Employment of peak area as instrumental datum for improving the voltammetric quantitative analysis of heavy metals in fresh and sea water in the presence of surfactants

Peak area instead of peak height is used for the simultaneous determination of Cu(II), Pb(II), Cd(II), and Zn(II) in fresh and sea water in the presence of anionic and cationic surfactants by differential pulse anodic stripping voltammetry. These species, if present, tend to make more irreversible the electrodic process of the metals. Because the employment of peak area for species having irreversible electrodic processes permits limits of detection about one or 2 orders of magnitude lower, this work shows the possibility of determining heavy metals at ultratrace level concentrations in the presence of surfactants, compounds which are always present in natural waters. The precision and accuracy of the analytical method were checked by the analysis of the standard reference materials (SRM) Fresh Water NIST-SRM 1643d, Sea Water BCR-CRM 403, and Estuarine Water BCR-CRM 505. The former, expressed as relative standard deviation (s(r)), and the latter, expressed as relative error (e), were satisfactory, being in all cases lower than 5%. The analytical procedure has been applied to fresh and sea water sampled in the Po river mouth area (Italy).     

Voltammetric Peak Area as Instrumental Datum. A Possibility to Improve the Determination at Ultratrace Level Concentration of Platinum Group Metals (PGMs) and Lead. Application to Particulate Matter

Peak area as instrumental datum for determining the concentration of metals in solution instead of peak height is proposed for analytical voltammetric determinations. In the case of species present at ultratrace concentration level or having low reversibility degree of the electrodic processes, the employment of peak area permits to achieve limits of detection lower even more of one order of magnitude. The present work shows the possibility of determining at ultratrace level concentration Pd(II), Pt(II), Rh(III) and Pb(II) by voltammetry in particulate matter; these species, linked to vehicle emissions, show to have, in the supporting electrolytes employed, a very high irreversibility degree of the electrodic processes.     

Crystallization impedance in the case of D.C. superposition

An expression for the impedance frequency dependence in the case of d.c. superposition has been derived and analyzed for the crystallization process of metals taking into account the two-dimensional diffusion of ions in solution and the surface diffusion of adatoms and using the simplest surface model with parallel step lines. It is shown that in the general case it is impossible to present the crystallization impedance as the sum of the surface diffusion impedance and the Warburg impedance. It has been found that in the case of anode polarization an unusual effect of the impedance increasing with agitation intensity takes place which is typical of this mechanism. Possible methods for determination of the model parameters from impedance measurements are discussed. The authors suggest an iteration procedure which permits to determine all the parameters of the model under consideration including the surface diffusion coefficient of adatoms and the spacing between the growth step lines.     

The Rayleigh‐Schrödinger perturbation series of quasi‐degenerate systems

We describe the first representation of the general term of the Rayleigh‐Schrödinger series for quasidegenerate systems. Each term of the series is represented by a tree and there is a straightforward relation between the tree and the analytical expression of the corresponding term. The combinatorial and graphical techniques used in the proof of the series expansion allow us to derive various resummation formulas of the series. A relation with several combinatorial objects used for special cases (degenerate or non‐degenerate systems) is established. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

A graphical symmetric group approach for a spin adapted full configuration interaction: partitioning of a configuration graph into sets of closed-shell and open-shell graphs

We developed a spin adapted full configuration interaction (FCI) method which was expected to be effective for parallel processing. The graphical symmetric group approach (GSGA) was employed, where a configuration graph was partitioned into several sets of closed-shell and open-shell graphs. The configuration state functions (CSFs) bearing the same number of closed-shells and open-shells were assembled in a configuration group. The graphical approach provided a number to identify each CSF in a sequential order within the group. Combination of this partitioning and an intermediate configuration-driven algorithm in calculating the so-called σ vectors allowed us to use symbolic coupling constants. Furthermore, this combination made it easy to implement an efficient algorithm suitable to task-distributed parallel procedure for evaluating σ vectors. A program was written and some test calculations were carried out with high parallel efficiency. The largest size of FCI used 10 million CSFs (20 million determinants).     

Voltammetric studies of indigo adsorbed on pre-treated carbon paste electrodes

The adsorptive behaviour of indigo on the surface of pre-treated carbon paste electrodes (CPE) has been studied in various aqueous media at different pH, by cyclic (CV) and alternating current voltammetry (ACV). Cyclic voltammograms of this molecule are characterised by two main electrodic processes in the potential range from −0.7 V to +0.9 V (vs. Ag/AgCl/sat. KCl), which are reversible in the pH range of 3–11, these being irreversible when both are recorded at pH 1. The first one at pH 1 becomes purely reversible when the applied potential range is from −0.2 V to +0.2 V and constitutes the best analytical signal. Due to these electrodic features a significant increase in sensitivity was achieved when the signal was recorded by alternating current voltammetry. Indigo was quantified by CV and ACV, in 0.1 M HClO₄ pH 1 and 0.1 M Tris-HCl pH 7.2 solutions. Limits of detection in the sub-nanomolar range were achieved for a 5-min accumulation time by ACV. These data provides useful information about the suitability of this electrodic process as a detection scheme in the development of alkaline phosphatase (AP) based voltammetric affinity devices, in which indigo is generated by the enzymatic hydrolysis of the 3-indoxyl phosphate substrate.     

