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141.
The absorbance signal in electrothermal atomic absorption spectrometry is a result of the processes of atom formation and dissipation and first of all depends on the initial generation of atoms from the surface. The rising part of the absorbance time profiles can be used for characterization of the release of the metals like Pd, Ag, Au etc. The method is applied to investigate the palladium release from a pyrocoated graphite support at different initial masses of the Pd. The kinetic parameters are evaluated and conclusions about the distribution of palladium on the graphite surface are made. The estimated values of apparent activation energies depend on the initial mass of analyte. The advantages and limitations of the present approach are discussed. 相似文献
142.
A method for evaluating Ea(loss), the apparent activation energy of the analyte loss process during the high-temperature thermal pretreatment stage in electrothermal atomization atomic absorption spectrometry (ETA-AAS) has been developed. The method is based on extracting information from the declining portion of thermal pretreatment curves (in this case presented as absorbance vs pyrolysis time at various fixed temperatures). Five volatile analytes (As, Pb, Sb, Se, and Sn) have been studied in the presence of tungsten chemical modifier (20 μg of W in H2O2). The data on Ea (loss) have been utilized together with those obtained from treating the atomization peaks Ea and with literature data on known values of bond energies and enthalpies of certain chemical reactions. Possible mechanisms of analyte losses and atomization are discussed. 相似文献
143.
F. Flores-Bazán C. Vera PhD Student 《Journal of Optimization Theory and Applications》2006,130(2):185-207
As a consequence of an abstract theorem proved elsewhere, a vector Weierstrass theorem for the existence of a weakly efficient solution without any convexity assumption is established. By using the notion (recently introduced in an earlier paper) of semistrict quasiconvexity for vector functions and assuming additional structure on the space, new existence results encompassing many results appearing in the literature are derived. Also, when the cone defining the preference relation satisfies some mild assumptions (but including the polyhedral and icecream cones), various characterizations for the nonemptiness and compactness of the weakly efficient solution set to convex vector optimization problems are given. Similar results for a class of nonconvex problems on the real line are established as well.Research supported in part by Conicyt-Chile through FONDECYT 104-0610 and FONDAP-Matemáticas Aplicadas II. 相似文献
144.
145.
Gerald G. Miller Kevin Brown Ronald B. Moore Malcolm S. McPhee Zhenjun J. Diwu Jixiang Liu Liren Huang J. William Lown David A. Begg Vera Chlumecky John Tulip 《Photochemistry and photobiology》1995,61(6):632-638
Hypocrellins are naturally occurring compounds with photosensitizing properties in biological systems. We have prepared synthetic derivatives of hypocrellin B, which have promise as photosensitizers in the clinical application of photodynamic therapy. The intracellular localization and uptake kinetics of hypocrellin B and several selected hypocrellin congeners were determined semiquantitatively by fluorescence confocal microscopy in monolayer cultures of EMT6/Ed murine tumor cells. Each compound had unique uptake kinetics. Although no compound tested to date has demonstrated nuclear labeling, most could be detected in lysosomes, Golgi, endoplasmic reticulum and, to a minor extent, in cellular membranes. No two compounds gave identical labeling distributions. The differences are assumed to originate in physicochemical properties characteristic of each compound, which may ultimately impact upon the primary modality of phototoxicity. 相似文献
146.
Rolf Zeisler Robert Dekner Vladimir Strachnov Hernan Vera Ruiz 《Fresenius' Journal of Analytical Chemistry》1995,352(1-2):14-18
For more than thirty years the International Atomic Energy Agency has been assisting laboratories in Member States in maintaining and improving the reliability, i.e. the quality of analyses of nuclear, industrial, environmental and biological materials, and materials of marine origin. Through the Analytical Quality Control Services (AQCS) the Agency initiates and supports improvements in the accuracy of analytical chemical and radiometric measurements and their traceability to basic standards. This is achieved by organizing worldwide and regional intercomparison exercises and by distributing reference materials. The latest biological and environmental reference and intercomparison materials are presented and general information is given on the preparation of materials, intercomparison runs, and the evaluation of reference data. Means to improve the quality of intercomparison results are presented and discussed. 相似文献
147.
For a sequence of partial sums ofd-dimensional independent identically distributed random vectors a corresponding multivariate renewal process is defined componentwise. Via strong invariance together with an extreme value limit theorem for Rayleigh processes, a number of weak asymptotic results are established for thed-dimensional renewal process. Similar theorems for the estimated version of this process are also derived. These results are suggested to serve as simultaneous asymptotic testing devices for detecting changes in the multivariate setting. 相似文献
148.
Christiane Albrecht Henning Böckemeier Mark Dennis Matthew D. Fletcher Vera Leitmann Richard Rowles Richard A. Stephenson Michael B. Hursthouse 《Tetrahedron letters》2008,49(1):185-188
The cyclisation of N-allyl- and N-homoallylguanidines using DMDO or I2/K2CO2 leading to novel heterocycles is reported. 相似文献
149.
Montserrat Vera Lourdes Franco Jordi Puiggalí 《Journal of polymer science. Part A, Polymer chemistry》2008,46(2):661-667
Poly(ester amide)s with a regular sequence and derived from diamine, dicarboxylic acid, and glycolic acid units can be easily synthesized by using a thermal polycondensation method based on the formation of metal halide salts as a driving force. These new poly(ester amide)s have tuneable properties depending on the number of methylene groups and constitute a group of materials with potential interest as biodegradable materials in applications such as surgical sutures. The present work reveals that the proposed synthesis can also be applied when esters of L ‐lysine are used as a diamine unit since no secondary reactions such as transesterification occur under the required polymerization conditions. The possibility of linking compounds with pharmacological activity to the carboxylic acid groups of lysine extends the interest of the referred materials, for example, as drug delivery systems. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 661–667, 2008 相似文献
150.
Juan I. Morán Vera A. Alvarez Viviana P. Cyras Analia Vázquez 《Cellulose (London, England)》2008,15(1):149-159
In this work a study on the feasibility of extracting cellulose from sisal fiber, by means of two different procedures was
carried out. These processes included usual chemical procedures such as acid hydrolysis, chlorination, alkaline extraction,
and bleaching. The final products were characterized by means of Thermogravimetric Analysis (TGA), Infrared Spectroscopy (FTIR),
X-Ray Diffraction (XRD), Differential Scanning Calorimetry (DSC) and Scanning Electronic Microscopy (SEM). The extraction
procedures that were used led to purified cellulose. Advantages and disadvantages of both procedures were also analyzed. Finally,
nanocellulose was produced by the acid hydrolysis of obtained cellulose and characterized by Atomic Force Microscopy (AFM). 相似文献