Investigation on the application of titania nanorod arrays to the determination of chemical oxygen demand

In the present paper, the TiO₂ nanorod arrays electrode was developed as a sensor for the determination of chemical oxygen demand (COD) based on a photoelectrochemical degradation principle. Effects of common parameters, such as applied potential, light intensity and pH on its analytical performance were investigated. Under the optimized conditions, the nanorod arrays electrode was successfully applied in the COD determination for both synthetic and real samples. In the COD determination, the proposed method can achieve a practical detection limit of 18.3 mg L⁻¹ and a linear range of 20–280 mg L⁻¹. Furthermore, the results obtained by the proposed method were well correlated with those obtained using the conventional (i.e., dichromate) COD determination method. The main advantages of this COD determination method were its simplicity, long term stability and environmental friendly (corrosive and toxic reagents not consumed). This work would open a new application area (COD determination) of the TiO₂ nanorod arrays.     

Steric and electronic contributions to conformational effects on chemical shifts of acyclic alcohols

Our calculations on bi- and polycyclic alcohols reveal that the Mulliken charge distribution and chemical shift patterns due to hyperconjugation of lone pairs on oxygen with neighboring groups break down or are attenuated for certain spatial relationships of the hydroxyl group. Since in strained ring systems other effects on these parameters may be present, we applied a similar analysis to acyclic alcohols. Calculations at the B3LYP/6-31G^* level on conformers of methanol, ethanol, 1- and 2-propanol, 2-methyl-1-propanol, 2-methyl-2-propanol, 2-butanol, 2-methyl-2-butanol, 1- 2- and 3-pentanol and 2-methyl-3-pentanol, where hyperconjugation may be present, reveal steric effects as modifiers of hyperconjugative patterns affecting carbon-13 chemical shifts in such alcohols. Contrary to what is observed in bi- and policyclic systems, where electrostatic effects interfere with effects due to hyperconjugation, these steric effects may be the main cause for the attenuation of deshielding of nuclei that are subject to hyperconjugation. Electrostatic effects are also present but they do not interfere with hyperconjugation by lone pairs. Conformational effects fall off sharply after the third carbon in the chain.     

Effects of room-temperature ionic liquids on the chemical vapor generation of gold: Mechanism and analytical application

To get insight into the mechanism of the effect of room-temperature ionic liquids (RTILs) on the chemical vapor generation (CVG) of noble metals, gold was taken as a model element, and eight RTILs were examined. All the RTILs resulted in 3-24 times improvement in sensitivity for Au, depending on their nature. For the RTILs with identical anion, the RTILs with the cations of short chain exhibited better enhancement effect than those with long alkyl chain length or complex branch chain. For the RTILs with identical cation, the RTILs with Br⁻ gave the best enhancement effect. The formation of ion pairs between the cation of RTILs and the anion species of gold via electrostatic interaction, and/or the substitution of the Cl⁻ in the anion species of gold by the anion of RTILs likely enabled a more effective CVG reaction to occur. The RTILs also facilitated the generation of small bubbles and provided an electrostatic stabilization to protect the unstable volatile gold species and to help fast isolation of volatile gold species from the reaction mixture. 1-Butyl-3-methylimidazolium tetrafluoroborate [C₄mim]Br gave the best improvement in the sensitivity (24 times) among the RTILs studied, and also reduced the interferences from common transition and other noble metals. Based on the enhancement effect of [C₄mim]Br, a novel flow injection-CVG-atomic fluorescence spectrometric method with a detection limit (3s) of 1.9 μg L⁻¹ and a precision of 3.1% (50 μg L⁻¹, RSD, n = 11) was developed for the determination of trace gold in geological samples.     

Mass spectrometry-based metabolomics applied to the chemical safety of food

Mass spectrometry (MS)-based metabolomics is emerging as an important field of research in many scientific areas, including chemical safety of food. A particular strength of this approach is its potential to reveal some physiological effects induced by complex mixtures of chemicals present at trace concentrations. The limitations of other analytical approaches currently employed to detect low-dose and mixture effects of chemicals make detection very problematic. Besides this basic technical challenge, numerous analytical choices have to be made at each step of a metabolomics study, and each step can have a direct impact on the final results obtained and their interpretation (i.e. sample preparation, sample introduction, ionization, signal acquisition, data processing, and data analysis). As the application of metabolomics to chemical analysis of food is still in its infancy, no consensus has yet been reached on defining many of these important parameters. In this context, the aim of the present study is to review all these aspects of MS-based approaches to metabolomics, and to give a comprehensive, critical overview of the current state of the art, possible pitfalls, and future challenges and trends linked to this emerging field.     

