Application of Nb2O5–SiO2 in Pre-Concentration and Determination of Copper and Cadmium by Flow System with Flame Atomic Absorption Spectrometry

 The use of niobium (V) oxide, chemically adsorbed on silica gel surface (Nb₂O₅–SiO₂), as an adsorbent in a pre-concentration system for copper (II) and cadmium (II) determination was proposed. The procedure is based on the large ion-exchange capacity for the adsorption of copper (II) and cadmium (II) ions on the mini-column packed with Nb₂O₅–SiO₂, followed by elution with 2.0 mol L⁻¹ nitric acid and determination by flame atomic absorption spectrometry. Chemical and flow variables were optimized. The results demonstrated that the sample solutions containing copper (II) or cadmium (II) in a concentration range of 3.0 to 600.0 μg L⁻¹ and 3.1 to 100.0 μg L⁻¹ respectively, in a solution of pH 5.0 could be determined using a pre-concentration time of 2 min. The enrichment factor when using a sample volume of 10.2 mL and 2 min of pre-concentration was 17.5 and 20.3 for copper (II) and cadmium (II), respectively. The limit of detection for copper (II) and cadmium (II) was 0.5 μg L⁻¹ and 1.0 μg L⁻¹, respectively. The relative standard deviation (RSD) was lower than 1.4% for copper (II) and cadmium (II) when using a concentration of 25.0 μg L⁻¹ for both metals. The method was tolerant to other ions usually present in water samples. Good accuracy was obtained by the analysis of water reference material and environment samples. Correspondence: Universidade Federal de Santa Catarina, Departamento de Química, Florianópolis SC, Brazil. e-mail: carasek@qmc.ufsc.br Received July 16, 2002; accepted October 25, 2002     

Application of fractional factorial experimental and Box-Behnken designs for optimization of single-drop microextraction of 2,4,6-trichloroanisole and 2,4,6-tribromoanisole from wine samples

In this paper a new method for the determination of 2,4,6-trichloroanisole (TCA) and 2,4,6-tribromoanisole (TBA) in wine samples is presented. Headspace single-drop microextraction (HS-SDME) was used for the extraction and preconcentration of the analytes, followed by analysis by gas chromatography and electron-capture detection (GC-ECD). The variables affecting extraction efficiency were optimized using fractional factorial experimental and Box-Behnken designs. The external calibration procedure was successfully carried out using a synthetic wine solution and diluted red wine samples. The method was also applied to white wine samples. Excellent detection limits of 8.1 and 6.1 ng L(-1) were achieved for TCA and TBA, respectively. Good precision and accuracy were obtained.     

Low temperature measurements by infrared spectroscopy in CoFe2O4 ceramic

This paper presents results of new far-infrared and middle-infrared measurements (wavenumber range of 4000?C100 cm^?1) of the CoFe₂O₄ ceramic in the temperature range from 300 K to 8 K. The band positions and their shapes remain constant across the wide temperature range. The quality of the sample was investigated by X-ray, EDS and EPMA studies. The CoFe₂O₄ retains the cubic structure (Fd - 3m) across the temperature range from 85 K to 360 K without any traces of distortion. Based on current knowledge the polycrystalline CoFe₂O₄ does not exhibit any phase transitions across the temperature range from 8 K to 300 K.     

Propellanes—From a Chemical Curiosity to “Explosive” Materials and Natural Products

Propellanes are a unique class of compounds currently consisting of well over 10 000 representatives, all featuring two more or less inverted tetrahedral carbon atoms that are common to three bridging rings. The central single bond between the two bridgeheads is significantly weakened in the smaller entities, which leads to unusual reactivities of these structurally interesting propeller-like molecules. This Review highlights the synthesis of such propellanes and their occurrence in material sciences, natural products, and medicinal chemistry. The conversion of [1.1.1]propellane into bridgehead derivatives of bicyclo[1.1.1]pentane, including oligomers and polymers with bicyclo[1.1.1]penta-1,3-diyl repeat units, is also featured. A selection of natural products with larger propellane subunits are discussed in detail. Heteropropellanes and inorganic propellanes are also addressed. The historical background is touched in brief to show the pioneering work of David Ginsburg, Günther Snatzke, Kenneth B. Wiberg, Günter Szeimies, and others.     

Determination of haloanisoles in paper samples for food packaging by solid-phase microextraction and gas chromatography

A method was developed for the determination of trichloroanisole, tribromoanisole and pentachloroanisol by solid-phase microextraction and gas chromatography in paper samples (Kraft liner, Test liner and Miolo). Four commercial SPME fibers were evaluated: Polydimethylsiloxane (PDMS), Polyacrylate (PA), Carbowax/Divinylbenzene (CW/DVB) and Divinylbenzene/Carboxen/Polydimethylsiloxane (DVB/CAR/PDMS). DVB/CAR/PDMS gave the best results and was therefore selected. Other variables involved in the extraction procedure were studied and optimized, such as: sample volume, salting-out effect, temperature and extraction time, effect of organic solvent and previous sample preparation. Optimum conditions were obtained using 20 mL of sample with 5 mol L⁻¹ NaCl in a 40 mL vial, extraction temperature of 70 °C and sonication and extraction time of 30 and 40 min, respectively. Detection limits ranged from 0.43 to 1.32 ng g⁻¹ for all analytes. Recoveries between 70 and 100% were obtained and relative standard deviation was below 10% for all compounds.     

