COMSOL Multiphysics®: Finite element software for electrochemical analysis. A mini-review

We discuss the use of the commercial finite element software COMSOL Multiphysics® for electrochemical analysis. Practical considerations relevant to finite element modelling are highlighted. A review of contemporary applications of this software is supplied; the subjects concerned reveal the particular suitability of general-purpose finite element methods for non-standard geometries, complex reaction chemistry, hydrodynamic electrochemistry, and rapid verification of standard results.     

Development of a micro-total analysis system (μ-TAS) for the determination of catecholamines

In this study, the first micro-total analysis system (μ-TAS) for catecholamines (dopamine, epinephrine, and norepinephrine) analysis in which preconcentration, separation, and determination steps were integrated on a microchip was developed. Electrophoresis microchips in a variety of channel lengths and designs were produced in borofloat glass for the μ-TAS studies. Chambers for the preparation of monolithic disks were formed in the microchips at the intersection of the injection and separation channels. Vinyl phenylboronic acid–ethylene glycol dimethacrylate polymers were prepared as monolithic disks in these chambers with a depth of 0.05 mm and a diameter of 2.1 mm. The microchips could be used more than 50 times if mechanical problems such as plugging or fracturing did not occur. Adsorption and elution of catecholamines were realized electrokinetically, with catecholamines determined via laser-induced native fluorescence detection following elution and electrophoretic separation. The most promising results were obtained with 100 mM phosphate buffer (pH 2) for elution with 25% propanol added to the separation buffer (100 mM phosphate, pH 3).     

A fully automated effervescence assisted dispersive liquid–liquid microextraction based on a stepwise injection system. Determination of antipyrine in saliva samples

A first attempt to automate the effervescence assisted dispersive liquid–liquid microextraction (EA-DLLME) has been reported. The method is based on the aspiration of a sample and all required aqueous reagents into the stepwise injection analysis (SWIA) manifold, followed by simultaneous counterflow injection of the extraction solvent (dichloromethane), the mixture of the effervescence agent (0.5 mol L⁻¹ Na₂CO₃) and the proton donor solution (1 mol L⁻¹ CH₃COOH). Formation of carbon dioxide microbubbles generated in situ leads to the dispersion of the extraction solvent in the whole aqueous sample and extraction of the analyte into organic phase. Unlike the conventional DLLME, in the case of EA-DLLME, the addition of dispersive solvent, as well as, time consuming centrifugation step for disruption of the cloudy state is avoided. The phase separation was achieved by gentle bubbling of nitrogen stream (2 mL min⁻¹ during 2 min).     

Application of flow injection analysis—photo-induced fluorescence (FIA-PIF) for the determination of α-cypermethrin pesticide residues in natural waters

Flow injection analysis combined with photo-induced fluorescence (FIA-PIF) has been applied for the determination of α-cypermethrin pesticide residues in Senegalese natural waters, using organic solutions and cyclodextrin (β-cyclodextrin and 2-hydroxypropyl-β-cyclodextrin) aqueous media. The α-cypermethrin insecticide has a very weak natural fluorescence, but it is converted into strongly fluorescent photoproduct(s) by UV irradiation. Cyclodextrins were found to enhance the PIF signal. FIA parameters, including mobile phase flow rate, injected volume, and reactor length, were optimized. Analytical performances of the FIA-PIF method for the determination of α-cypermethrin were satisfactory, with concentration linear dynamic ranges over one to two orders of magnitude and with rather low limits of detection and limits of quantification, in the ng mL⁻¹ range, and relative standard deviations comprised between 1.2% and 3.8%. Application of FIA-PIF for the analysis of fortified natural water samples collected from Senegal yielded good recovery values (84–112%). Because of its high sampling rate, the FIA-PIF method constitutes a rapid analytical tool, useful for quantification of α-cypermethrin residues in natural waters.     