The effect of substitution on the properties of a chemical group

The effect of the substituent R(R = F, OH, NH₂, CH₃) on the electron distribution of the C=C group of ethylene and acrylonitrile may be fairly well represented in terms of classical interactions between the and components of this group and the molecular remainder. A distinction between direct and indirect effects is made, and an estimate of the relative importance of conjugative versus inductive effects is done.This analysis has been performed on a set of 22 SCF wavefunctions calculated with the 4-31G basis set.     

Quantifying temperature and flow rate effects on the performance of a fixed-bed chromatographic reactor

Chromatographic reactors are based on coupling chemical reactions with chromatographic separation in fixed-beds. Temperature and flow rate are important parameters for the performance of such reactors. Temperature affects mainly adsorption, chemical equilibria, mass transfer and reaction kinetics, whereas flow rate influences residence time and dispersion. In order to evaluate the mentioned effects, the hydrolysis reactions of methyl formate (MF) and methyl acetate (MA) were chosen as case studies. These reactions were performed experimentally in a lab-scale fixed-bed chromatographic reactor packed with a strong acidic ion exchange resin. The chosen reactions can be considered to represent a relative fast (MF) and a relative slow (MA) reaction. The processes which take place inside the reactor were described and simulated using an isothermal equilibrium dispersive model. The essential model parameters were determined experimentally at different temperatures and flow rates. The performance of the chromatographic reactor was evaluated at several discrete constant temperature levels by quantifying product purity, productivity and yield. The work provides insight regarding the influence of temperature and flow rate on values of the model parameters and the performance criteria.     

铀在地下水中的化学形态模拟研究

用EQ3/6程序模拟研究了某场址地下水中U的化学形态,并模拟了在不同的pH、温度T、总碳?∑C?浓度、U的浓度ρ(UO22 )等条件下U的化学形态,分析了其化学形态变化规律。结果表明,在该场址地下水中,铀主要以碳酸铀酰络合物形式存在。pH、T、∑C浓度变化对铀的化学形态影响较大。     

Donnan dialysis preconcentration coupled with ion chromatography and electrocatalytic oxidation for the determination of cyanide

An electrocatalytic amperometric detector for the ion chromatographic determination of CN⁻ is described. A conducting composite that is based on a graphite-loaded sol–gel material comprises the working electrode. The composite is doped with a Ru^II metallodendrimer which is demonstrated to promote the electrochemical oxidation of CN⁻ at potentials positive of 0.5 V vs. Ag/AgCl. In 6 mM NaOH, 0.05 M NaCl flowed at 1.0 ml min⁻¹, a 5-point calibration curve with the following linear least squares parameters is obtained over the range, 1.0–30 M CN⁻: slope, 24.2±0.1 nA M⁻¹; intercept, −6±2 nA; and r, 0.9997. The detection limit, 0.7 μM CN⁻, compares favorably to that obtained by amperometry at a silver electrode, 0.5 μM CN⁻, under comparable experimental conditions. A 60-min preconcentration by Donnan dialysis increases the sensitivity by a factor of 23.6.     

Photoelectro-synergistic catalysis combined with a FIA system application on determination of chemical oxygen demand

In this paper, photoelectro-synergistic catalysis oxidation of organics in the water on Ti/TiO₂/PbO₂ electrode was investigated. The prepared TiO₂ film was investigated with Atomic force micrograph (AFM). Furthermore, the results were compared with those obtained from electrocatalysis (EC) and electro-assisted photocatalysis (PC). The method proposed employed photoelectro-synergistic catalysis (PEC), together with flow injection analysis, to determine the chemical oxygen demand (COD) values. It was shown that the method of photoelectro-synergistic catalysis had lower detection limit (15.0 mg l⁻¹) and wider linear range (30.0–2500.0 mg l⁻¹) than the methods of electro-assisted photocatalysis and electrocatalysis. The results obtained by the proposed method and conventional one were compared by carrying out the experiment on 20 wastewater samples and also agreed well by high correlation (R = 0.9912).     

