Optimization of thiamethoxam adsorption parameters using multi-walled carbon nanotubes by means of fractional factorial design.

The aim of this work was to evaluate significant factors affecting the thiamethoxam adsorption efficiency using oxidized multi-walled carbon nanotubes (MWCNTs) as adsorbents. Five factors (initial solution concentration of thiamethoxam in water, temperature, solution pH, MWCNTs weight and contact time) were investigated using 2V(5-1) fractional factorial design. The obtained linear model was statistically tested using analysis of variance (ANOVA) and the analysis of residuals was used to investigate the model validity. It was observed that the factors and their second-order interactions affecting the thiamethoxam removal can be divided into three groups: very important, moderately important and insignificant ones. The initial solution concentration was found to be the most influencing parameter on thiamethoxam adsorption from water. Optimization of the factors levels was carried out by minimizing those parameters which are usually critical in real life: the temperature (energy), contact time (money) and weight of MWCNTs (potential health hazard), in order to maximize the adsorbed amount of the pollutant. The results of maximal adsorbed thiamethoxam amount in both real and optimized experiments indicate that among minimized parameters the adsorption time is one that makes the largest difference. The results of this study indicate that fractional factorial design is very useful tool for screening the higher number of parameters and reducing the number of adsorption experiments.

Trace level voltammetric determination of lead and cadmium in sediment pore water by a bismuth-oxychloride particle-multiwalled carbon nanotube composite modified glassy carbon electrode.

Two multiwalled carbon nanotubes-based composites modified with bismuth and bismuth-oxychloride particles were synthesized and attached to the glassy carbon electrode substrate. The resultant configurations, Bi/MWCNT-GCE and BiOCl/MWNT-GCE, were then characterized with respect to their physicochemical properties and electroanalytical performance in combination with square-wave anodic stripping voltammetry (SWASV). Further, some key experimental conditions and instrumental parameters were optimized; namely: the supporting electrolyte composition, accumulation potential and time, together with the parameters of the SWV-ramp. The respective method with both electrode configurations has then been examined for the trace level determination of Pb(2+) and Cd(2+) ions and the results compared to those obtained with classical bismuth-film modified GCE. The different intensities of analytical signals obtained at the three electrodes for Pb(2+) and Cd(2+) vs. the saturated calomel reference electrode had indicated that the nature of the modifiers and the choice of the supporting electrolyte influenced significantly the corresponding stripping signals. The most promising procedure involved the BiOCl/MWCNT-GCE and the acetate buffer (pH 4.0) offering limits of determination of 4.0 µg L(-1) Cd(2+) and 1.9 µg L(-1) Pb(2+) when accumulating for 120 s at a potential of -1.20 V vs. ref. The BiOCl/MWCNT electrode was tested for the determination of target ions in the pore water of a selected sediment sample and the results agreed well with those obtained by graphite furnace atomic absorption spectrometry.

Hydrodynamic chronoamperometric method for the determination of H2O2 using MnO2-based carbon paste electrodes in groundwater treated by Fenton and Fenton-like reagents for natural organic matter removal.

A simple hydrodynamic chronoamperometric method based on the application of an unmodified carbon paste electrode (CPE) and bulk-modified with different contents of MnO2 was investigated for the determination of H2O2. The optimized method involving the CPE with 5% of MnO2 was applied for the determination of the H2O2 consumption in samples of groundwater from the Central Banat region (Province of Vojvodina, Serbia) treated by the Fenton (Fe(2+) and H2O2) and Fenton-like (Fe(3+) and H2O2) reagents to remove natural organic matter at different initial concentrations of iron species, and of their ratios to the initial concentration of H2O2. Under optimized conditions, with a working potential of 0.40V vs. the saturated calomel electrode and a phosphate buffer solution (pH 7.5) as supporting electrolyte, the method enabled the quantitation of H2O2 in the concentration interval from 1.4 to 65 µg mL(-1) with a relative standard deviation of less than 10%. The results obtained for the H2O2 consumption are in good agreement with those obtained by parallel measurements related to the efficiency of organic matter removal.

Conditioning of renewable silver amalgam film electrode for the characterization of clothianidin and its determination in selected samples by adsorptive square-wave voltammetry.

