A new simple sensitive differential pulse polarographic method for the determination of acrylamide in aqueous solution

A new simple sensitive differential pulse polarographic (DPP) method was investigated for the determination of acrylamide (AA) directly in a neutral aqueous solution. The AA showed a well-defined and well-resolved peak in pure aqueous LiCl at −1.84 V in the potential range from −1.6 V to −1.97 V at nitrogen pressure of 0.5 kg cm⁻². Among the various electrolytes studied, the AA showed good DPP response in the presence of LiCl and tetra methyl ammonium iodide, while it showed poor response in the presence of tetra butyl ammonium hydroxide and tetra butyl ammonium bromide due to their strong adsorption on the surface of electrode which hindered its reduction. The effect of LiCl concentration, the cyclic voltammetric response and the drop time study showed that AA exhibited an irreversible adsorptive electrochemical behavior. The good electrochemical response in pure aqueous medium suggested that hydrogen bonding might be involved which may favor the electrode reaction. Under optimized conditions, the peak current was linear in the entire concentration range from 0.2 mg L⁻¹ to 20 mg L⁻¹ with the correlation coefficient of R² = 0.9998. The method showed good reproducible results with R.S.D. of 0.3% (n = 16). The detection limit (LOD) was 27 μg L⁻¹. The influence of various interfering agents was also studied. The method was applied successfully for the quantification of AA in water samples without any interference effect from alkali metals.     

Spectrofluorometric, thermal, and molecular mechanics studies of the inclusion complexation of selected imidazoline-derived drugs with β-cyclodextrin in aqueous media

The inclusion complexes of selected imidazoline-derived drugs, namely Antazoline (AN), Naphazoline (NP) and Xylometazoline (XM) with β-cyclodextrin (β-CD) were investigated using steady-state fluorescence spectroscopy, differential scanning calorimetry (DSC), and molecular mechanics (MM) calculations and modeling. The modified form of the Benesi-Hildebrand relation was employed for estimating the formation constant (K_f) of the 1:1 inclusion complexes, which was applied based on measuring the variation in the fluorescence intensity of the guest molecule as a function of growing β-CD concentration. On the other hand, the formation of the inclusion complexes was verified by analyzing solid samples of the complexes using DSC. The thermodynamics of the inclusion complexation, standard enthalpy (ΔH°) and entropy changes −(ΔS°) were obtained from the temperature-dependence of K_f. Obtained values of ΔH° and ΔS° indicated that the inclusion process favorably proceeds through enthalpy changes that was sufficiently predominant to compensate for the unfavorable entropy changes. MM calculations revealed that the proposed drugs molecules can form 1:1 inclusion complexes with β-CD that are stabilized predominantly through van der Waals forces. In addition, MM calculation provided the energetically favored configuration of the inclusion complexes, where NP and XM can be included inside the β-CD cavity through its wide rim, whereas AN can penetrate through the narrow rim of the β-CD cavity.     

Hemin Modified Carbon Fat Electrode for Analyzing Antimalarial Endoperoxide Artemisinin in Artemisia annua

A hemin bulk modified carbon electrode with Adeps neutralis (solid fat) as binder was developed for the determination of antimalarial endoperoxide artemisinin in plant matrix. The hemin modified electrode showed significant catalytic activity for the electrochemical reduction of artemisinin at about ?380 mV vs. Ag/AgCl in phosphate buffer solution of pH 7 by using cyclic and differential pulse voltammetry. Under optimized conditions strict linearity between artemisinin concentration and height of the cathodic catalytic current peak was observed in 4.8×10^?6–7.8×10^?5 M concentration range (R=0.9991) when using differential pulse voltammetry. The detection limit was calculated as 1.4×10^?6 M of artemisinin. The developed electroanalytical device is suitable for the determination of artemisinin in Artemisia annua extracts.     

Development of a Creatinine Sensor Based on a Molecularly Imprinted Polymer‐Modified Sol‐Gel Film on Graphite Electrode

An electrochemical creatinine sensor based on a molecularly imprinted polymer (MIP)‐modified sol‐gel film on graphite electrode was developed. The surface coating of MIP over sol‐gel was advantageous to obtain a porous film with outwardly exposed MIP cavities for unhindered selective rebinding of creatinine from aqueous and biological samples. A fast differential pulse, cathodic stripping voltammetric response of creatinine can be obtained after being preanodized the sensor in neutral medium containing appropriate amount of creatinine at +1.8 V versus SCE for 120 s. A linear response over creatinine concentration in the range of 1.23 to 100 μg mL^?1 was exhibited with a detection limit of 0.37 μg mL^?1 (S/N=3).     

