Cristina Ariño

Multivariate curve resolution with alternating least squares optimisation: a soft-modelling approach to metal complexation studies by voltammetric techniques

Abstract Voltammetry of metal ions in the presence of ligands is usually interpreted by means of hard-modelling approaches. Soft-modelling approaches have been scarcely applied to electroanalytical data, especially in comparison with their use in spectroscopy. In recent years, the multivariate curve resolution–alternating least squares (MCR–ALS) method, based on factor analysis, has been shown to be a powerful tool in the analysis of metal complexation processes by a variety of voltammetric techniques. In this article, an overview of the method and its applications is presented. For this purpose, either numerically simulated or experimental data corresponding to different characteristic systems are analysed. A very interesting feature of MCR–ALS is that, by using different constraints, both the pure voltammograms of the different electrochemical processes involved and the corresponding concentration profiles are easily obtained. This type of information cannot be reached by means of classical univariate data analysis techniques. From calculated concentration profiles, the model of complexation and the equilibrium stability constants are estimated.

Antimony film screen-printed carbon electrode for stripping analysis of Cd(II), Pb(II), and Cu(II) in natural samples.

An in-situ antimony film screen-printed carbon electrode (in-situ SbSPCE) was successfully used for the determination of Cu(II) simultaneously with Cd(II) and Pb(II) ions, by means of differential pulse anodic stripping voltammetry (DPASV), in a certified reference groundwater sample with a very high reproducibility and good trueness. This electrode is proposed as a valuable alternative to in-situ bismuth film electrodes, since no competition between the electrodeposited copper and antimony for surface sites was noticed. In-situ SbSPCE was microscopically characterized and experimental parameters such as deposition potential, accumulation time and pH were optimized. The best voltammetric response for the simultaneous determination of Cd(II), Pb(II) and Cu(II) ions was achieved when deposition potential was -1.2 V, accumulation time 120 s and pH 4.5. The detection and quantification limits at levels of μg L(-1) suggest that the in-situ SbSPCE could be fully suitable for the determination of Cd(II), Pb(II) and Cu(II) ions in natural samples.

Differential pulse voltammetric study of the complexation of Cd(II) by the phytochelatin (γ-GluCys)2Gly assisted by multivariate curve resolution

Abstract A multivariate curve resolution method by alternating least-squares (MCR-ALS) is applied to differential pulse voltammograms measured on the Cd(II)+(γ-GluCys) 2 Gly system as a model of metal–phytochelatin interactions at concentrations of both components in the range 10 −7 –10 −5 mol l −1 . The course of complexation is different when peptide is titrated with metal from that when metal is titrated with peptide. The combined analysis of both matrices from titrations of peptide with metal and of metal with peptide allowed the resolution of the system. The analysis of the resulting pure voltammograms and concentration profiles of the resolved components suggested the presence of four different types of bound Cd(II) and made possible the formulation of a complexation model.

Heavy Metal Binding by Tannic Acid: A Voltammetric Study

The interactions between tannic acid (TA) and the heavy metal ions Cu(II), Pb(II), Cd(II), and Zn(II), have been studied by several voltammetric techniques as a model for the metal binding of tannin substances and also to evaluate the possible effects of such compounds on the voltammetric analysis of natural samples. The study has shown quite a different behavior of the systems considered: Cd(II) is practically not complexed by TA, whereas Pb(II) and Zn(II) produce labile complexes and Cu(II) seems to form an inert complex with TA. In the cases of the Zn(II)+TA and Pb(II)+TA systems, it was possible to give a rough estimation of the stability constants and the relative diffusion coefficients of the complexes. From these values, it appears that the affinity for TA increases in the order Cd(II)<Zn(II)<Pb(II). In contrast, the possible stabilization of Cu(I) species during the voltammetric measurement hindered a quantitative study of the complexation of Cu(II) by TA.

Thermodynamics of Cd2+ and Zn2+ binding by the phytochelatin (γ-Glu-Cys)4-Gly and its precursor glutathione

Isothermal titration calorimetry (ITC) was used to study the binding of Cd(2+) and Zn(2+) by glutathione (GSH) and phytochelatins (PC(n)), the metal sequestering compounds in plants and algae. The results are compared with those obtained by differential pulse polarography (DPP) assisted by multivariate curve resolution with alternating least squares (MCR-ALS) and by electrospray ionization mass spectrometry (ESI-MS). ITC allows one to determine (i) the stoichiometries of the different complexes (and confirms those found by DPP/MCR-ALS and ESI-MS) and (ii) their binding and thermodynamic parameters. For Cd-PC(4), the sequential binding sites model with two identical sites yields the best fitting of ITC curves and confirms the presence of CdPC(4) and Cd(2)PC(4) complexes. For Zn-PC(4), exothermic formation of ZnPC(4) is reported. Conditional stability and formation constants for Cd-GSH and Zn-GSH are determined from the fitting of the proper model to experimental ITC curves. The effect of different buffers in the complexation processes shows the key role of the choice of the buffer in calorimetric study.