A new square-wave adsorptive stripping voltammetric (SWAdSV) method was developed for the determination of the neonicotinoid insecticide clothianidin (Clo), based on its reduction at a renewable silver amalgam film electrode (Hg(Ag)FE). The key point of the procedure is the pretreatment of the Hg(Ag)FE by applying the appropriate conditioning potential (-1.70 V vs. Ag/AgCl reference electrode). Under the optimized voltammetric conditions, such pretreatment resulted in the peak for the Clo reduction in Britton-Robinson buffer pH 9.0 at about -0.60 V, which was used for the analytical purpose. The developed SWAdSV procedure made it possible to determine Clo in the concentration range of 6.0×10(-7)-7.0×10(-6) mol L(-1) (LOD=1.8×10(-7) mol L(-1), LOQ=6.0×10(-7) mol L(-1)) and 7.0×10(-6)-4.0×10(-5) mol L(-1) (LOD=1.3×10(-6) mol L(-1), LOQ=4.2×10(-6) mol L(-1)). The repeatability, precision, and the recovery of the method were determined. The effect of common interfering pesticides was also investigated. Standard addition method was successfully applied and validated for the determination of Clo in spiked Warta River water, corn seeds samples, and in corn seeds samples treated with the commercial formulation PONCHO 600 FS.

Multi-residue method for determination of selected neonicotinoid insecticides in honey using optimized dispersive liquid-liquid microextraction combined with liquid chromatography-tandem mass spectrometry.

The objective of this study was to develop analytical method based on optimized dispersive liquid-liquid microextraction (DLLME) as a pretreatment procedure combined with reversed phase liquid chromatographic separation on C18 column and isocratic elution for simultaneous MS/MS determination of selected neonicotinoid insecticides in honey. The LC-MS/MS parameters were optimized to unequivocally provide good chromatographic separation, low detection (LOD, 0.5-1.0 µg kg(-1)) and quantification (LOQ, 1.5-2.5 µg kg(-1)) limits for acetamiprid, clothianidin, thiamethoxam, imidacloprid, dinotefuran, thiacloprid and nitenpyram in honey samples. Using different types (chloroform, dichloromethane) and volumes of extraction (0.5-3.0 mL) and dispersive (acetonitrile; 0.0-1.0 mL) solvent and by mathematical modeling it was possible to establish the optimal sample preparation procedure. Matrix-matched calibration and blank honey sample spiked in the concentration range of LOQ-100.0 µg kg(-1) were used to compensate the matrix effect and to fulfill the requirements of SANCO/12495/2011 for the accuracy (R 74.3-113.9%) and precision (expressed in terms of repeatability (RSD 2.74-11.8%) and within-laboratory reproducibility (RSDs 6.64-16.2%)) of the proposed method. The rapid (retention times 1.5-9.9 min), sensitive and low solvent consumption procedure described in this work provides reliable, simultaneous, and quantitative method applicable for the routine laboratory analysis of seven neonicotinoid residues in real honey samples.

Spectroscopic monitoring of photocatalytic degradation of the insecticide acetamiprid and its degradation product 6-chloronicotinic acid on TiO2 catalyst.

Two spectroscopic methods, (1)H NMR and FTIR, were developed for the monitoring of the photocatalytic degradation of acetamiprid, a widely used pyridine-based neonicotinoid insecticide, in UV-irradiated aqueous suspensions of O(2)/TiO(2). The (1)H NMR method allowed also the identification of the intermediates such as 6-chloronicotinic and formic acids, as well as separate monitoring of the kinetics of degradation of acyclic and aromatic moieties based on the different chemical shifts of the protons belonging to the methyl group of the acyclic and selected proton of the heterocyclic aromatic moiety. The FTIR procedure enabled the monitoring of the kinetics of degradation of the cyano group of the compound. The obtained results are in good agreement with the comparative HPLC-DAD and HPLC-MS/MS measurements, which also enabled the identification of certain intermediates. To get a deeper insight into the complex photocatalytic process, the photocatalytic degradation of 6-chloronicotinic acid, a stable degradation intermediate of acetamiprid, was also investigated by (1)H NMR and HPLC-DAD methods. Based on the obtained data, a tentative reaction mechanism was proposed for the photocatalytic degradation of acetamiprid.

Molecular interactions between selected sodium salts of bile acids and morphine hydrochloride.

