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991.
A fast method for the measurement of metabolites of pyrene in urine was improved by HPLC with fluorescence detector using “heart‐cut” technique. This method can quantify the total amount of pyrene metabolites corresponding to glucuronic acid and sulfate conjugates as well as free 1‐OH‐Py. The hydrolyzed biological fluid was directly injected into the chromatographic system, via a column‐switching system. Pre‐treatment and analysis were performed within 0.5 and 9 min, respectively. Enzymatic hydrolysis has been optimized to not exceed 2 h. The best response function, in the 0.2–10 ng/mL range, is the linear regression, bringing simplicity, good accuracy, and low LOQ. The intra‐ and interday precision values were inferior to 1 and 2%, respectively. The proposed method provided a simple, convenient, and practical procedure to determine the level of the main urinary pyrene metabolites in biological samples. 相似文献
992.
José Luis Pérez Pavón Miguel del Nogal Sánchez María Esther Fernández Laespada Bernardo Moreno Cordero 《Analytical and bioanalytical chemistry》2009,394(5):1463-1470
A sensitive method for the fast analysis of filbertone in spiked olive oil samples is presented. The applicability of a headspace
(HS) autosampler in combination with a gas chromatograph (GC) equipped with a programmable temperature vaporizer (PTV) and
a mass spectrometric (MS) detector is explored. A modular accelerated column heater (MACHTM) was used to control the temperature of the capillary gas chromatography column. This module can be heated and cooled very
rapidly, shortening total analysis cycle times to a considerable extent. The proposed method does not require any previous
analyte extraction, filtration and preconcentration step, as in most methods described to date. Sample preparation is reduced
to placing the olive oil sample in the vial. This reduces the analysis time and the experimental errors associated with this
step of the analytical process. By using headspace generation, the volatiles of the sample are analysed without interference
by the non-volatile matrix, and by using injection in solvent-vent mode at the PTV inlet, most of the compounds that are more
volatile than filbertone are purged and the matrix effect is minimised. Use of a liner packed with Tenax-TA? allowed the compound
of interest to be retained during the venting process. The limits of detection and quantification were as low as 0.27 and
0.83 μg/L, respectively, and precision (measured as the relative standard deviation) was 5.7%. The method was applied to the
determination of filbertone in spiked olive oil samples and the results revealed the good accuracy obtained with the method. 相似文献
993.
Miguel Guerrero Josefina Pons Mercè Font-Bardia Teresa Calvet Josep Ros 《Journal of chemical crystallography》2011,41(5):721-726
Abstract
The crystal structure of the title compound C22H30N4O2·H2O (L), has been determined using X-ray diffraction at 293 K. The crystal of 1,2-bis[4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-oxobutyl]benzene is in triclinic crystal system with space group P(−1) (Z = 2), lattice parameters a = 8.225(6) ?, b = 10.967(6) ?, c = 12.903(6) ?, V = 1119.1(11) ?3. Analyses of single crystals of L, crystallized from dichloromethane/diethyl ether (1:1), revealed that the molecules are arranged in couples, which adopt a pseudo chair conformation, by means of intermolecular O–H···N hydrogen bonding interactions. Moreover, the extended structure revealed a 1D chain caused by several C–H···N intermolecular interactions. 相似文献994.
Yigui Ou Author Vitae Qian Zhou Haichan Lin 《Journal of Computational and Applied Mathematics》2009,232(2):318-326
In this paper, a new trust region algorithm is proposed for solving unconstrained optimization problems. This method can be regarded as a combination of trust region technique, fixed step-length and ODE-based methods. A feature of this proposed method is that at each iteration, only a system of linear equations is solved to obtain a trial step. Another is that when a trial step is not accepted, the method generates an iterative point whose step-length is defined by a formula. Under some standard assumptions, it is proven that the algorithm is globally convergent and locally superlinear convergent. Preliminary numerical results are reported. 相似文献
995.
