Codimension two submanifolds with 2-nonnegative curvature operator

In this paper we obtain a classification of compact n-submanifolds of the Euclidean space with 2-nonnegative curvature operator. Received: 4 April 2007     

Identifying quadric bundle structures on complex projective varieties

In this paper we characterize smooth complex projective varieties that admit a quadric bundle structure on some dense open subset in terms of the geometry of certain families of rational curves.      

Structural,magnetic and transport properties of ion beam deposited Co thin films

Magnetic nanostructures display new and interesting physical phenomena and are currently used in a large variety of applications. We studied the structural, magnetic and transport properties of Co thin films deposited by ion beam sputtering. We probed the influence of the buffer layer material (Al, Cu, Ru or Ta) and thickness (10–100 Å) on the structural properties of Co thin films. Using X-ray diffraction we observed that textured fcc Co films can be grown on amorphous Ta as thin as 20 Å but for the other buffer layers no texture is observed. We also studied by magneto-optical Kerr effect (MOKE) the magnetic properties of the Co thin films as a function of Co thickness (100–1000 Å). Finally, the electrical resistivity and anisotropic magnetoresistance (AMR) of our Co thin films (on a Ta buffer) was obtained as a function of Co thickness.     

Study of the Ternary Complex Formation between Vanadium(III)-Cysteine and Small Blood Serum Bioligands

The complex species formed between vanadium(III) cysteine and small blood serum bioligands: lactic, oxalic, citric and phosphoric acids were studied in aqueous solution by means of electromotive force measurements, Emf(H), at 25 °C and 3.0 mol⋅dm⁻³ KCl as the ionic medium. The ternary complexes were studied by potentiometric measurements and the data analyzed using the least-squares computational program LETAGROP to obtain the respective stability constants and stoichiometric coefficients. The UV-Vis spectrophotometric measurements were done in order to make a qualitative characterization of the complexes formed in aqueous solution.     

Determination of copper in powdered chocolate samples by slurry-sampling flame atomic-absorption spectrometry

Chocolate is a complex sample with a high content of organic compounds and its analysis generally involves digestion procedures that might include the risk of losses and/or contamination. The determination of copper in chocolate is important because copper compounds are extensively used as fungicides in the farming of cocoa. In this paper, a slurry-sampling flame atomic-absorption spectrometric method is proposed for determination of copper in powdered chocolate samples. Optimization was carried out using univariate methodology involving the variables nature and concentration of the acid solution for slurry preparation, sonication time, and sample mass. The recommended conditions include a sample mass of 0.2 g, 2.0 mol L^–1 hydrochloric acid solution, and a sonication time of 15 min. The calibration curve was prepared using aqueous copper standards in 2.0 mol L^–1 hydrochloric acid. This method allowed determination of copper in chocolate with a detection limit of 0.4 g g^–1 and precision, expressed as relative standard deviation (RSD), of 2.5% (n=10) for a copper content of approximately 30 g g^–1, using a chocolate mass of 0.2 g. The accuracy was confirmed by analyzing the certified reference materials NIST SRM 1568a rice flour and NIES CRM 10-b rice flour. The proposed method was used for determination of copper in three powdered chocolate samples, the copper content of which varied between 26.6 and 31.5 g g^–1. The results showed no significant differences with those obtained after complete digestion, using a t-test for comparison.     

Finite element solution of flow between eccentric cylinders with viscous dissipation

A computational study of viscous flow between two eccentrically rotating cylinders is presented in which the effect of viscous dissipation is taken into account. The space discretization is based on piecewise linear finite elements with velocity stabilization, while the method of characteristics is used for time integration. Numerical results illustrate the efficiency of the adopted approach.     

Kähler-Einstein metrics for some quasi-smooth log del Pezzo surfaces

Recently Johnson and Kollár determined the complete list of anticanonically embedded quasi-smooth log del Pezzo surfaces in weighted projective -spaces. They also proved that many of those surfaces admit a Kähler-Einstein metric, and that some of them do not have tigers.

The aim of this paper is to settle the question of the existence of Kähler-Einstein metrics and tigers for those surfaces for which the question was left open. In order to do so, we will use techniques developed earlier by Nadel, Demailly and Kollár.

     

Isometric Shifts and Metric Spaces

 Let M be a complete metric space. If admits an isometric shift, then M is separable. (Received 2 February 2001; in revised form 9 April 2001)     

A new indirect method based on square-wave voltammetry for ceftiofur determination in bovine milk using an alkaline degradation product

In this paper we present a new electroanalytical method for determination of ceftiofur based on the hydrolysis of this antibiotic in 0.04 mol L⁻¹ Britton–Robinson buffer at pH 10 and 60 °C for 60 min (reduction peak at − 0.70 V). Conditions were optimized for complete hydrolysis and quantitative determination of ceftiofur in milk. The method can be successfully used for determination of the antibiotic directly from samples of fluid milk and powder milk spiked to concentrations of 6.0 × 10⁻⁸, 8.0 × 10⁻⁸, and 10 × 10⁻⁸ mol L⁻¹, respectively. Recovery test ranged from 98.28% to 100.83%. The limits of detection and quantification were 3.73 × 10⁻¹⁰ and 1.24 × 10⁻⁹ mol L⁻¹, respectively. The method has the advantage of eliminating interference from proteins present in the sample, thus obviating the need for exhaustive extraction, which often renders other procedures unfeasible in terms of time and reagent cost. In addition, waste generation was found to be lower than in other methods.