive tools for trace metal detection. Analytical results showed that these electrodes, modified by the membrane 3 which the composition is 15/6/125 (in mg) (TT12C4–KTCPB–polystyrene), were able to detect simultaneously some ␮g L−1 of Pb2+ and Cd2+ , with good sensitivity and reproducibility, at different pH. Moreover, the direct analysis in raw samples without pretreatment offered a first and rapid approach of metal speciation. These sensors appeared therefore as a promising tool to aid environmental decision making. Acknowledgments The authors acknowledge Laurence Denaix and Val´erie Sappin-Didier of the Soil plant transfer and cycle of nutrients and trace elements INRA (Bordeaux, France) for their collaboration. References [1] L. Sigg, W. Stumm, R. Behra, Chimie des milieux aquatiques, Masson ed., Paris, 1992. [2] W. Davison, H. Zhang, Nature 367 (1994) 546. [3] E.J.M. Temminghoff, A.C.C. Plette, R. Van Eck, W.H. Van Riemsdijk, Anal. Chim. Acta 417 (2000) 149. [4] C. Erkey, J. Supercrit. Fluids 17 (2000) 259. [5] D. Wang, J. Xing, J. Peng, C. Wu, J. Chromatogr. A 1005 (2003) 1. [6] Z. Zeng, W. Qiu, M. Yang, X. Wei, Z. Huang, F. Li, J. Chromatogr. A 934 (2001) 51.

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[7] S. Katsuta, F. Tsuchiya, Y. Takeda, Talanta 51 (2000) 637. [8] M. Shamsipur, M.H. Mashhadizadeh, Talanta 53 (2001) 1065. [9] M. Shamsipur, M.H. Mashhadizadeh, G. Azimi, Sep. Purif. Technol. 27 (2002) 155. [10] M.R. Yaftian, A.A. Zamani, M. Parinejad, E. Shams, Sep. Purif. Technol. 42 (2005) 175. [11] V.K. Gupta, M. Al Khayat, A.K. Minocha, P. Kumar, Anal. Chim. Acta 532 (2005) 153. [12] V.K. Gupta, S. Chandra, R. Mangla, Electrochim. Acta 47 (2002) 1579. [13] V.K. Gupta, P. Kumar, Anal. Chim. Acta 389 (1999) 205. [14] J. Wang, Stripping Analysis—Principles, Instrumentation, and Applications; Inc., Florida ed., VCH Publishers, Deerfield Beach, 1985. [15] T. Ugur, S. Yucel, E. Nusret, U. Yasemin, P. Kadir, Y. Attila, J. Electroanal. Chem. 570 (2004) 6. [16] I. Palchetti, S. Laschi, M. Mascini, Anal. Chim. Acta 530 (2005) 61. [17] K.Z. Brainina, A.V. Ivanova, N.A. Malakhova, Anal. Chim. Acta 349 (1997) 85. [18] C.M.A. Brett, D.A. Fungaro, J.M. Morgado, M.H. Gil, J. Electroanal. Chem. 468 (1999) 26. [19] C.M.A. Brett, A. Maria Oliveira Brett, F.-M. Matysik, S. Matysik, S. Kumbhat, Talanta 43 (1996) 2015. [20] M.P. Hurst, K.W. Bruland, Anal. Chim. Acta 546 (2005) 68. [21] H.-J. Kim, K.-S. Yun, E. Yoon, J. Kwak, Electrochim. Acta 50 (2004) 205. [22] S. Zhang, W. Huang, Anal. Sci. 17 (2001) 983. [23] L. Authier, B. Sch¨ollhorn, B. Limoges, Electroanalysis 10 (1998) 1255. [24] L. Authier, B. Sch¨ollhorn, J. Moiroux, B. Limoges, J. Electroanal. Chem. 488 (2000) 48. [25] O. Bagel, B. Limoges, B. Sch¨ollhorn, C. Degrand, Anal. Chem. 69 (1997) 4688. [26] L. Authier, C. Grossiord, P. Brossier, B. Limoges, Anal. Chem. 73 (2001) 4450. [27] E. Santoyo, S.P. Verma, J. Chromatogr. A 997 (2003) 171. [28] G. Lagger, L. Tomaszewski, M.D. Osborne, B.J. Seddon, H.H. Girault, J. Electroanal. Chem. 451 (1998) 29.