New method for a graphical portrayal of the electrochemical impedance

A new method of a graphical representation of the electrode impedance is proposed. The method allows one to construct the impedance of a complicated circuit by summing up impedance plots for series and parallel branches. Rules for constructing complex representations are presented. New shapes of impedance plots for some of the most typical circuits are analyzed. The method opens additional possibilities for analyzing how the experiment accuracy impacts the limits of discovery of individual elements of complicated circuits. The method gives grounds to introduce the notion of the information potential of an experimental setup. Conditions for a complete decidability of equivalent circuits are shown by the example of a system that comprises several parallel branches consisting of the Frumkin-Melik-Gaikazyan impedances.     

Impedance modelling of two-phase solid-state ionic conductors. Part I—theoretical model and computer simulations

An equivalent circuit model is introduced to account for the impedance properties of solid state ionic conductors, composed of two distinct phases. The model is developed on the basis of physical arguments, regarding the micrometer-scale structure of the two-phase material system and the comparison of different possible equivalent circuit representations. The final equivalent circuit reduces to two simpler circuits, suitable for fitting experimental impedance spectra. Computer simulations are provided to demonstrate the non-Arrhenius behaviour, which is observed in the temperature dependence of the ohmic elements of the equivalent circuits used for data analysis. This complex dual-slope behaviour of the Arrhenius plot is in agreement with the predictions of the model. Finally, with the aid of mathematical calculations and illustrated by computer simulations, a modified Arrhenius plot evaluation procedure was developed to derive correctly the electrical properties of the individual constituent phases from impedance measurements.     

Multiwalled carbon nanotube modified screen-printed electrodes for the detection of p-aminophenol: optimisation and application in alkaline phosphatase-based assays

Carboxylated multiwalled carbon nanotubes (MWCNT-COOH) were used to modify the working electrode surface of different screen-printed electrodes. The effect of this modification on the electrodic characteristics (double layer capacitance, electroactive area and heterogeneous rate constants for the electron transfer) was evaluated and optimized for the cyclic voltammetric determination of p-aminophenol. The enzymatic hydrolysis of p-aminophenylphosphate was employed for the quantification of alkaline phosphatase, one of the most important label enzymes in immunoassays. Finally, ELISA assays were carried out to quantify pneumolysin using this enzymatic system. Results obtained indicated that low superficial densities of MWCNT-COOH (0.03-0.06 μg mm⁻²) yielded the same electrodic improvements but with better analytical properties.     

New voltammetric method useful for water insoluble or weakly soluble compounds: application to p<Emphasis Type="Italic">K</Emphasis>a determination of hydroxyl coumarin derivatives

Here, we reveal a different way of doing the voltammetric experiments that considers the electroactive species packaged in the electrodic phase instead of dissolved in solution. In this way, it is possible to obtain voltammograms of insoluble species. In this work, the method is exemplified by obtaining voltammograms for weakly soluble coumarins but it could be extrapolated to other weakly soluble compounds. We have studied a comprehensive series of 3-acetyl-hydroxycoumarins derivatives which are insoluble in aqueous medium but capable of being trapped in a three-dimensional multi-walled carbon nanotubes (MWCNT) network. Consequently, an electrodic phase composed of an MWCNT modified glassy carbon electrode (GCE) containing the coumarin derivative is prepared. The voltammetric experiment is performed with the above electrodic phase and an aqueous medium as the solution phase. All the coumarin derivatives show one anodic peak due to the oxidation of the hydroxyl group in the phenyl ring. The oxidation peaks follow a one-electron, one-proton irreversible, pH-dependent process for all monohydroxylated compounds. The Ep values are closely dependent of the substituent effect being the 7,8-cum derivative more easily oxidized due to both the electron donor effect of neighboring hydroxyls groups and hydrogen bonding interaction between them. On the other hand, the hydroxyl of the 7-cum derivative is the most difficult to oxidize due to the electron-attracting effect of the lactone carbonyl group at position 2 and acetyl carbonyl at position 3 which is conjugated with OH at 7 positions. From the breaks in the graphs Ep versus pH, we estimate the voltammetric pKa values for all the studied coumarin derivatives.     

Angular distributions for vibrationally inelastic nonresonant scattering of electrons by molecules

Polar graphs for differential cross section (dcs) called spatial dcs maps are presented as graphical representation of the angular distribution of vibrationally inelastic electron scattering by polyatomic molecules. The objective of this paper is to show that an intuitive understanding of the principal features of these graphs can be obtained from a simple analysis of the normal modes of vibration of the target molecule and plane-wave functions representing the scattering electron. The procedure is illustrated on the H2 and CH4 molecules.     

