Determination of Arsenic in Food Samples by Hydride Generation – Atomic Absorption Spectrometry

A method for the determination of trace amounts of arsenic in food samples using flow injection analysis and atomic absorption spectrometry with hydride generation (FI-HG AAS) was developed. The parameters of the flow injection system and the hydride generation were optimized with respect to reagent concentrations, atomization temperature, injection volume, reaction coil length and carrier flow rate. The limits of detection and quantification were 0.34µgL^–1 and 1.2µgL^–:1, respectively, and the analytical curve is linear up to 30.0µgL^–1 arsenic. The relative standard deviation for 12 replicates varies between 5% for 4.0µgL^–1 As and 1.8% for 30.0µgL^–1 As, with an injection frequency of up to 135h^–1. Interferences from Ni(II), Cu(II), Fe(III), Cr(III), Mo(II), Bi(III), Se(IV), Se(VI), Sb(III) and Sb(V) could be masked with a mixture of ascorbic acid-KI in a 5.0molL^–1 HCl solution. The accuracy of the proposed method was evaluated by using certified reference materials of biological samples, and the method was used to determine the content of arsenic in fish and coffee beans.     

Photoluminescence of osmocene and decamethylosmocene

Osmocene (and decamethylosmocene) is characterized by a lowest-energy ligand field triplet which occurs at _max=372nm (374nm) in absorption and 567nm (572nm) in emission. This orange–yellow phosphorescence is rather intense at 77K but is also visible at r.t.     

Electrochemical Behavior of Epinephrine at a Penicillamine Self-Assembled Gold Electrode,and its Analytical Application

The fabrication and electrochemical characteristics of a penicillamine (PCA) self-assembled monolayer modified gold electrode were investigated. The self-assembled electrode shows obvious electrocatalytic activity for the oxidation of epinephrine (EP). In phosphate buffer (pH 7.73), a sensitive oxidation peak was observed at 0.190V with the PCA modified Au electrode. The peak current is proportional to the concentration of EP in the range of 2.0×10^–56.0×10^–4molL^–1 and 5.0×10^–6 2.0×10^–4molL^–1 for cyclic voltammetry (CV) and differential pulse voltammetry (DPV) with the detection limits of 1.8×10^–7 and 1.3×10^–7molL^–1, respectively. The possible reaction mechanism is also discussed. The PCA self-assembled monolayer modified gold electrode is highly stable and can be applied to the determination of EP in practical injection samples. Application is simple, rapid and produces accurate results.     

Determination of Attogram Quantities of Silver via Quenching of the Solid Substrate Room-Temperature Phosphorescence of Rhodamine B Based on the Catalysis of its Oxidation by Potassium Persulfate

A new method of SS-RTP for the determination of trace silver has been established. This method is based on the fact that Ag⁺, when activated by ,-bipyridyl (bipy) in a media of HAc–NaAc (pH=4.9), can catalyze the reaction of Rhodamine B (RhoB) oxidized by K₂S₂O₈, thus causing the Solid Substrate Room-Temperature Phosphorescence (SS-RTP) of RhoB to be quenched. The activating efficiency of bipy is 6.7 times higher than that of o-phenanthroline (phen). The reduction of the phosphorescence intensity (I_p) of RhoB is directly proportional to the concentration of Ag⁺ ions in the range of 1.6016.0agspot^–1 (0.40µLspot^–1). The regression equation of the working curve can be expressed as I_p=18.78+5.100m_Ag+ (agspot^–1) (r=0.9994, n=6), the detection limit is 0.28agspot^–1. This rapid, accurate and sensitive method has been successfully applied to the determination of trace silver in tea and human hair samples, and the results agree well with the Atomic Absorption Spectroscopy (AAS) method. The mechanism of the catalyzing reaction is also discussed.     

