Cadmium analysis in soil by microwave acid digestion and graphite furnace atomic absorption spectrometry

The need to determine micronutrients and toxic elements in soils has grown in recent years and cadmium is of special interest. A method has been developed for the determination of cadmium in soils based on a prior acid digestion of the samples with nitric acid in closed Teflon vessels, into a microwave over. The cadmium determination was carried out by graphite furnace atomic absorption spectrometry (GFAAS) with Lvov platform. Optimum operating conditions, analyte modifiers and matrix interferences have been investigated. The best matrix modifier was found to be ammonium dihydrogen phosphate. The interferences are greatly reduced under these operating conditions and calibration can be performed with simple aqueous solutions of the metal standard. The method is rapid and provides accurate and precise results that agree with certified values for two reference materials: BCR 141 (calcareous loam soil) and BCR 277 (estuarine sediment).     

Effect of X-rays on the surface chemical state of Al2O3, V2O5, and aluminovanadate oxide

The surface composition of Al(2)O(3), V(2)O(5), and aluminovanadate oxide, "V-Al-O", was studied by X-ray photoelectron spectroscopy (XPS), using Mg K(alpha) to reveal time-dependent irradiation damage of samples. Spectral parameters such as peak intensity and width and absolute and relative peak binding energies were evaluated along with the Auger parameter. Irradiation of Al(2)O(3) was found to cause partial dehydration of the surface hydroxide film, while sputter-cleaned alumina turned out to be resistant to X-rays. In V(2)O(5), a small fraction of V(4+) species was seen to form during X-ray exposure. X-ray induced damage in Al(2)O(3) and V(2)O(5) was compared to that caused by bombardment with 500 eV argon ions. The V-Al-O material which is used as a precursor of oxynitride catalysts for ammoxidation turned out to be most susceptible and could be damaged by low X-ray doses. An appreciable reduction from the V(5+) to the V(4+) formal oxidation state (the latter increases from 20 to 45% after 150 min time of exposure to Mg K(alpha) at 150 W) was found along with the decomposition of aluminum hydroxide which is believed to act as an amorphous support in this catalyst. Gas-phase analysis during X irradiation demonstrated desorption of oxygen and water molecules. X-ray induced damage is believed to be caused by electron-hole pair generation and Auger decay rather than by thermal effects since the sample surface temperature increased only slightly.     

Speciation of inorganic and organic arsenic in marine sediments from La Coruña estuary

The speciation of As(III), As(V), MMA and DMA in marine sediments from La Coruña estuary is described. The arsenic species have been separated by ion-exchange chromatography and detected by hydride generation atomic absorption spectrometry (HGAAS). The redox potential has been determined in order to relate the concentration of arsenic species to this parameter.     

Optimization of microwave-assisted extraction of hydrocarbons in marine sediments: comparison with the Soxhlet extraction method

Microwave energy was applied to extract polycyclic aromatic hydrocarbons (PAHs) and linear aliphatic hydrocarbons (LAHs) from marine sediments. The influence of experimental conditions, such as different extracting solvents and mixtures, microwave power, irradiation time and number of samples extracted per run has been tested using real marine sediment samples; volume of the solvent, sample quantity and matrix effects were also evaluated. The yield of extracted compounds obtained by microwave irradiation was compared with that obtained using the traditional Soxhlet extraction. The best results were achieved with a mixture of acetone and hexane (1:1), and recoveries ranged from 92 to 106%. The extraction time is dependent on the irradiation power and the number of samples extracted per run, so when the irradiation power was set to 500 W, the extraction times varied from 6 min for 1 sample to 18 min for 8 samples. Analytical determinations were carried out by high-performance liquid chromatography (HPLC) with an ultraviolet-visible photodiode-array detector for PAHs and gas chromatography (GC) using a FID detector for LAHs. To test the accuracy of the microwave-assisted extraction (MAE) technique, optimized methodology was applied to the analysis of standard reference material (SRM 1941), obtaining acceptable results.     

Aptamer-based folding fluorescent sensor for cocaine.