Preparation and application of NiTi alloy coated with ZrO(2) as a new fiber for solid-phase microextraction

In this study a NiTi alloy was applied as an SPME support due to its superelasticity and shape memory properties. This new metallic support was coated with ZrO(2) by electrodeposition using chronoamperometry. It was then evaluated for extraction of three classes of compounds from gaseous samples: alcohols, BTEX and trihalomethanes (THM). For the optimization of the parameters affecting the extraction efficiency of the target compounds, the univariate approach was used. Five fibers were electrodeposited to evaluate the reproducibility of the coating procedure, resulting in a relative standard deviation lower than 11.9%. The repeatability for one fiber (n=6) was lower than 8.5%. The detection limits were lower than 28.1, 20.8 and 0.18 microgL(-1) for alcohols, BTEX and THM, respectively, and the correlation coefficients were higher than 0.996. Taking into account the amount extracted per unit volume, the NiTi-ZrO(2) fiber showed a better extraction profile in comparison with the commercial fibers 7 microm PDMS, 85 microm PA and 30-50 microm DVB/CAR/PDMS. The new SPME fiber has a lifetime of over 300 extractions. Thus, it is a promising alternative for low-cost analysis, as it is robust, and easily and inexpensively prepared.     

Use of Fe(3+) ion probe to study the stability of urea-intercalated kaolinite by electron paramagnetic resonance

The effect of mechanical and chemical activation in processes of urea intercalation in the interlayer spacing of kaolinite and the effect of varying the temperature of the intercalation product between 100 and 200 degrees C were studied using Fe(3+) ions as a probe in electron paramagnetic resonance (EPR) spectroscopy. Other techniques were also used to characterize the samples. Monitoring the heating of urea-intercalated kaolinite, FTIR, and XRD revealed that the product obtained was stable up to a temperature of 150-160 degrees C. The EPR data indicated that the intercalation process promoted an approximation and increase of the magnetic interactions among the Fe(3+) ions. The DRUV-vis analysis of the product before heating showed an absorption band at 680 nm that was absent in the raw kaolinite. This band was attributed to the transition A(1)6-->T(2)4(G4) in the adjacent Fe(3+) ions, intensified by magnetic coupling among these ions. We suggest that intercalated urea forms hydrogen bonds between the carbonyl's oxygen and the hydroxyls bound to the Fe(3+) ions of the kaolinite structure. This would cause the approximation of the Fe(3+) ions, maximizing magnetic couplings and intensifying concentrated centers of Fe(3+), as was visible by EPR spectroscopy.     

Solvent‐enhanced headspace sorptive extraction in the analysis of the volatile fraction of matrices of vegetable origin

The solvent‐enhanced headspace sorptive extraction technique aims at modifying PDMS polarity using a solvent to increase its concentration capability. In solvent‐enhanced headspace sorptive extraction, a PDMS tubing closed at both ends by small glass stoppers and filled with an organic solvent is suspended in the sample headspace for a fixed time. After sampling, the sampled analytes are recovered from the PDMS tubing by thermal desorption and online transferred to a GC–flame ionization detector or GC‐MS system for analysis. Cyclohexane, iso‐octane, ethyl acetate, acetone, acetonitrile and methanol were tested as PDMS modifiers to sample the volatile fractions of sage (Salvia lavandulifolia Vahl.), thyme (Thymus vulgaris L.) and roasted coffee. Ethyl acetate was found to be the most effective PDMS modifier for all matrices investigated; although to a lesser extent, cyclohexane also increased component recoveries with sage and thyme. Acetone, acetonitrile and methanol did not increase PDMS recovery, while isooctane was excluded because of its interaction with the polymer. The results show that solvent‐modified PDMS extends the range of sampled headspace components with different polarities, increases the recovery of many of them, improves sensitivity in trace analysis, speeds up recovery and gives repeatability comparable with that of unmodified PDMS.     

Dehydrated halloysite intercalated mechanochemically with urea: Thermal behavior and structural aspects

Urea has been intercalated mechanochemically into dehydrated halloysite and analyzed by X-ray powder diffraction (XRPD), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance ultraviolet/visible spectroscopy (DRUV–VIS), thermal analysis (TGA/DTA), transmission electron microscopy (TEM), and electron paramagnetic resonance (EPR). The basal distance expands from 7.4 to 10.7 Å and the interaction of urea to adjacent layers of halloysite through hydrogen bonds increases the structural order of the matrix. After heat treatment in air at different temperatures, decomposition products begin to appear starting from 100 °C. Although the basal distance remains constant up to 160 °C and collapses to the original value at 200 °C, urea and the decomposition products are still present in the sample. Starting from 125 °C, urea decomposition products reduce halloysite structural Fe³⁺ centers to Fe²⁺, as indicated by DRUV–VIS and EPR spectroscopy.     

Physicochemical characterization of the vegetal drug and nebulized extract of the roots from Apodanthera congestiflora Cogn. (Cucurbitaceae)

Journal of Thermal Analysis and Calorimetry - Apodanthera congestiflora Cogn. is a Brazilian Cucurbitaceae, known for its anti-inflammatory and antiparasitic activities. The objective of this work...