Structural studies and T c dependence in La2−x Dy x Ca y Ba2Cu4+y O z type mixed oxide superconductors

A new series of mixed oxide superconductors with the stoichiometric composition La_2−x Dy _x Ca _y Ba₂Cu_4+y O _z (x=0.0 − 0.5, y=2x) has been studied for structural and superconductiong properties. Our earlier studies on La_2−x (Y/Er) _x Ca _y Ba₂Cu_4+y O _z series, show a strong dependence of T _c on hole concentration (p _sh). In the present work, the results of the analysis of the neutron diffraction measurements at room temprerature on x=0.3 and 0.5 samples are reported. It is interesting to know that Ca substitutes for both La and Ba site with concomitant displacement of La onto Ba site. Superconductivity studies show that maximum T _c is obtained for x=0.5, y=1.0 sample (T _c ∼ 75 K), for La_1.5Dy_0.5Ca₁Ba₂Cu₅O _z (La-2125).     

Behaviour of several solid adsorbents for sampling tetraalkyllead compounds in air

The behaviour of various solid adsorbents for collecting tetraalkyllead compounds has been studied. A synthetic atmosphere containing a known concentration of tetraethyllead was produced and the tetraethyllead compound was trapped in a glass tube containing the solid adsorbent. The trapped compound was extracted with hexane in an ultrasonic bath, and the resulting solution was analysed by GC-MS. Porapak and Tenax, with retention efficiencies of 92 and 96%, respectively, were shown to be the more efficient at trapping this alkyllead compound than Chromosorb, active charcoal, Amberlites and polyurethane foam. The behaviour of both Porapak and Tenax for trapping other tetraalkyllead compounds in the presence of gasoline vapour was also studied.     

Gravimetric and spectrophotometric determination of palladium with 2,3,4-pentanetrione trioxime

A study of the use of 2,3,4-pentanetrione trioxime (PTT) in the gravimetric and spectrotrophotometric determination of palladium is reported. PTT has been applied to the determination of palladium in various synthetic and standard samples. The reagent has some advantages over dimethylglyoxime.     

Densification of sol–gel silica thin films induced by hard X‐rays generated by synchrotron radiation

In this article the effects induced by exposure of sol–gel thin films to hard X‐rays have been studied. Thin films of silica and hybrid organic–inorganic silica have been prepared via dip‐coating and the materials were exposed immediately after preparation to an intense source of light of several keV generated by a synchrotron source. The samples were exposed to increasing doses and the effects of the radiation have been evaluated by Fourier transform infrared spectroscopy, spectroscopic ellipsometry and atomic force microscopy. The X‐ray beam induces a significant densification on the silica films without producing any degradation such as cracks, flaws or delamination at the interface. The densification is accompanied by a decrease in thickness and an increase in refractive index both in the pure silica and in the hybrid films. The effect on the hybrid material is to induce densification through reaction of silanol groups but also removal of the organic groups, which are covalently bonded to silicon via Si—C bonds. At the highest exposure dose the removal of the organic groups is complete and the film becomes pure silica. Hard X‐rays can be used as an efficient and direct writing tool to pattern coating layers of different types of compositions.     

Biogenic amine determination in wines using solid-phase extraction: A comparative study

Biogenic amines in wine usually are analyzed by high-performance liquid chromatography after direct derivatization. A method of isolation based on solid-phase extraction (SPE) with mixed-mode resins (Oasis MCX, reverse-phase and ion exchange) was developed. The different stages of the isolation process (loading, elution and washing) were optimized to obtain a simple procedure that yields a clean chromatogram. The relative standard deviation (%RSD) of the retention times and relative areas was less than 0.3% and 6%, respectively. Limits of quantification were lower than 0.16 mg L⁻¹ for all the amines and the linear range of concentration was 0.16–8 mg L⁻¹ for putrescine, cadaverine and tyramine, and up to 10 mg L⁻¹ for histamine.     

Determination of styrene in olive oil by an automatic purge-and-trap system coupled to gas chromatography-mass spectrometry

Summary A new gas chromatographic method using an automatic purge-and-trap system coupled to a GC with mass selective detection to analyze styrene at the parts-per-trillions (ng kg⁻¹) level is described. The method shows a good sensitivity and the detection limit is 10 ng kg⁻¹ with a relative standard deviation (RSD) of 4.7 % for 164 ng kg⁻¹ styrene in olive oil. This analytical method has been successfully applied to the analysis of styrene in extra-virgin olive oil from the European market.     

Spin filtering of free electrons by magnetic multilayers: towards an efficient self-calibrated spin polarimeter

An asymmetrical ferromagnetic cobalt bilayer (18 nm Au/0.8 nm Co/2.2 nm Au/1.3 nm Co/1.5 nm Au) operates as a self-calibrated spin polarimeter with a high spin selectivity for free electrons injected at a few eV above the Fermi level. We present the analysis of transmitted currents as a function of the incident energy, based on a model of spin polarization dilution into the first gold layer and ballistic transport close to the vacuum level throughout the sample.     

Finite element error bounds for a curve shrinking with prescribed normal contact to a fixed boundary

We approximate the evolution of a curve subject to motion bycurvature by linear finite elements. The curve evolves insidea given domain and meets orthogonally. We derive optimal boundsfor the error with respect to the L²- and H¹-norms and presentsome computed examples.     

Simple strategy for the optimization of solid-phase extraction procedures through the use of solid-liquid distribution coefficients application to the determination of aliphatic lactones in wine

A practical strategy for the optimization of solid-phase extraction (SPE) systems is presented. Critical SPE volumes (sample loaded, rinsing and elution solvent) are calculated from solid–liquid extraction coefficients and from basic bed parameters determined in simple experiments, using the Lövkist Jonsson model and other expressions derived from the general theory of chromatography. The agreement between calculated and measured volumes is satisfactory, which makes it possible to consider different sorbents and rinsing and elution solvents in the SPE optimization with a relatively low experimental effort. The strategy has been successfully applied to the optimization of a SPE method directed to the selective extraction of aliphatic lactones from wine. Six different reversed-phase sorbents were studied and the one showing maximum extraction selectivity was selected. Wine (50 ml) is extracted in a 200 mg cartridge filled with Bond Elut-ENV resins. Interferences are removed with 20 ml of methanol–water (40:60) with 1% NaHCO₃. Elution is carried out with 1.8 ml of dichloromethane. The extract is concentrated to 0.15 ml and analyzed by GC–ion trap MS. Eight odor-active aliphatic γ and δ lactones (with 8–12 C atoms) from wine are recovered (R>75%) in an extract free from wine major volatiles. Detection limits are in the 40–300 ng/l range, well below the odor detection threshold of these compounds. Linearity (r²>0.996) and precision (average R.S.D. 3.5%) are satisfactory in all cases. The levels in wine of some of these lactones (γ-octa, undeca and dodecalactones) are reported by first time and results demonstrate that can be present at concentrations above or close to their corresponding odor thresholds.