Automated determination of verapamil and norverapamil in human plasma with on-line coupling of dialysis to high-performance liquid chromatography and fluorometric detection

A fully automated method for the simultaneous determination of verapamil and its main metabolite norverapamil in human plasma is described. This method is based on on-line sample preparation using dialysis followed by clean-up and enrichment of the dialysate on a precolumn and subsequent HPLC analysis with fluorometric detection. All sample handling operations were performed automatically by a sample processor equipped with a robotic arm (ASTED system). The plasma samples were dialysed on a cellulose acetate membrane (cut-off: 15 kD) and the dialysate was purified and enriched on a short pre-column filled with cyanopropyl silica. Before starting dialysis, this trace enrichment column (TEC) was first conditioned with the HPLC mobile phase and then with pH 3.0 acetate buffer. 370 μl of plasma sample spiked with the internal standard (gallopamil) were dialysed in the static-pulsed mode. The solution at the donor side was pH 3.0 acetate buffer containing Triton X-100 while the acceptor solution was made of the same acetate buffer. When dialysis was discontinued, the analytes were desorbed from the TEC by the HPLC mobile phase and transferred to the C₁₈ analytical column by means of a switching valve. This mobile phase consisted of a mixture of acetonitrile, pH 3.0 acetate buffer and 2-aminoheptane. The influence of different parameters of the dialysis process on the recovery of verapamil and norverapamil has been studied. The effect of the volume, the aspirating and dispensing flow-rates of the dialysis solution has been investigated. The recoveries of verapamil and norverapamil in plasma were close to 75% and the limits of quantification were 5 ng/ml for both analytes. The method was found to be linear in the concentration range from 5 to 500 ng/ml (r²: 0.9996 for both analytes). The intra-day and inter-day reproducibilities at a concentration of 100 ng/ml were 2.3% and 5.6% for verapamil and 1.7% and 5.1% for norverapamil, respectively.     

Solid-State nuclear magnetic resonance: performance of point-charge distributions to model intermolecular effects in 19F chemical shifts

This contribution presents results from applying two different charge models to take into account intermolecular interactions to model the solid-state effects on the ¹⁹F NMR chemical-shift tensors. The density functional theory approach with the B3LYP gradient-corrected exchange correlation functional has been used because it includes electron correlation effects at a reasonable cost and is able to reproduce chemical shifts for a great variety of nuclei with reasonable accuracy. The results obtained with the charge models are compared with experimental data and with results obtained from employing the cluster model, which explicitly includes neighboring molecular fragments. The results show that the point-charge models offer similar accuracy to the cluster model with a lower cost. Received: 3 October 1999 / Accepted: 3 February 2000 / Published online: 5 June 2000     

A new hydride generation system applied in determination of arsenic species with ion chromatography-hydride generation-atomic fluorescence spectrometry (IC-HG-AFS)

A novel procedure was developed for the determination of arsenite (As(III)), arsenate (As(V)), monomethylarsonic (MMA) and dimethylarsinic acid (DMA) with ion chromatography-hydride generation-atomic fluorescence spectrometry (IC-HG-AFS) by employing a new gas-liquid separator (GLS). The effective separation of the four arsenic species was achieved in about 12 min. With a sample loading volume of 20 μl, the measurable minimum for As(III), DMA, MMA and As(V) were 0.02, 0.045, 0.043 and 0.166 ng, respectively, along with relative standard deviations of 1.1, 1.1, 1.7 and 2.2% at the 100 μg l⁻¹ level (n = 6) for As(III), DMA, MMA and As(V), respectively. The present procedure was applied for the speciation of arsenic in underground water and in urine samples, and the sum of the four arsenic species by IC-HG-AFS was in good agreement with the total value by HG-AFS.     

Practical applications of the chemical strain effect in ionic and mixed conductors

Abstract  The chemical strain effect in solids is the deviation from linear elasticity due to the association and dissociation of point defects. Although to date this effect has been observed and studied only in Ce_0.8Gd_0.2O_1,9, one may expect that it will be found in other ionic and mixed conductors containing a large concentration of point defects. In this work, some practical applications of materials exhibiting the chemical strain effect are discussed. Based on the example of Ce_0.8Gd_0.2O_1,9, mechanical structures built from these materials should exhibit exceptional mechanical stability and are therefore very attractive for use as components of solid oxide fuel cells (SOFC) or other devices subjected to large and frequent temperature variations. The ability of these materials to withstand large strain without accumulating large stress also makes them potentially useful as flexible elements in micro-electromechanical systems (MEMS). Graphical abstract        

Design and fabrication of a low-cost flow-through cell for the determination of acetaminophen in pharmaceutical formulations by flow injection cyclic voltammetry