The objective of this study was to understand the prolonged analgesic action of morphine hydrochloride observed in the presence of sodium 12-oxochenodeoxycholanate. Based on literature, this phenomenon may be due to the formation of aggregates in the cell between the molecules of bile acids and morphine. In addition to the sodium 12-oxochenodeoxycholanate, the present investigation also included salts of cholic and 7-oxodeoxycholic acids. Saturation transfer difference NMR experiments showed that morphine binds to the bile acid molecule close to the aromatic protons H1 and H2 provided that the concentration of the bile acid salt approaches the critical micellar concentration (CMC). The spin-lattice relaxation times (T(1)) of the affected protons decrease significantly in the presence of micellar solutions of the bile acid salts, and the most pronounced change in T(1) was observed for sodium 7-oxodeoxycholate. Diffusion-ordered NMR experiments suggested that morphine hydrochloride can interact only with sodium 7-oxochenodeoxycholate. It can be supposed that the molecular ratio of sodium 7-oxodeoxycholate and morphine hydrochloride in the mixed micelle is 2:1. The CMC values of mixed micelles do not differ from the CMC values of the micelle constituents, which suggests that the binding of morphine hydrochloride does not perturb the hydrophobic domain of the bile acid molecule. In the presence of bile acids, the transfer rate constant (k(12)) of morphine hydrochloride from the buffered aqueous solution to chloroform (model of the cell membrane) shows a decrease. A significant decrease of the k(12) was also observed in the presence of micellar solutions. Kinetic measurements indicated that, in addition to micellar interaction between morphine hydrochloride and sodium salts of bile acids, a complex may also be formed in chloroform via hydrogen bonds formed between the drug and bile acid molecules.

Voltammetric determination of the herbicide Linuron using a tricresyl phosphate-based carbon paste electrode.

This paper summarises the results of voltammetric studies on the herbicide 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea (Linuron), using a carbon paste electrode containing tricresyl phosphate (TCP-CPE) as liquid binder. The principal experimental conditions, such as the pH effect, investigated in Britton-Robinson buffer solutions (pH 2.0-7.0), the peak characteristics for the analyte of interest, or instrumental parameters for the differential pulse voltammetric mode were optimized for the method. As found out, the best electroanalytical performance of the TCP-CPE was achieved at pH 2.0, whereby the oxidation peak of Linuron appeared at ca. +1.3 V vs. SCE. The analytical procedure developed offers good linearity in the concentration range of 1.25-44.20 µg mL(-1) (1.77 × 10(-4)-5.05 × 10(-6) mol L(-1)), showing-for the first time-the applicability of the TCP-CPE for anodic oxidations in direct voltammetry (without accumulation). The method was then verified by determining Linuron in a spiked river water sample and a commercial formulation and the results obtained agreed well with those obtained by the reference HPLC/UV determination.

Bismuth modified carbon-based electrodes for the determination of selected neonicotinoid insecticides.

Two types of bismuth modified electrodes, a bismuth-film modified glassy carbon (BiF-GCE) and a bismuth bulk modified carbon paste, were applied for the determination of selected nitroguanidine neonicotinoid insecticides. The method based on an ex situ prepared BiF-GCE operated in the differential pulse voltammetric (DPV) mode was applied to determine clothianidin in the concentration range from 2.5 to 23 µg cm⁻³ with a relative standard deviation (RSD) not exceeding 1.5%. The tricresyl phosphate-based carbon paste electrodes (TCP-CPEs), bulk modified with 5 and 20 w/w% of bismuth, showed a different analytical performance in the determination of imidacloprid, regarding the peak shape, potential window, and noise level. The TCP-CPE with 5% Bi was advantageous, and the developed DPV method based on it allowed the determination in the concentration range from 1.7 to 60 µg cm⁻³ with an RSD of 2.4%. To get a deeper insight into the morphology of the bismuth-based sensor surfaces, scanning electron microscopic measurements were performed of both the surface film and the bulk modified electrodes.

Antimony-film electrode for the determination of trace metals by sequential-injection analysis/anodic stripping voltammetry.

