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71.
Tatiana Visnevschi-Necrasov Sara C. Cunha Eugénia Nunes M. Beatriz P.P. Oliveira 《Journal of chromatography. A》2009,1216(18):3720-3724
A method based on matrix solid-phase dispersion (MSPD) has been developed for the determination of 12 isoflavones in Trifolium pratense L. Dried leaf samples were blended with C18, placed in small columns and isoflavones extracted with dichloromethane–methanol. Analyses were performed by high performance liquid chromatography with diode array detection (HPLC-DAD) with 2-methoxyflavone as internal standard. Several dispersants, eluents and clean-up steps were tested during the optimization of the process in order to obtain the best selectivity and yields. Mean recoveries ranged from 70% to 119%, with relative standard deviations <18%. The limits of detection were between 0.006 mg/l for biochanin A and 0.108 mg/l for daidzin. The performance of the optimized method in real samples was compared with a conventional method based in solid–liquid extraction (SLE). 相似文献
72.
Fresco-Cala Beatriz Cárdenas Soledad Herrero-Martínez José Manuel 《Mikrochimica acta》2017,184(6):1863-1871
Microchimica Acta - A copolymer was prepared from glycidyl methacrylate, ethylene glycol dimethacrylate and oxidized single-walled carbon nanohorns via photo-polymerization and used in spin columns... 相似文献
73.
Ashish Kumar Yahya E. Jad Ayman El-Faham Beatriz G. de la Torre Fernando Albericio 《Tetrahedron letters》2017,58(30):2986-2988
Herein, we report the use of γ-valerolactone (GVL) and N-formylmorpholine (NFM) as DMF substitutes in polystyrene based SPPS. The solubility of selected amino acids and coupling reagents were studied in GVL and NFM, followed by their use in the successful synthesis of Aib-enkephalin pentapeptide (H-Tyr-Aib-Aib-Phe-Leu-NH2) and Aib-ACP decapeptide (H-Val-Gln-Aib-Aib-Ile-Asp-Tyr-Ile-Asn-Gly-NH2). 相似文献
74.
Vapor–liquid equilibria (VLE) data at 100 kPa have been determinated for the ternary system cyclohexane + cyclohexene + morpholine and two constituent binary systems cyclohexane + morpholine and cyclohexene + morpholine. The thermodynamic consistency of experimental data has been verified. Both binary systems deviate moderately from ideality without the presence of an azeotrope. The VLE data have been well correlated using local composition models (Wilson, NRTL and UNIQUAC) and have been also predicted with the original UNIFAC. 相似文献
75.
Éder José dos Santos Amanda Beatriz Herrmann Charyane Satie Sato Tatiane Andrade Maranhão Adilson José Curtius 《Microchemical Journal》2010,96(1):27-216
Mercury and lead were determined in an extraction solution applied to compact fluorescent lamps by axial view inductively coupled plasma optical emission spectrometry (ICP OES). The solution contained 50% v/v HNO3 and 10% v/v H2O2. The compact fluorescent lamps were treated by removing the socket and breaking the rest of the lamp inside the solution, followed by placing the mixture in an ultrasonic bath for 1 h. The mixture was filtrated in a coarse paper filter and the resulting extraction slurry was analyzed. The determination of Pb in the slurry required calibration by the analyte addition technique, while for Hg, external calibration was adequate. The quantification limits (10 s, n = 7), for Hg (194.164 nm) and Pb (220.353 nm) were, respectively, 10 and 30 µg per lamp. Analysis of spiked samples demonstrated reasonable accuracy of the method, with recoveries in the range from 99 to 120% for Hg and from 91 to 107% for Pb. Fifteen analyzed samples showed Hg masses per lamp in the range from 1.6 to 27 mg, and six samples were above the limit allowed by the European Community that is 5 mg per compact lamp. The values for Pb were between 0.07 and 0.75 mg per lamp. By filtrating the slurry in a membrane filter, and analyzing the resulting solution, it was found that all Hg was extracted to the liquid phase of the slurry, while a fraction of about 40% m/v of Pb is retained in the solid particles. The lamp cover glass, after extraction of the phosphor layer, was also analyzed for Hg and Pb. The concentration of Hg in the glass is quite low; however, the Pb content is high. 相似文献
76.
Hesham A. Habib Beatriz Gil-Hernández Khalid Abu-Shandi Joaquín Sanchiz Christoph Janiak 《Polyhedron》2010
Hydrothermal synthesis with MCl2 (M = Fe, Cu, and Zn) and disodium 5-ammonium-1-hydroxypentylidene-1,1-bisphosphonate, (Na+)2[+H3N(CH2)4C(OH)(PO32−)(PO3H−)] (Na2HAC5OHP2) or sodium 3-ammonium-1-hydroxypropylidene-1,1-bisphosphonate hydrate, Na+[+H3N(CH2)2C(OH)(PO3H–)(PO3H–)]·H2O (NaH2PAM·H2O) the sodium salt of pamidronic acid, H3PAM) yielded the one-dimensional (1D) iron, molecular copper and two-dimensional (2D) zinc compounds 1D-{[Fe(μ3-η5-HAC5OHP2)]·H2O}, 1, [Cu(η2-H2AC5OHP2)2], 2, 2D-{[Zn2(μ5-η7-AC5OHP2)Cl], 3, and 2D-{[Zn(μ2-η3-H2PAM)2], 4, respectively. The bisphosphonate ligand bridges (μn) between 2–5 metal atoms and uses 2–7 oxygen donor atoms towards metal coordination (ηn). The zwitterionic nature of the now bis- or tetrakis-deprotonated ammonium–bisphosphonate is retained in the metal complexes. From the reaction of NiCl2 and Na2HAC5OHP2 the zwitterionic 5-ammonium-1-hydroxypentylidene-1-phosphonic acid, +H3N(CH2)4CH(OH)PO3H−, 5 was obtained as a product of the ligand P–C bond hydrolysis. Adjacent strands, molecules or layers in 1–4, respectively are organized through the Coulomb attraction between the positive ammonium group and the negative phosphonate groups, supported by hydrogen-bonding. Each protic H atom on the C–OH, NH3+ and –PO3H− group is involved in charge-assisted hydrogen-bonding. The ammonium-pentylidene groups act as hydrophobic separators between the hydrophilic units with the polar M{C(OH)(PO3)2} and {NH3} units. Bond valence sum calculations support the Fe(II) oxidation state in 1, which was experimentally determined from a quantitative polarographic Fe(II)/Fe(III) speciation analysis as well as a temperature variable magnetic study. 相似文献
77.
