Zorana Grabarić

Application of multivariate curve resolution to the voltammetric data Factor analysis ambiguities in the study of weak consecutive complexation of metal ion with ligand

Abstract Different techniques for factor analysis (FA) have been applied in the study of weak consecutive complexes of Cd(II) propanoate by analysing experimental matrices obtained from four electroanalytical techniques: normal pulse polarography (NPP), reverse normal pulse polarography (RNPP), differential pulse polarography (DPP) and differential pulse anodic stripping voltammetry (DPASV). All experiments were performed at a constant temperature, 25 ± 1 °C, in aqueous solutions having a constant cadmium ion concentration (10 and 20 μM), varying propanoate concentrations in the range 10–400 mM, a constant c (propanoic acid)-to- c (sodium propanoate) ratio (1: 50), and a constant ionic strength (2 M), obtained by addition of NaClO 4 . From the experimental signals using DeFord and Humes model, to calculate the experimental F 0 function model identification, β i estimates and corresponding standard error of stability constants were obtained using conventional fitting of polynomial (univariate hard modelling) to the experimental data, together with the recently proposed evolving polynomial fitting and optimized least squares extrapolation of F i functions. The same experimental data matrices were analysed by multivariate soft modelling approach using the following FA techniques: principal component analysis (PCA), evolving factor analysis (EFA) and constrained alternating least squares (ALS) optimization of individual and global (augmented) data matrices.

Resolution of global signals using ratio differential pulse polarograms: Determination of p-nitroaniline and p-nitrotoluene in their mixture

Abstract Ratio differential pulse polarograms obtained by dividing the multianalyte and single analyte signals are proposed as a tool for resolution of global signals and quantification of the analytes from a qualitatively known mixture by differential pulse polarography (DPP) and related electroanalytical techniques. The influences of shape and position of the resolving function (DP polarograms of individual analyte) on the efficiency of resolution are discussed on simulated and experimental results. The method is applied for the determination of p-nitroaniline (NA) and p-nitrotoluene (NT) from their mixture in N,N′-dimethylformamide solutions with 0.1 M tetrabutylammonium iodide as supporting electrolyte, using an external calibration diagram and internal standard addition methods. NA and NT give one-electron DP polarographic peaks with 93 mV of peak separation and, therefore, show significant overlapping which depends on the concentration ratio of NA and NT in the mixture. The method is especially suitable for quantification of one analyte in the presence of a large excess of another analyte, because by division the component in excess is removed and the pseudo-ratio DPP of the minor component is clearly revealed in a way which is not possible by deconvolution using polynomial division or deconvolution by Fourier transforms.

Signals ratio method for resolution enhancement in differential pulse polarography and related techniques

An efficient and simple resolution method based on signals ratio in time domain is proposed for differential pulse (DP) polarographic and related techniques. The ratio pseudo-DP polarograms are obtained dividing the multianalyte DP polarogram with individual DP polarograms of a single analyte. Using the same procedure for DP polarograms recorded for calibration diagrams and for DP polarograms of unknown binary mixtures, well resolved ratio pseudo-DP polarograms are obtained. The method is applied to the model binary system of lead and thallium, having approximately 70 mV of DP polarographic peaks separation depending of the medium used. DP polarograms of these two metal ions show significant overlapping which depends on their concentration ratio. The method proposed is able to resolve lead from at least 40-fold excess of thallium, and thallium from at least 20-fold excess of lead at micromolar concentration range with reproducibility better than 5%.

Optimisation of resolution function in signals ratio method and deconvolution by polynomial division - quantitation of Cd(II) and In(III) from their global signals obtained at carbon fibre disk ultramicroelectrode

In order to investigate the applicability of the carbon fibre disk ultramicroelectrode (CFDUME) as a global electrochemical sensor for metal ions, several resolution methods have been tested for quantitation of Cd(II) and In(III) ions in their mixture. The signal ratio resolution method, as well as conventional deconvolution and deconvolution by fast Fourier transform have been compared using signals from different mixtures of Cd(II) and In(III) ions, obtained by differential pulse anodic stripping voltammetry (DPASV) at CFDUME. For the first two methods an efficient parameter is introduced which we named parameter of the difference between the mean of the mean of absolute and non-absolute values of pseudo-signals (or deconvoluted signals), PDMMV (PS). Using many different resolving functions, this parameter enables to select the optimised one, obtaining therefore reliable concentrations, independent of the resolving signal used. Fourier transform deconvolution was also compared, but in its conventional form, it seems to be unsuitable for quantitation because it introduces deconvolution noise which is too high. The PDMMV parameter was tested on simulated differential pulse polarographic data and experimental DPASV results.

Determination of small amounts of analytes in the presence of a large excess of one analyte from multi-analyte global signals of differential-pulse voltammetry and related techniques with the signal ratio resolution method

The signal ratio resolution method was applied to the determination by differential-pulse voltammetry and related techniques of two analytes in presence of a large excess of a third analyte in ternary mixtures of known qualitative composition. Using the signal ratio resolution method, a global signal is divided by the individual signal of the major component, multiplied by a factor, in order to obtain a height as close as possible to the estimated contribution of the major component to the global signal. To establish possible influence of resolving the signal on the quantification, simulated differential-pulse polarograms (DPPs) were divided by the individual major component DPP having a different peak current height and different current translation values. Using theoretically simulated DPPs, it was found that two minor components can be determined with a relative error <1% for different resolving parameters (peak current height of resolving functions, current translation values and peak separations). The method was then verified on a system containing Cd at 0.3, Tl at 200 and Pb at 0.5 µmol l–1 using differential-pulse anodic stripping voltammograms. It was shown that using this method and a system with favourable reversibility and amalgam formation, cadmium can be determined in a 200-fold and lead in a 120-fold excess of thallium, with a relative error of <10%. The main advantage of the method over other deconvolution methods is the removal of the contribution of one component from the global signal, revealing the contributions of the other components in a peak-shaped form that is linearly proportional to the analyte concentrations.

Evolving polynomial regression and error analysis in model identification and determination of consecutive stability constants using voltammetric methods

Abstract Error analysis in polarographic model identification and stability constant determination of consecutively formed complexes has been performed on simulated data. On the basis of the results obtained on simulated data several constraints have been proposed in order to obtain more reliable model identification and stability constants having physico-chemical meaning. Ledens function extrapolation has been optimized using least squares in order to minimize the possible cumulative errors. Evolving polynomial regression has also been proposed as a method for better model identification. The proposed procedure was tested on experimental data of Pb(II) and Cd(II)-propanoate, measured using differential pulse polarography in aqueous solution of constant ionic strength of 2 M (NaClO4) and constant temperature of 25 ± 1 °C. An evaluation of the data for the Cd(II)-thiocyanate system published in the literature has also been attempted.