Miguel Ramos 《Journal of Mathematical Analysis and Applications》2009,352(1):246-258
We study the existence, multiplicity and shape of positive solutions of the system −ε2Δu+V(x)u=K(x)g(v), −ε2Δv+V(x)v=H(x)f(u) in RN, as ε→0. The functions f and g are power-like nonlinearities with superlinear and subcritical growth at infinity, and V, H, K are positive and locally Hölder continuous. 相似文献
996.
Given a set-valued optimization problem (P), there is more than one way of defining the solutions associated with it. Depending on the decision maker’s preference, we consider the vector criterion or the set criterion. Both criteria of solution are considered together to solve problem (P) by reducing the feasible set. 相似文献
997.
The likelihood of vector GARCH models is ill-conditioned because of two facts. First, when the series display high correlations,
as often happens with financial data, some eigenvalues of the conditional covariance matrix are close to zero. Second, the
likelihood function is very flat in the neighborhood of the optimum due to the functional form of the GARCH process. These
facts explain the instability of multivariate GARCH estimation procedures. Building on this analysis, we suggest a data transformation
which moves the critical eigenvalues far from zero and, therefore, improves the stability of iterative optimization methods.
The transformed values are re-scaled principal components, so their interpretation is straightforward. The application of
this technique is illustrated by modeling the short-run conditional correlations of four nominal exchange rates.
相似文献
998.
999.
Fast 2D NMR-based screening can be achieved using Hadamard encoded spectroscopy to focus on the signals of interest (e.g., enzyme active or ligand recognition sites). By recording a set of Hadamard spectra (a "Hadamard constellation") with relative offsets comparable to the excitation bandwidth, quantitative ligand-induced shifts can be obtained from peak intensities. 相似文献
1000.
Baya M Maresca O Poli R Coppel Y Maseras F Lledós A Belkova NV Dub PA Epstein LM Shubina ES 《Inorganic chemistry》2006,45(25):10248-10262
The isomerization of complex [Cp*Fe(dppe)(eta2-H2)]+, generated in situ by low-temperature protonation of Cp*Fe(dppe)H with either HBF4 or CF3COOH, to the dihydride tautomer trans-[Cp*Fe(dppe)(H)2]+ is irreversible and follows first-order kinetics in the -10 to +15 degrees C range with Delta H double dagger = 21.6 +/- 0.8 kcal mol(-1) and DeltaS double dagger = 5 +/- 3 eu. The isomerization rate constant is essentially independent of the nature and quantity of a strong acid. Density functional theory (DFT) calculations on various models, including the complete system at both the quantum mechanics/molecular mechanics (QM/MM) and full QM levels, probe the relative importance of steric and electronic effects for the relative stability of the nonclassical and classical isomers and identify two likely isomerization mechanisms: a "direct" pathway involving simultaneous H-H bond breaking and cis-trans isomerization and a "via Cp" pathway involving agostic C5Me5H intermediates. Both pathways are characterized by activation energies in close correspondence with the experimental value (21.3 and 22.2 kcal mol(-1), respectively). Further kinetic studies were carried out for the Cp*Fe(dppe)H + CF3COOD and Cp*Fe(dppe)D + CF3COOD systems at 273 K. The [Cp*Fe(dppe)(eta2-HD)]+ complex establishes a very rapid isotope redistribution equilibrium with the eta2-H2 and eta2-D2 analogues. The equilibrium constant value (K = 3.3 +/- 0.3) indicates a significant equilibrium isotope effect. Simulation of the rate data provides access to the individual isomerization rate constants kHH, kHD, and kDD for the three isotopomers, yielding kinetic isotope effects: kHH/kHD = 1.24 +/- 0.01 and kHD/kDD = 1.58 +/- 0.01 (and, consequently, kHH/kDD = 1.96 +/- 0.02). The analysis of the DFT-calculated frequencies, using the [Cp*Fe(dhpe)H2]+ model system, for the [Cp*Fe(dhpe)(eta2-XY)]+ isotopomers as well as transition states for the "direct" (TSdir) and "via Cp" (TSrot) pathways (X = H, D) allowed the computation of the expected isotope effects. A comparison with the experiment strongly suggests that the mechanism occurs via the "direct" pathway for the present system, although the small difference in the calculated energy barriers suggests that the "via Cp" pathway may be preferred in other cases. 相似文献