Beatriz Jurado-Sánchez Evaristo Ballesteros Mercedes Gallego 《Analytical and bioanalytical chemistry》2010,396(5):1929-1937
Humans are exposed to endogenous or exogenous formation of aromatic amines (AAs) and N-nitrosamines (NAms), which are considered to be potent carcinogens. The objective of this study was to monitor AAs and NAms
in human urine to obtain a way to assess exposure. However, while NAms can be directly detected in urine samples, AAs require
hydrolysis to convert their conjugates into free amines. A semiautomatic flow-base method is proposed for the simultaneous
determination of aliphatic and aromatic NAms, anilines and chloroanilines in human urine in one analytical run. Conjugated
AAs are released from urine by on-line microwave-assisted acid hydrolysis without degradation of NAms; all amines were then
preconcentrated using solid-phase extraction. Separation/determinations are carried out by gas chromatography and mass spectrometry
operating in selected ion monitoring mode. The method is fast (∼15 min for 25 mL of sample) and provides low limits of detection
(from 2 to 26 ng/L) with good precision (relative standard deviation within and between days less than 7%). Finally, the proposed
method was successfully applied to check AAs and NAms in the human urine of exposed and unexposed researchers. The kinetics
of amine excretion in the urine of the researchers exposed is calculated after termination of the exposure and shows half-life
times between 1.3 and 2.1 h, and that the dosage absorbed was eliminated within 6 h after exposure. 相似文献
78.
The carbamoylating agents carbamoyl azides and carbamoyl cyanides (aka cyanoformamides) react with hydroxylamine in different ways, leading in the first case to N-hydroxyureas and, in the case of carbamoyl cyanides, to carbamoyl amidoxime derivatives. The synthetic procedure developed for the latter type of compound, which represents an interesting precursor for heterocyclic structures, allowed the highly efficient preparation of a wide selection of examples. The Z configuration of the double bond in the amidoxime moiety was proposed on the basis of comparison between experimental and calculated (13)C and (15)N NMR chemical shift values for the isopropyl and benzyl derivatives. 相似文献
79.
Devika Kannan Ana Beatriz Lopes de Sousa Jabbour Charbel José Chiappetta Jabbour 《European Journal of Operational Research》2014
Due to an increased awareness and significant environmental pressures from various stakeholders, companies have begun to realize the significance of incorporating green practices into their daily activities. This paper proposes a framework using Fuzzy TOPSIS to select green suppliers for a Brazilian electronics company; our framework is built on the criteria of green supply chain management (GSCM) practices. An empirical analysis is made, and the data are collected from a set of 12 available suppliers. We use a fuzzy TOPSIS approach to rank the suppliers, and the results of the proposed framework are compared with the ranks obtained by both the geometric mean and the graded mean methods of fuzzy TOPSIS methodology. Then a Spearman rank correlation coefficient is used to find the statistical difference between the ranks obtained by the three methods. Finally, a sensitivity analysis has been performed to examine the influence of the preferences given by the decision makers for the chosen GSCM practices on the selection of green suppliers. Results indicate that the four dominant criteria are Commitment of senior management to GSCM; Product designs that reduce, reuse, recycle, or reclaim materials, components, or energy; Compliance with legal environmental requirements and auditing programs; and Product designs that avoid or reduce toxic or hazardous material use. 相似文献
80.
A useful application for copula functions is modeling the dynamics in the conditional moments of a time series. Using copulas, one can go beyond the traditional linear ARMA (p,q) modeling, which is solely based on the behavior of the autocorrelation function, and capture the entire dependence structure linking consecutive observations. This type of serial dependence is best represented by a canonical vine decomposition, and we illustrate this idea in the context of emerging stock markets, modeling linear and nonlinear temporal dependences of Brazilian series of realized volatilities. However, the analysis of intraday data collected from e‐markets poses some specific challenges. The large amount of real‐time information calls for heavy data manipulation, which may result in gross errors. Atypical points in high‐frequency intraday transaction prices may contaminate the series of daily realized volatilities, thus affecting classical statistical inference and leading to poor predictions. Therefore, in this paper, we propose to robustly estimate pair‐copula models using the weighted minimum distance and the weighted maximum likelihood estimates (WMLE). The excellent performance of these robust estimates for pair‐copula models are assessed through a comprehensive set of simulations, from which the WMLE emerged as the best option for members of the elliptical copula family. We evaluate and compare alternative volatility forecasts and show that the robustly estimated canonical vine‐based forecasts outperform the competitors. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献