Determination of Heavy Metal Ions in Chinese Herbal Medicine by Microwave Digestion and RP-HPLC with UV-Vis Detection

A new method for the simultaneous determination of heavy metal ions in Chinese herbal medicine by microwave digestion and reversed-phase high-performance liquid chromatography (RP-HPLC) has been developed. The Chinese herbal medicine samples were digested by microwave digestion. Lead, cadmium, mercury, nickel, copper, zinc, and tin ions in the digested samples were pre-column derivatized with tetra-(4-chlorophenyl)-porphyrin (T₄-CPP) to form the colored chelates which were then enriched by solid phase extraction with C₁₈ cartridge and eluted from the cartridge with tetrahydrofuran (THF). The chelates were separated on a Waters Xterra RP₁₈ column by gradient elution with methanol (containing 0.05molL^–1 pyrrolidine-acetic acid buffer salt, pH=10.0) and THF (containing 0.05molL^–1 pyrrolidine-acetic acid buffer salt, pH=10.0) as mobile phase at a flow rate of 0.5mLmin^–1 and detected with a photodiode array detector in the range of 350–600nm. In the original samples the detection limits of lead, cadmium, mercury, nickel, copper, zinc and tin are 4ngL^–1, 3ngL^–1, 6ngL^–1, 5ngL^–1, 2ngL^–1, 6ngL^–1, and 4ngL^–1, respectively. This method was applied to the determination of lead, cadmium, mercury, nickel, copper, zinc and tin in Chinese herbal medicine samples with good results.     

Non-Aqueous Capillary Electrophoresis with Indirect Photometric Detection for Direct Analysis of Pb2+, Zn2+ and Cd2+ Ions

A new approach, based on non-aqueous capillary electrophoresis separation and indirect photometric detection, was established for the determination of the transition metal ions Pb²⁺, Zn²⁺ and Cd²⁺. Under optimized conditions, the method produced baseline separation of these three metal ions. The linear range and detection limits were 1050µM, 1.9µM for Cd²⁺; 1050µM, 2.1µM for Zn²⁺; and 20100µM, 3.8µM for Pb²⁺, respectively.     

Voltammetric Determination of Trace Amounts of Fenitrothion on a Novel Nano-TiO2 Polymer Film Electrode

A novel nano-TiO₂ polymer modified glassy carbon (GC) electrode was developed for the determination of an organophosphorous pesticide, fenitrothion (-NO₂), in citrate buffer solution. The electrochemical behavior of fenitrothion was characterized by using cyclic voltammetry. An irreversible form, -NO₂, was transformed into a reversible redox couple (-NHOH/-NO), and it can be used to determine trace fenitrothion by square wave voltammetry. The experimental parameters, such as film thickness, pH value, accumulation potential and time were optimized. Interestingly, a cyclic voltammetric scan was observed to be more effective than a constant potential for the accumulation of fenitrothion. A linear response over a fenitrothion concentration of 2.5×10^–8 to 1.0×10^–5M was exhibited, with a detection limit of 1.0×10^–8M (S/N=3). The high sensitivity and selectivity of this film electrode was demonstrated by its practical application to the determination of trace amounts of fenitrothion in lake water and apple samples.     

Modelling of quantitative DTA equipment I. Electric analog approach

For an electric model, relationships have been established as a function of the DTA baseline shift, the intensity of the signal, the slope of the leading edge, the heating rate, the thermal resistance of the heating disc built into the DTA apparatus, the contact resistance between the disc and the sample holders, and the heat resistance between the two measuring sites. Some of the relationships obtained supported knowledge acquired earlier by other approaches, and some furnished new information. The limits of use of this simple model are indicated.

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Zusammenfassung Unter Anwendung eines elektrischen Modells wurden Zusammenhänge als Funktion der Verschiebung der Basislinie der DTA, der Signalintensität, der Steile der Leitkante, der Aufheizgeschwindigkeit, des Wärmewiderstandes der in das DTA-Gerät eingebauten Heizscheibe, des Kontaktwiderstandes, zwischen Scheibe und Probenbehälter und des Wärmewiderstandes zwischen den beiden Meßstellen ermittelt. Ein Teil der erhaltenen Zusammenhänge bestätigte von anderen Annäherungen her bereits vorliegende Kenntnisse, ein anderer Teil lieferte neue Informationen. Auf die Grenzen und der Nutzen dieses einfachen Modells wurden hingewiesen.