A low-cost electrochemical flow-through cell is designed and fabricated to use in conjunction with a flow injection (FI) system. This detector cell used a centrosymmetric radial flow thin-layer geometry with a stainless steel auxiliary electrode and a reference electrode (Ag/AgCl) without a salt bridge. The 5H pencil lead electrode used as a working electrode in the home-made cell is an extremely inexpensive electrode which performs as well as the expensive commercial glassy carbon electrode. Optimum conditions for determining acetaminophen using the proposed FI manifold was investigated. Appropriate volume of sample and/or standard solution containing acetaminophen in pH 2.2 Mcllvaine buffer solution was injected into the proposed FI system and mixed with the flowing stream of supporting electrolyte (pH 2.2 Mcllvaine buffer solution) at an optimum flow rate of 1 ml min⁻¹. The cyclic voltammograms were recorded over the potential range from −0.5 to +2.0 V with a scan rate of 40 mV s⁻¹. Linear calibration curve over the range of 0.1–5 mM acetaminophen was established with the regression equation Y=3.68X+1.0157 (r²=0.9964). The recommended method has been applied to the determination of acetaminophen in 8 commercial pharmaceutical preparations. The percentage recoveries of the spiked acetaminophen in four tablet samples were ranging from 103 to 112 with the relative standard deviation in the range of 0.1–1.3%.     

Novel flow injection-fluorometric method for the determination of trace silicate and its application to ultrapurified water analysis

A highly sensitive fluorescence quenching method for the determination of silicate based on the formation of an ion associate between molybdosilicate and Rhodamine B (RB) in nitric acid medium was developed. A flow injection system coupled with a fluorescence detector was used for the measurement of fluorescence intensity at 560 and 580 nm as excitation and emission wavelengths, respectively. The calibration graph for Si showed a linear range of 0.1–5 ng cm⁻³ with correlation coefficient of 0.9999, and the detection limit of 0.06 ng cm⁻³. The proposed method was successfully applied to the determination of silicate in ultrapurified water with satisfactory results.     

Surface characterization using chemical force microscopy and the flow performance of modified polydimethylsiloxane for microfluidic device applications

The widespread interest in micro total analysis systems has resulted in efforts to develop devices in cheaper polymer materials such as polydimethylsiloxane (PDMS) as an alternative to expensive glass and silicon devices. We describe the oxidation of the PDMS surface to form ionizable groups using a discharge from a Tesla coil and subsequent chemical modification to augment electroosmotic flow (EOF) within the microfluidic devices. The flow performance of oxidized, amine-modified and unmodified PDMS materials has been determined and directly compared to conventional glass devices. Exact PDMS replicas of glass substrates were prepared using a novel two step micromolding protocol. Chemical force microscopy has been utilized to monitor and measure the efficacy of surface modification yielding information about the acid/base properties of the modified and unmodified surfaces. Results with different substrate materials correlates well with expected flow modifications as a result of surface modification. Oxidized PDMS devices were found to support faster EOF (twice that of native PDMS) similar to glass while those derivatized with 3-aminopropyl triethoxysilane (APTES) showed slower flow rates compared to native PDMS substrates as a result of masking surface charge. Results demonstrate that the surface of PDMS microdevices can be manipulated to control EOF characteristics using a facile surface derivatization methodology allowing surfaces to be tailored for specific microfluidic applications and characterized with chemical force microscopy.     

地塞米松磷酸钠的流动注射-化学发光分析法的研究

实验了地塞米松磷酸钠在K3Fe（CN）6-鲁米诺、KMnO4-鲁米诺、KIO4-鲁米诺、H2O2-鲁米诺体系中的化学发光现象。结果表明,地塞米松磷酸钠能够显著增强K3Fe（CN）6-鲁米诺体系和KMnO4-鲁米诺体系的化学发光强度,而在另外两个体系中没有增强作用。结合流动注射技术,分别在K3Fe（CN）6-鲁米诺体系和KMnO4-鲁米诺体系中建立了测定地塞米松的新方法。方法的线性范围分别为1.0×10^-7-1.0×10^-5g/mL和6.0×10^-8-1.0×10^-5g/mL,检出限分别为3.0×10^-8g/mL和2.0×10^-8g/mL,相对标准偏差分别为1.6%和1.9%（1.0×10^-6g/mL地塞米松,n=11）。此法已用于针剂和片剂样品的测定,结果与药典方法没有显著性差异。     

On-line ion-exchange preconcentration in a flow injection system coupled to capillary electrophoresis for the direct determination of UV absorbing anions

On-line ion-exchange preconcentration, performed in a flow injection analysis system, has been integrated with capillary electrophoresis via a specially designed interface, and a sensitive and selective method for the determination of nitrite, nitrate, bromide and iodide using direct UV absorbance detection has been developed. Fivefold enrichment of these aforementioned anions can be realised. Separation conditions such as carrier electrolyte and concentration of electroosmotic modifier were investigated. Limits of detection were ca. 10 ng ml⁻¹ for nitrite and nitrate in aqueous samples, and the overall relative standard deviation was about 5%.