We adapted in two steps a deoxyribonucleotide-based aptamer to signal the recognition of cocaine: an instability was engineered in one stem of a three-way junction that forms the cocaine-binding pocket and the resulting short stem was end labeled with a fluorophore and a quencher. In the absence of cocaine, two stems are open, but in its presence they close and the three-way junction forms. This major structural change brings fluorophore and quencher together thereby signaling the presence and concentration of ligand. The sensor is selective for cocaine over its metabolites, can operate in serum, and is useful for the screening of cocaine hydrolases.     

Characterization of subwavelength elastic cylinders with the decomposition of the time-reversal operator: theory and experiment

The decomposition of the time-reversal operator provides information on the scattering medium. It has been shown [Chambers and Gautesen, J. Acoust. Soc. Am. 109, 2616-2624 (2001)] that a small spherical scatterer is in general associated with four eigenvalues and eigenvectors of the time-reversal operator. In this paper, the 2D problem of scattering by an elastic cylinder, imbedded in water, measured by a linear array of transducers is considered. In this case, the array response matrix has three nonzero singular values. Experimental results are obtained with linear arrays of transducers and for wires of different diameters smaller that the wavelength. It is shown how the singular value distribution and the singular vectors depend on the elastic velocities cL, cT, the density rho of each wire, and on the density rho0 and velocity c0 of the surrounding fluid. These results offer a new perspective towards solution of the inverse problem by determining more than scattering contrast using conventional array processing like that used in medical ultrasonic imaging.     

Development of a fast analytical tool to identify oil spillages employing infrared spectral indexes and pattern recognition techniques

A fast analytical tool based on attenuated total reflectance mid-IR spectrometry is presented to evaluate the origin of spilled hydrocarbons and to monitor their fate on the environment. Ten spectral band ratios are employed in univariate and multivariate studies (principal components analysis, cluster analysis, density functions - potential curves - and Kohonen self organizing maps). Two indexes monitor typical photooxidation processes, five are related to aromatic characteristics and three study aliphatic and branched chains. The case study considered here comprises 45 samples taken on beaches (from 2002 to 2005) after the Prestige carrier accident off the Galician coast and 104 samples corresponding to weathering studies deployed for the Prestige's fuel, four typical crude oils and a fuel oil. The univariate studies yield insightful views on the gross chemical evolution whereas the multivariate studies allow for simple and straightforward elucidations on whether the unknown samples match the Prestige's fuel. Besides, a good differentiation on the weathering patterns of light and heavy products is obtained.     

Integrability properties of a generalized reduction of the KdV6 equation

We consider the integrability properties of a generalized version of a similarity reduction of the so-called KdV6 equation, an equation that has recently generated much interest. We give a linear problem for this generalized reduction and show that it satisfies the requirements of the Ablowitz-Ramani-Segur algorithm. In addition we give a Bäcklund transformation to a related equation, giving also an auto-Bäcklund transformation for this last. Our results mirror those for the Korteweg-de Vries equation itself, which has a similarity reduction to an ordinary differential equation which is related by a Bäcklund transformation to the second Painlevé equation, this last having an auto-Bäcklund transformation.     

Comparison of oil spillages using mid-IR indexes and 3-way procrustes rotation, matrix-augmented principal components analysis and parallel factor analysis

Three different approaches for 3-way analyses, namely, Procrustes rotation, parallel factor analysis (PARAFAC) and matrix-augmented principal component analysis (MA-PCA), have been used to compare six different oil spillages made under controlled conditions (one of them corresponding to the heavy oil released after the sunk of the Prestige tanker off the Galician coast-NW Spain on November 2002). Each spillage was monitored during three and a half months by attenuated total reflectance (ATR) mid-IR spectroscopy. Ten characteristic band ratios were defined. Results showed that the three 3-way chemometric techniques lead to essentially the same conclusions, where from it was concluded that the most relevant pattern defining the oil weathering was related to ‘total aromaticity’, i.e., the total number of CC bonds in the molecules which form the products. In addition, weathering of the samples got clearly characterized by a steady evolution on the scores (sample weights), with a clear increase after 11-14 days. Differentiation of the products (slices of the data cube) was also possible due to their intrinsic characteristics as, in general, heavy products oppose to the lightest ones.