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Résumé A l'aide d'un modèle électrique, on a établi des relations décrivant l'influence de la dérive de la ligne de base ATD, de l'intensité du signal, de la pente du bord du pic, de la vitesse de chauffage, de la résistance thermique du disque de chauffage incorporé dans l'appareil ATD, de la résistance de contact entre le disque et les porte-échantillons et enfin, de la résistance thermique entre les deux points de mesure. Une partie des relations obtenues confirme les connaissances déjà acquises par d'autres approches, l'autre partie fournit des renseignements nouveaux. Les limites d'utilisation de ce modèle simple sont indiquées.

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Reversible Optical Sensor Membrane for Hydrogen Peroxide Using an Immobilized Fluorescent Probe,and its Application to a Glucose Biosensor

An optical sensor for hydrogen peroxide (HP) based on the probe europium tetracycline (EuTc) incorporated into a polyacrylonitrile-co-polyacrylamide polymer matrix is described. Upon optical excitation with 400-nm light, the EuTc in the membrane displays fairly strong fluorescence peaking at 616nm. The fluorescence intensity increases up to 3-fold if the sensor is exposed to solutions containing HP. The effect is reversible and can thus be used for continuous sensing. The largest signal changes with HP are observed at pH levels between 6.5 and 7.5, and the range of the response is between 10 and 300ppm (w/w) of HP, equal to 0.3 to 10mmolL^–1). At concentrations above 0.3% of HP, decomposition of the EuTc in the membrane is observed. The limit of detection is 15ppm (0.45mmolL^–1). The response is fully reversible and rather slow (10min) in both directions, but the reverse response may be accelerated by addition of a reducing agent such as thiosulfate. Alkali ions and most anions remain inert, but phosphate and citrate interfere, as do Cu²⁺ ions, which quench fluorescence. In order to image the spatial distribution of HP concentrations, sensor membranes were placed on the bottom of the wells of a microtiter plate, and their fluorescence was imaged using an LED-based device based on the measurement of the luminescence decay time of EuTc. If glucose oxidase is immobilized on the sensor layer, a glucose sensor is obtained in which the HP sensor acts as the transducer and which can quantify glucose in concentrations between 0.1 and 5mmolL^–1. The limit of detection for glucose is 0.2mmolL^–1.     

Use of 2,4-dinitrophenylhydrazone of glyoxylic acid for the determination of glyoxylic acid by the chromatographic-spectrophotometric method and by differential pulse polarography

Two different methods (one based on chromatography and spectrophotometry and the other on polarography) have been developed for the determination of glyoxylic acid in the form of a derivative with 2,4-dinitrophenylhydrazine (DNPHGA). The TLC method allows the separation of two DNPHGA isomers (trans and cis). Spectrophotometric measurements of the eluents of the separated compounds (=360 nm) allow the determination of GA within the range from 4 to 30 g. Using differential pulse polarography, the conditions of DNPHGA formation were examined. The reduction peak of this derivative (E_P=–0.430 V), observed by dpp, was used to develop a polarographic determination of GA within the concentration range from 110^-4 to 710^–4 mol/l.     

Solid-Phase Extraction with Slurry Injection of the Resin Into ETAAS for Trace Determination of Thallium in Natural Water