Design of Micro‐interdigitated Electrodes and Detailed Impedance Data Analysis for Label‐free Biomarker Quantification

Electrical impedance based biosensing is a label‐free technique that is gaining momentum in biology/medicine. The electrical impedance, typically measured using an array of micro‐fabricated interdigitated electrode array (IDE), is a byproduct of the interaction between electric fields and target bio‐molecules/cells. In current impedance based biosensing, it has been focused on utilizing the magnitude of the impedance (|Z|) to detect/quantify bio‐molecules. There were no reports on designing IDE electrodes, sensitivity analysis and detailed impedance data analysis. To address this issue, we have designed and fabricated IDE array and performed model experiments. We have found that depending on the frequency of the external electric potential, there is a variation of electric field across the array of IDEs from first pair to last pair. We then developed impedance data analysis technique (using (|Z|) and its phase (φ)) to analyze the complex impedance data, and finally, we have utilized Warburg theoretical circuit model to calculate the capacitance and resistance of the individual IDE pairs in the constant phase impedance region. Using the capacitance values, we have developed a procedure to determine the sensitivity of the IDE array. We have found that sensitivity of the IDE array does not depend on the sample conductivity.     

Voltammetric monitoring of laccase-catalysed mediated reactions

Six different compounds capable of mediating laccase-catalysed reactions have been tested by cyclic voltammetry. They exhibited quasi-reversible electrodic behaviour with formal redox potentials ranging from 150 to 800 mV (E(0)' vs. SCE). The immersion of a laccase-coated glassy carbon electrode (GCE) in mediator solutions generated large cathodic catalytic currents easily recorded by cyclic voltammetry at low-potential scan rates. This current showed two well-defined pH profiles, which correlated with the variation of the mediator redox potentials at the pH range tested. The relevant effect of temperature on the activity of laccase has been assessed here. Likewise, it was shown that the current record varied with the substrate concentration. This trend fitted Michaelis-Menten kinetics, which allowed us to give an estimation of the affinity of the fungal laccase for the different mediators.     

An orbital localization criterion based on the topological analysis of the electron localization function at correlated level

This work describes a procedure for localizing orbitals based on the topological analysis of the electron localization function at correlated level. The decomposition of the overlap matrix according to the partitioning of the three dimensional physical space into basins provided by that function allows us to define a localization index to be maximized using isopycnic orbital transformations. The localization algorithm has been computationally implemented and its efficiency tested on selected molecular systems at equilibrium, stretched, and twisted geometries. We report results which allow to analyze the influence of the correlated and uncorrelated treatments on the orbital localization.     

Advantages of Odd Random Phase Multisine Electrochemical Impedance Measurements

Electrochemical impedance spectroscopy (EIS) is a powerful technique to study electrochemical processes and to perform screening tasks. Recently an integrated measuring and modeling methodology for EIS based on a multisine excitation signal was developed. A key issue in this methodology is the data analysis, allowing us to rapidly quantify the reliability of the measured data. In this paper, a comparison is made between classical single‐sine and the proposed multisine measurements on the same system. The fitting of the impedance data obtained by single‐or multisine excitation and using different weighting factors is also discussed. In addition to the advantages reported in earlier work, it is concluded that, of all investigated frequencies, the odd random phase multisine excitation yields the highest quality data in the shortest measurement time.     

Response of nanoparticle-based one-dimensional photonic crystals to ambient vapor pressure

Herein we report an analysis of the variation of the optical properties of different nanoparticle-based one-dimensional photonic crystal architectures versus changes in the ambient vapor pressure. Gradual shift of the optical response provides us with information on the sorption properties of these structures and allow us to measure precise adsorption isotherms of these porous multilayers. The potential of nanoparticle-based one-dimensional photonic crystals as base materials for optical sensing devices is demonstrated in this way.     

Automatic assignment of 1H‐NMR spectra of small molecules

A novel data‐evaluation procedure for the automatic atom to peak or multiplet assignment of ¹H‐NMR spectra of small molecules has been developed using a fast and robust expert system. The applicability and reliability of the method are demonstrated by comparison of a manually assigned database of ¹H‐NMR spectra with the assignments produced by the automatic procedure. The results of this analysis show an excellent success ratio, indicating that this new algorithm can have a major impact as a time saving tool for the organic chemist. A new graphical feature used to illustrate both the stability and quality of the elementary assignments is also introduced. Copyright © 2013 John Wiley & Sons, Ltd.     

Analysis of lipid peroxidation kinetics. 1. Role of recombination of alkyl and peroxyl radicals

The kinetics of the lipid peroxidation reaction is only partly understood. Although the set of reactions constituting the overall reaction are believed to be known, it has not been possible to predict how the reaction will respond to a change of one or more of the parameters, e.g., initial concentrations of reactants or different ways of initiating the reaction, nor has it been possible to predict the time dependence of the products. The reason for these problems is the complicated structure of the kinetic scheme, which includes a chain reaction. In this work we perform an in-depth analysis of the importance of the individual reaction steps, and we introduce a new quasi-stationary concentration method based on the assumption that one or more concentrations vary much slower than the others. We show that it is justified to use a quasi-stationary concentration approximation for the alkyl radical L (*), but not for the peroxyl radicals LO 2 (*) as assumed in previous works. The method allows us to derive manageable analytical expressions. On the basis of literature values of the rate constants, we are able to introduce specific simplifications that allow us to obtain simple analytical expressions for the time dependence of all concentrations involved in the process.