Chemometrics in Electroanalytical Chemistry

The use of chemometrics in electroanalytical chemistry is not as popular as in spectroscopy, although recently, application of these methods for mathematical resolution of overlapping signals, calibration and model identification have been increasing. Self-modelling curve resolution and multivariate analysis have been shown to be very powerful for in the analysis of electroanalytical data, especially for multianalyte calibration and modelling in multicomponent dynamic systems. In this paper, an overview on the application of chemometrics to electroanalytical data is presented, with special attention to the contributions of the last decade.

Sputtered bismuth screen-printed electrode: a promising alternative to other bismuth modifications in the voltammetric determination of Cd(II) and Pb(II) ions in groundwater.

A commercially available sputtered bismuth screen-printed electrode (BispSPE) has been pioneeringly applied for the simultaneous determination of Cd(II) and Pb(II) ions in a certified groundwater sample by means of differential pulse anodic stripping voltammetry (DPASV) as an alternative to more conventional bismuth screen-printed carbon electrodes (BiSPCEs). BispSPEs can be used for a large set of measurements without any previous plating or activation. The obtained detection and quantification limits suggest that BispSPEs produce a better analytical performance as compared to In-situ BiSPCE for Pb(II) and Cd(II) determination, but also to Ex-situ BiSPCE for Cd(II) determination. The results confirm the applicability of these devices for the determination of low level concentrations of these metal ions in natural samples with very high reproducibility (0.7% and 2.5% for Pb(II) and Cd(II) respectively), and good trueness (0.3% and 2.4% for Pb(II) and Cd(II) respectively).

Differential pulse polarographic study of the Pb(II) complexation by glutathione

Differential pulse polarography (DPP) is applied to the study of the complexes of Pb(II) with glutathione (GSH). As occurred in previous studies on Cd(II)– and Zn(II)–GSH complexes, the system presented overlapping of the peaks of free metal and metal complexes but, additionally, the interference of the anodic signals of mercury in the presence of GSH. This necessitated, even more than in previous work, the use of a multivariate curve resolution method (MCR-ALS) to extract quantitative information from the system. Despite the important interference of anodic signals, the combined use of DPP and MCR-ALS suggested the presence of two kinds of Pb(II)–GSH bonds and allowed the prediction of stoichiometries for the complexes formed.

Glutathione modified screen-printed carbon nanofiber electrode for the voltammetric determination of metal ions in natural samples.

This work reports the development of a glutathione modified electrode via electrografting on a screen-printed carbon nanofiber substrate (GSH-SPCNFE). GSH-SPCNFE was compared to a classical screen-printed carbon electrode modified with glutathione (GSH-SPCE) for the simultaneous voltammetric determination of Cd(II) and Pb(II). Their electrochemical characterization and analytical performance suggest that SPCNFE could be a much better support for GSH immobilization. The applicability of GSH-SPCNFE for the determination of low concentration levels of Pb(II) and Cd(II) ions in environmental samples was successfully tested in a certified wastewater reference material by means of stripping voltammetry with a very high reproducibility and good trueness.

Stripping analysis of heavy metals in tap water using the bismuth film electrode

AbstractA commercially available screen-printed carbon electrode coated with an ex situ deposited bismuth film (BiSPCE) has been applied to the determination of Pb(II) and Zn(II) ions in tap water (Barcelona water distribution network) by means of stripping voltammetry (SV) and stripping chronopotentiometry (SCP). A good reproducibility of the measurements and a satisfactory agreement between SV and SCP data were observed for both heavy metal ions. Although, in principle, the procedure could be also suited to the determination of Cd(II), this species was not detected. The results were also consistent with the routine ICP-OES measurements of the water distribution company, thus confirming the potential usefulness of such BiSPCE disposable devices for the analysis of heavy metals in natural waters. FigureDPASV measurements of Pb(II)-Cd(II)-Zn(II) system in 0.01 mol L-1 KNO3 on bismuth film coated screen-printed carbon electrode (BiSPCE