Thallium in natural water samples was determined by electrothermal atomic absorption spectrometry after 1000-fold enrichment by mini solid-phase extraction from a 100-mL sample solution. A Tl-pyrrolidine-1-carbodithioate complex formed in a sample solution of pH 1.6 was extracted on fine particles of a cellulose nitrate resin dispersed in the sample solution. The cellulose nitrate resin was then collected on a membrane filter (25mm^ø) by filtration under suction using a glass funnel with an effective filtration area of 0.64cm². As a result, a circular thin layer of the resin phase with a diameter of 9mm was obtained. Then the resin phase was carved out by an acrylate resin puncher with a 10-mm^ø hole to put it into a sample cup containing 100µL of 10mM HNO₃ containing 0.5mM NaCl. The resin phase was suspended in the solution by ultrasonication. 1000-fold enrichment was thus attained within 15min, and the suspension was delivered to electrothermal atomic absorption spectrometry. The linear calibration graph was obtained in the range of 0–4ng of Tl in 100mL of a sample solution. The detection limit obtained by 3 method was 0.19ng. The proposed method was applied to the determination of Tl in natural water samples. The results showed the concentration of Tl in seawater was 12.1±1.8pgmL^–1 for the calibration graph method and 12.6±1.4pgmL^–1 for the standard addition method. A snowmelt sample contained 20.7±1.0pgmL^–1 of Tl.     

Energy Transfer Electrogenerated Chemiluminescence for the Determination of Sulfite

A simple and novel electrogenerated chemiluminescence (ECL) method for the determination of sulfite has been developed based on the energy transfer ECL process. It was found that a weak ECL signal of sulfite was electrochemically generated on a platinum electrode in neutral aqueous solution. The signal was strongly enhanced by rhodamine B as an energy receptor and further enhanced by the neutral surfactant Tween 80. In 0.10M phosphate buffer solution (pH=7.5) containing 2.0×10^–6gmL^–1 rhodamine B and 0.4% (v/v) Tween 80, the ECL response to the concentration of sulfite at a potential of 0.82V was linear over a range of 1.0×10^–7gmL^–1 to 8.0×10^–6gmL^–1, and the detection limit was 5×10^–8gmL^–1. The relative standard deviation (n=11, 1.0×10^–6gmL^–1) was 3.8%. The proposed method has been successfully applied to the determination of sulfite in pharmaceutical injections and white sugar samples.     

2,3-Dihydroxypyridine Loaded Amberlite XAD-2 (AXAD-2-DHP): Preparation, Sorption–Desorption Equilibria with Metal Ions, and Applications in Quantitative Metal Ion Enrichment from Water, Milk and Vitamin Samples

2,3-Dihydroxypyridine loaded (via –N=N–linker) Amberlite XAD-2 (AXAD-2-DHP) was prepared and characterized by elemental analyses, TGA and FT-IR spectra. It (1g packed in a column of 1cm diameter; surface area 135.5m²g^–1) was found to be an effective solid phase sorbent for enriching Zn²⁺, Mn²⁺, Ni²⁺, Pb²⁺, Cd²⁺, Cu²⁺, Fe³⁺ and Co²⁺ at pH 3.5 to 7.0 using flow rates between 1.0–5.0mLmin^–1. For desorption (recovery 97.0–99.8%) of the metal ions, 8 to 10mL of 2.0molL^–1 HCl or 1.5molL^–1 HNO₃ at a flow rate of between 2.0 and 4.0mLmin^–1 were found most suitable. The t_1/2 (time for 50% sorption) is between 2 and 10min when a 50mL solution (containing a total amount of metal of 2mg) was equilibrated with 0.5g of resin. Sorption of all metal ions except Pb²⁺ follows the Langmuir model, whereas for Pb the data fits with the Freundlich model. The sorption capacity is between 60.7 (for Cd) and 406.7 (for Cu) µmolg^–1. The resin can withstand an acid concentration of 6molL^–1 and can be reused for thirty cycles of sorption–desorption. The preconcentration factor varies between 100 and 300. For Cd, Ni and Cu the sorption capacity of 2,3-dihydroxypyridine loaded cellulose is lower than that of the present resin. The tolerance limits of electrolytes, humic acid, complexing agents, Ca²⁺ and Mg²⁺ in the enrichment of all metal ions are reported. The limits of detection are 3.88, 5.37, 8.72, 13.88, 4.71, 1.24, 0.59 and 0.30µgL^–1 for Zn²⁺, Mn²⁺, Ni²⁺, Pb²⁺, Cd²⁺, Cu²⁺, Fe³⁺ and Co²⁺, respectively. The calibration curves for flame AAS determination were linear in the ranges 0.018–1.0, 0.067–5.0, 0.2–5.0, 0.9–20, 0.028–2.0, 0.077–5.0, 0.19–10 and 0.1–3.5µgmL^–1, respectively. All the eight metal ions in river and synthetic water samples, Co in vitamin tablets and Zn in milk samples have been quantitatively enriched with Amberlite XAD-2-DHP and determined by flame atomic absorption spectrometry.     