The possibility of applying antimony-film modified glassy carbon electrode in sequential-injection analysis (SIA) was investigated with the objective of determining Pb(II) and Cd(II) by anodic stripping voltammetry (ASV). The conditions of antimony-film deposition concerning composition of the plating/carrier solutions, concentrations of Sb(III) and hydrochloric acid, effects of different supporting electrolyte salts, and plating potential were optimized. It was found that the antimony-film deposition on glassy carbon substrate in a sample solution consisting of 750 microg L(-1) Sb(III), 0.5 mol L(-1) HCl at -1.5 V (vs. Ag/AgCl/3 mol L(-1) KCl) yielded a modified electrode suitable for the determination of Pb(II) and Cd(II) at the microg L(-1) level. The reproducibility of the analytical signals was characterized by a relative standard deviation lower than 2.8%, and the calculated values of detection limits were 1.2 microg L(-1) for Pb(II) and 1.4 microg L(-1) for Cd(II). The presence of KSCN in the sample solution offers the possibility of detecting ions with more negative oxidation potentials like Zn(II), Mn(II) or Cr(III). The developed SIA-ASV procedure was compared with the commonly used batch method, and its applicability was tested on a spiked tap water sample.

Determination of critical micellar concentrations of two monoketo derivatives of cholic acid.

Critical micellear concentrations (CMC) were determined for two novel promoters of membrane permeability-7-monoketocholic acid (7-MKC) and 12-monoketocholic acid (12-MKC), using two non-invasive ((1)H NMR relaxation experiment and conductometry) and two invasive (spectral shift and partition coefficient of the probe molecule) methods. Studies by the former methods suggest the different aggregation abilities of the investigated bile acid derivatives. In an aqueous solution, 7-MKC has a somewhat lower CMC value (43 mM) than 12-MKC (50 mM). Further, it was found that, in addition to primary micelles, 7-MKC forms also secondary micelles. In the experiments with probe (hydrophobic) molecules, the aggregation properties of investigated bile acids did not differ in water, whereas the presence of urea altered the aggregation of 7-MKC. Based on the CMC value, 7-MKC is more hydrophobic than 12-MKC. The apparent hydrophobicity of 7-MKC is a consequence of the formation of secondary micelles, shifting the monomer equilibrium to the direction of primary micelles, which is manifested as a decrease in the CMC value.

Formation of hydrogen-bonded complexes between bile acids and lidocaine in the lidocaine transfer from an aqueous phase to chloroform.

Bile acids are amphiphilic molecules, which, in addition to their physiological role, have also acquired increasingly more important pharmacological applications. It has been shown that these compounds have a promoting effect on the transport of many drugs through the cell membrane. Pharmacodynamic studies showed that they exerted a significant effect on the analgesic action of lidocaine. This study is concerned with the determination of the constants of hydrogen-bonded complexes formed between the investigated bile acids and lidocaine. It was found that a prerequisite for forming such a complex is the existence of at least two OH groups or one OH group and one keto group in the bile acid molecule at an appropriate mutual distance. If a keto group is involved in lidocaine binding, the resulting complex has a larger equilibrium constant. A model--multiple linear regression equation--was constructed, relating the molecular descriptors to the equilibrium constant of hydrogen-bonded complex. It was also shown how the complex formed between lidocaine and bile acid influences the rate constant of the decrease of lidocaine concentration in the aqueous phase during its transfer to the chloroform solution of a bile acid. It was found that the complex formed between lidocaine and bile acids plays an important role in the appearance of the depot effect of lidocaine.

Phosphorus-doped and undoped glassy carbon indicator electrodes in controlled-current potentiometric titrations of bromide- or chloride-containing active ingredients in some pharmaceutical preparations.

Phosphorus-doped glassy carbon (as a novel material) and glassy carbon (Sigri commercial sample) were applied as potentiometric indicator electrodes in the titrimetric determination of active components with bromide or chloride in their molecules in different pharmaceutical preparations (Buscopan, Prostigmine, Isoptin, Bedoxin, Akineton and Trodon). After the necessary pre-treatment of the electrode surfaces and sample dissolution, the halide was titrated with a standard solution of silver nitrate (indirect determination). Amounts of 10-20 micromol of the investigated active ingredients per titration were determined with a relative standard deviation that, depending on the nature of indicator electrode, determined molecules and filler components, was in the range of 0.3-2.7%. The results obtained were compared with those of the official methods and with those obtained by potentiometric titrations using silver electrode. The titrimetric procedures developed are relatively fast, easy, economical and can be used to analyse of a large number of pharmaceutical products.