A novel concept for the improvement of the Ni–Mo/Al2O3-based nanocatalyst system: design and analysis

A new Ni?CMo/Al₂O₃-based nano catalyst composition was developed and manufactured by a proprietary catalyst preparation technology for diesel hydrotreatment. The nanocatalyst has been performing commercially since September 2011, consistently producing ultra low-sulfur diesel of Euro-IV/V standards from a feedstock containing 1.75?wt% sulfur. In addition to lowering sulfur content, the catalyst also enhances cetane number and reduces boiling end-point to obtain diesel with better quality. The nanocatalyst was characterized by X-ray photoelectron spectroscopy to ascertain the electronic state of metal species. The morphological characterization of the nanocatalyst carried out by TEM revealed the presence of nano-sized MoS₂ slab structures. The performance of the nanocatalyst is mainly attributed to MoS₂ slabs with increased stacking which in turn are generated from the customized metal-sulfide precursors.     

Development of a sequential injection analysis system for the potentiometric determination of nitrite in meat products by using a Gran’s plot method

A method was developed for in vitro dissolution and ICP-AES based determination of cadmium (Cd) for evaluating its bioaccessibility in herbal medicines to investigate the utility of this test for toxicological assessments. The bioaccessible Cd content of five commonly consumed and Cd-accumulating herbal medicines was determined by in vitro dissolution test with a biorelevant medium simulating gastric conditions. Cd levels were determined by a validated ICP-AES method. In all 5 herbal medicines, the total amount of Cd in herbal medicines was not proportional to the bioaccessible Cd content, which varied from 23.0% to 54.9%. The Cd tolerance limit was exceeded by four of the five herbal medicines in terms of total Cd content, but by only one of these in terms of bioaccessible Cd content. Assessments based on total content may overestimate the risk of Cd toxicity from herbal medicines. Assessing bioaccessibility by in vitro dissolution test coupled with ICP-AES method may be a more appropriate means of determining the safety of herbal medicines containing Cd.     

Exploiting π-acceptors for the determination of thyroid hormones (T3 and T4) using a single interface flow system

A fully automated methodology was developed for the determination of the thyroid hormones levothyroxine (T4) and liothyronine (T3). The proposed method exploits the formation of highly coloured charge-transfer (CT) complexes between these compounds, acting as electron donors, and π-acceptors such as chloranilic acid (CLA) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). For automation of the analytical procedure a simple, fast and versatile single interface flow system (SIFA) was implemented guaranteeing a simplified performance optimisation, low maintenance and a cost-effective operation. Moreover, the single reaction interface assured a convenient and straightforward approach for implementing Job's method of continuous variations used to establish the stoichiometry of the formed CT complexes.Linear calibration plots for levothyroxine and liothyronine concentrations ranging from 5.0 × 10⁻⁵ to 2.5 × 10⁻⁴ mol L⁻¹ and 1.0 × 10⁻⁵ to 1.0 × 10⁻⁴ mol L⁻¹, respectively, were obtained, with good precision (R.S.D. <4.6% and <3.9%) and with a determination frequency of 26 h⁻¹ for both drugs. The results obtained for pharmaceutical formulations were statistically comparable to the declared hormone amount with relative deviations lower than 2.1%. The accuracy was confirmed by carrying out recovery studies, which furnished recovery values ranging from 96.3% to 103.7% for levothyroxine and 100.1% for liothyronine.     

A miniaturized flow-injection analysis ( μ FIA) system with on-line chemiluminescence detection for the determination of iron in estuarine water