ESR spectra of47Ti-enriched titanium catalysts for polymerization

For the first time ESR spectra of⁴⁷Ti-enriched Ziegler system Ti-TIBA in toluene have been studied. For the ratio Ti/Al1, more accurate h. f. s. constants have been found. For Ti/Al1/15 no h. f. s. have been observed, which is ascribed to ordering due to the cooperative Jahn-Teller effect.

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TiCl₄+ ⁴⁷Ti. Ti/Al1 . Ti/Al1/15 , -.

     

Fluorescence Quenching Studies of Curcumin by Hydrogen Peroxide in Acetonitrile Solution

Summary. Steady state quenching studies of curcumin, 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione, fluorescence by hydrogen peroxide were conducted in acetonitrile solution. A quenching rate constant, k_q, of 1.05×10¹⁰M^–1·s^–1 was obtained with a short fluorescence lifetime of 347ps. The reaction rate constant, which is within the diffusion-limited regime, is activation-controlled. The rate constant of deactivation of the thermally excited curcumin was 1.2 orders of magnitude more nonradiative (2.67×10⁹s^–1) than radiative (2.16×10⁸s^–1). The reaction was exothermic with a G° of –1.97eV and solvent reorganization energy of 1.37eV. These values indicate that the electron transfer reaction is solvent-mediated with electron transfer rate constant, k_ET, of 2.16×10¹⁰s^–1.     

Determination of Caffeine Using a Simple Graphite Pencil Electrode with Square-Wave Anodic Stripping Voltammetry

A simple commercial graphite pencil electrode (GPE) was utilized for monitoring caffeine using the square-wave anodic stripping voltammetry (SWASV) method. This method was applied to determine the caffeine levels in several tea samples, which yielded a relative error of 1% in the concentrations. Caffeine was deposited at 0.0V (vs. Ag/AgCl), then reduced at +1.40V to strip it on the GPE. Optimal experimental conditions for the analysis were found to be as follows: pH value of 9 for the medium; deposition potential of 0.0V; deposition time of 120s; SW frequency of 25Hz; SW amplitude of 45mV, and step potential of 6mV. Given these optimum conditions, a linear range was observed within the concentration of 0500mgL^–1. At caffeine concentrations of 50.0, 250.0, and 500.0mgL^–1, the relative standard deviations in measured concentrations (n=12) were 0.19, 0.09, and 0.11%, respectively. The detection limit was found to be 9.2mgL^–1, which is comparable with the result obtained using a carbon paste electrode, equivalent to 8.2mgL^–1.     

Effect of acid-base properties of oxide catalysts in the oxidative dehydrodimerization of olefins

The dependence of the activity of tin-containing oxide catalysts on their basicity and acidity has been examined. The rate of oxidative dehydrodimerization of olefins is shown to be determined by the mobility of allyl compounds depending on the catalyst acid properties.

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Multimedia Analysis of Radon in 10 mL of Air for in-Laboratory Quality Assurance

A new, simple, reliable and self-consistent procedure has been developed to produce, handle and measure radon in 10mL of air in the three media air, water and charcoal simultaneously. Equal and low activities of radon (200Bq, 4ag) are transferred with a 10mL gas syringe from a 3.8L natural, harmless and constant radon source to three receivers: a 48L Peli case with outdoor air, an 0.5L bottle with tap water, and a 22mL tube with activated charcoal. Concentrations, i.e. activities in the three media are measured with different methods providing independent results. Hence, in-laboratory cross-method calibration and traceability to a certified standard is available for radon in air and in water for the first time and with minimum effort.