Carbon Paste Electrode Incorporating 1‐[4‐(Ferrocenyl Ethynyl) Phenyl]‐1‐Ethanone for Electrocatalytic and Voltammetric Determination of Tryptophan

A carbon paste electrode spiked with 1‐[4‐ferrocenyl ethynyl) phenyl]‐1‐ethanone (4FEPE) was constructed by incorporation of 4FEPE in graphite powder‐paraffin oil matrix. It has been shown by direct current cyclic voltammetry and double step chronoamperometry that this electrode can catalyze the oxidation of tryptophan (Trp) in aqueous buffered solution. It has been found that under optimum condition (pH 7.00), the oxidation of Trp at the surface of such an electrode occurs at a potential about 200 mV less positive than at an unmodified carbon paste electrode. The kinetic parameters such as electron transfer coefficient, α and rate constant for the chemical reaction between Trp and redox sites in 4FEPE modified carbon paste electrode (4FEPEMCPE) were also determined using electrochemical approaches. The electrocatalytic oxidation peak current of Trp showed a linear dependent on the Trp concentrations and linear calibration curves were obtained in the ranges of 6.00×10^?6 M–3.35×10^?3 M and 8.50×10^?7 M–6.34×10^?5 M of Trp concentration with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods, respectively. The detection limits (3σ) were determined as 1.80×10^?6 M and 5.60×10^?7 M by CV and DPV methods. This method was also examined as a selective, simple and precise new method for voltammetric determination of tryptophan in real sample.     

Simultaneous Quantitative Determination of Cadmium,Lead, and Copper on Carbon‐Ink Screen‐Printed Electrodes by Differential Pulse Anodic Stripping Voltammetry and Partial Least Squares Regression

Water is a vital commodity for every living entity on the planet. However, water resources are threatened by various sources of contamination from pesticides, hydrocarbons and heavy metals. This has resulted in the development of concepts and technologies to create a basis for provision of safe and high quality drinking water. This paper focuses on the simultaneous quantitative determination of three common contaminants, the heavy metals cadmium, lead and copper. Multivariate calibration was applied to voltammograms acquired on in‐house printed carbon‐ink screen‐printed electrodes by the highly sensitive electrochemical method of differential pulse anodic stripping voltammetry (DPASV). The statistically inspired modification of partial least squares (SIMPLS) algorithm was employed to effect the multivariate calibration. The application of data pretreatment techniques involving range‐scaling, mean‐centering, weighting of variables and the effects of peak realignment are also investigated. It was found that peak realignment in conjunction with weighting and SIMPLS led to the better overall root mean square error of prediction (RMSEP) value. This work represents significant progress in the development of multivariate calibration tools in conjunction with analytical techniques for water quality determination. It is the first time that multivariate calibration has been performed on DPASV voltammograms acquired on carbon‐ink screen‐printed electrodes.     

Simultaneous Determination of Acetaminophen and Ascorbic Acid at an Unmodified Boron‐Doped Diamond Electrode by Differential Pulse Voltammetry in Buffered Media

A boron‐doped diamond electrode (BDDE) was used for the simultaneous anodic determination of L ‐ascorbic acid (AA) and acetaminophen (AC) in aqueous buffered media by differential pulse voltammetry (DPV). Linear calibration plots of anodic current peaks versus concentration were obtained for both analytes in the concentration range 0.01–0.1 mM with very high correlation coefficients. RSD of 2–3% and high sensitivities were obtained from DPV data in single and dicomponent systems. The potential applicability of the DPV technique associated with standard addition was illustrated by simultaneous determination of AA and AC in real sample solutions made up from pharmaceutical products.     

On normal families and differential polynomials for meromorphic functions

We consider the normality criterion for a families F meromorphic in the unit disc Δ, and show that if there exist functions a(z) holomorphic in Δ, a(z)≠1, for each z∈Δ, such that there not only exists a positive number ε₀ such that |an(a(z)−1)−1|?ε₀ for arbitrary sequence of integers an(n∈N) and for any z∈Δ, but also exists a positive number B>0 such that for every f(z)∈F, B|f^′(z)|?|f(z)| whenever f(z)f^″(z)−a(z)(f^′2(z))=0 in Δ. Then is normal in Δ.     

On the stability of invariant sets of systems with impulse effect

A system of differential equations with impulse effect is considered. It is assumed that this system has an invariant set M$M$. By means of the direct Lyapunov method, the necessary and sufficient conditions of its uniform asymptotic stability are obtained. The conditions on the perturbations of right hand sides of differential equations and impulse effects, under which the uniform asymptotic stability of the invariant set M$M$ of the “nonperturbed” system implies the uniform asymptotic stability of the invariant set of the “perturbed” system, are obtained. The stability properties of invariant sets of periodic systems are also studied.     

On <Emphasis Type="Italic">t</Emphasis><Superscript>2</Superscript>-like solutions of certain second order differential equations

Via an integral transformation, we establish two embedding results between the Emden-Fowler type equation , t ≥ t ₀ > 0, with solutions x such that as , , and the equation , u > 0, with solutions y such that for given k > 0. The conclusions of our investigation are used to derive conditions for the existence of radial solutions to the elliptic equation , , that blow up as in the two dimensional case.