A miniaturized flow-injection analysis system constructed from a glass base plate and polydimethylsiloxane (PDMS) top plate was employed for the determination of iron in river water. Two designs were investigated, one utilizing a syringe pump and the other utilizing EOF pumping with a mini-filtration system incorporated. The syringe pump system was used to optimize the analytical method on chip, where the pump was used to deliver both the analyte and the reagents to the reactor chip. The highly sensitive chemiluminescence reaction between alkaline luminol (3-aminophthalhydrazide) and 0.1?M of hydrogen peroxide (H₂O₂) in the presence of iron(II) was utilized. The bright blue light (λ _max?～?440?nm) emitted was detected using a miniaturized photomultiplier tube interfaced directly under the chip. The light intensity signals were recorded, and the corresponding concentration of iron(II) concentration was determined. The calibration for iron(II) standards was linear up to 0.75?µg?mL^?1 (y?=?5.7839x?+?0.0378, r² ?=?0.9939) with a precision value of up to 3.72% RSD, for n?=?3. The limits of detection (blank?+?3s _y/x) were found to be 28?ng?mL^?1. The system which utilized EOF pumping and incorporated a minifiltration unit provided a linear calibration for 0–5?µg?mL^?1 (y?=?3.316x?+?0.1831; correlation coefficient, r ²?=?0.9996) over a working range of 0.0–0.5?µg?mL^?1. This system provided lower limits of detection 5.1?ng?mL^?1 and better repeatability (%RSD less than 0.5% for n?=?4), but problems occurred with the mini-filtration system at higher iron(II) concentrations. The EOF pumping system provided slightly higher results for the concentration of iron(II) in the Humber estuary (0.058?µg?mL^?1), but these results were in line with the results expected by the Environment Agency.     

A LabVIEW® program for determining electron number density from Stark broadening measurements of the hydrogen-beta line

A useful plasma diagnostic is the measurement of electron number density. One way to accomplish such measurements is to determine the contribution to the broadening of a spectral line due to the Stark effect. To simplify and extend such electron density measurements across computer platforms, a program that calculates electron number density from the Stark-broadened hydrogen-beta line has been written for the LabVIEW^® environment. This program calculates electron number densities from the field strength that would be exerted on a hydrogen atom immersed in a plasma. Using the new program, the electron number density in a glow discharge is calculated for two different operating conditions. Not surprisingly, the results indicate that to increase the current density in the discharge, the source electrodes must be reduced in surface area. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by one disk with an executable program (written for Apple Macintosh), data and text files including a manual.     

Simple and selective method for determination of iodide in pharmaceutical products by flow injection analysis using the iodine–starch reaction

This work exploited the well-known iodine–starch reaction for development of a simple flow-injection (FI) method for determination of iodide in pharmaceutical samples. Iodide in an injected zone was oxidized to iodine. A gas diffusion unit enables selective permeation of iodine through a hydrophobic membrane. Detection was made very selective for elemental iodine by employing formation of the I₃ ^––starch complex. The detection limit (3S/N) of the system was 1 mg I L^–1. For a liquid patent medicine used for asthma treatment we suggested modification of the system. Direct injection of this sample, which contains a particularly high concentration level of iodide (ca. 9000 mg I L^–1), can be achieved by coupling a dialysis unit to the FI system. This has increased the working range to 6000–10,000 mg I L^–1 without employing complicated nanoliter injection.     

Rapid speciation of Se(IV) and Se(VI) by flow injection–capillary electrophoresis system with contactless conductivity detection

A flow injection–capillary electrophoresis system with contactless conductivity detection and hydrostatic-pressure-generated flow was used for the fast and sensitive speciation of Se(IV) and Se(VI). The sample throughput was 25 samples per hour using a background electrolyte solution containing 8.75 mM l-histidine (His) adjusted to pH 4.00 with acetic acid. The repeatability of peak areas (n=8) was better than 1.41% and the limits of detection were 190 g L^–1 and 7.5 g L^–1 for Se(IV) and Se(VI), respectively. The interference from carbonate, typically present in water samples, was eliminated by using a low-pH electrolyte in which carbonate is uncharged and migrates at the EOF front. The method was applied to the analysis of Se(IV) and Se(VI) in soil samples that were spiked with both selenium species and the results for recovery of both selenium species were in good agreement with their introduced concentrations.     

A capillary column packed with a zirconium(IV)-based organic framework for enrichment of endogenous phosphopeptides

A zirconium(IV)-based metal organic framework (Zr-MOF) was deposited on polydopamine-coated silica microspheres to form microspheres of type SiO₂@PDA@Zr-MOF. These were packed into capillary columns for enrichment of phosphopeptides. The column was off-line coupled to both matrix-assisted laser desorption/ionization time of flight mass spectrometry and LC-ESI-MS/MS. The method has a detection limit as low as 4 fmol of β-casein digest and a selectivity as high as 1:1000 (molar ratio of β-casein and BSA digest). It was applied to the analysis of human saliva. In total, 240 endogenous phosphopeptides were identified in only 25 μL human saliva.

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Graphical abstract A zirconium-based metal organic framework (Zr-MOF) was modified outside of polydopamine-coated silica microspheres to form microspheres named SiO₂@PDA@Zr-MOF. Then they were packed in capillary columns for selective enrichment of phosphopeptides via interaction between Zr-O clusters and phosphate groups. The pre-concentration resulted in a better detection of phosphopeptides by mass spectrometry. Tris: Tris(hydroxymethyl)aminomethane; DMF: Dimethyl Formamide; Zr-MOF: Zirconium(IV)-organic framework; MOAC: Metal oxide affinity chromatography.

     

Nafion® as advanced immobilisation substrate for the voltammetric analysis of electroactive microparticles: the case of some artistic colouring agents

Voltammetry of microparticles is applied to characterise and to identify solid analytes of interest in the field of cultural heritage. Nafion® is used for the immobilisation of solid microparticles onto the surface of a glassy carbon electrode by exploiting the deposition onto the electrode surface of a micro-volume of a suspension of the microsample in polymeric solution. Cyclic voltammetry and square wave voltammetry are applied to characterise and to identify the microparticles immobilised in the Nafion® coating. The analyte studied in this work is Prussian Blue as a typical inorganic pigment, with a relatively simple electrochemical behaviour. The proposed method is applied to a sample of Venetian marmorino plaster. The performance of Nafion® for this analysis is compared with that of the polymer Paraloid B72.

Amberlyst®-15: a reusable heterogeneous catalyst for the dehydration of tertiary alcohols

Tertiary alcohols react under mild conditions in the presence of Amberlyst^®-15 (dry) (solid-supported sulfonic acid) to give predominantly the most stable alkene in very good yield. The dehydration of tertiary alcohol functionality occurs without observation of rearrangement and polymerization products, and with outstanding substrate tolerance, which include the NHCBz, NHBoc, OSEM, OTBDMS, OBOM and ethylene ketal functional groups. Amberlyst^®-15 (dry) can be easily recovered from the reaction medium and reused for five cycles, maintaining the catalytic efficiency. In addition, the dehydration can occur under continuous operation.     

A General QSAR Model for Predicting the Toxicity of Organic Chemicals to Luminescent Bacteria (Microtox® test)

Abstract A large data set of Microtox? toxicity results was used to derive a general QSAR model. Chemicals were described by means of a modified autocorrelation method. The autocorrelation vectors were generated from atomic contributions encoding the hydrophobicity and molar refractivity of the molecules. A three-layer backpropagation neural network was used to design the model. The obtained results were compared with those obtained from a principal components regression analysis.     

A comparison of the reduction of dinitrogen by a vanadium(ii)—catechol system with that by the active centre of nitrogenase

A comparison of the action of a nitrogenase active centre with that of the vanadium(II)—catechol system in water and alcohol as far as N₂ and C₂H₂ reduction, H₂ evolution and the inhibiting effect of CO are concerned demonstrates the existence of many similarities, thus enabling some tentative conclusions to be made regarding the mechanism governing the functioning of nitrogenase.     

A reusable enantioselective catalytic system for the Kharasch–Sosnovsky allylic oxidation of alkenes based on a ditopic azabis(oxazoline) ligand

An easily recoverable and reusable enantioselective catalytic system based on the self-assembly of a coordination polymer through the use of a chiral ditopic ligand, is described for the allylic oxidation of cycloalkenes with tert-butyl perbenzoate. Upon addition of the substrates and a coordinating solvent (acetone or acetonitrile) the coordination polymer disassembles, allowing the catalysis to take place in homogeneous phase. After completion of the reaction, the catalyst reassembles as a coordination polymer upon solvent evaporation and addition of hexane, and is recovered as an insoluble solid. This way, good enantioselectivities and yields are obtained in seven consecutive operation cycles.