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排序方式: 共有141条查询结果,搜索用时 140 毫秒
41.
42.
Cavalcanti Gde A Garrido BC Leal FD Padilha MC de la Torre X Pereira HM de Aquino Neto FR 《Rapid communications in mass spectrometry : RCM》2010,24(22):3297-3302
Trimethylsilylation of anabolic agents and their metabolites is frequently achieved by using the derivatization mixture N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA)/NH(4)I/2-mercaptoethanol. Nevertheless, artifacts were formed when this mixture was employed in the monitoring of exemestane and its main metabolite 17β-hydroxyexemestane prior to GC-MS analysis. These artifacts were identified as the N-methyltrifluoroacetamide (MTFA) and trimethylsiloxyethylmercapto products of the respective trimethylsilyl (TMS) derivatives. Furthermore, artifact formation was evaluated taking the structure (1,4-diene-3-keto-6-exomethylene) of the compounds into account. Although these artifacts are relevant for investigations regarding the derivatization process and may be of interest in many fields, they are detrimental to cope with the requirements of the World Anti-Doping Agency (WADA) in terms of the limits of detection (LODs) required. To overcome this issue, a method using an alternative derivatization was proposed: formation of methyloxime-TMS derivatives through double derivatization using O-methylhydroxylamine/pyridine and MSTFA/TMS imidazole after enzymatic hydrolysis and liquid-liquid extraction. Samples from an excretion study after administration of exemestane to healthy volunteers were analyzed by the proposed method and detection of both exemestane and its main metabolite was possible. This method showed excellent results for both analytes meeting the LODs required for antiestrogenic agents (50 ng/mL) established by WADA. The method was validated for the main metabolite, it was robust and cost-effective for qualitative and quantitative purposes, with LOD and LOQ of 10 ng/mL and 25 ng/mL, respectively. 相似文献
43.
A. F. F. Vasconcelos M. B. Dantas M. G. R. Filho R. Rosenhaim E. H. S. Cavalcanti N. R. Antoniosi Filho F. S. M. Sinfrônio I. M. G. Santos A. G. Souza 《Journal of Thermal Analysis and Calorimetry》2009,97(2):657-660
The influence of drying processes in the biodiesel oxidation was investigated by means of the oxidative induction time obtained
from differential scanning calorimetry data. For this purpose, corn biodiesel was dried by different methods including: chemical
(anhydrous sodium sulfate) and thermal (induction heating, heating under vacuum and with microwave irradiation). The drying
efficiency was evaluated by monitoring IR absorption in the 3,500–3,200 cm−1 range and by the AOCS Bc 2-49 method. In general, the oxidative induction times increased inversely to the heating degree,
except that of microwave irradiation, which was selective to water evaporation and caused low impact over the unsaturation
of biodiesel. The DSC technique was shown to be a powerful tool to evaluate with high level of differentiation the influence
of the drying process on the oxidative stability of biodiesel. 相似文献
44.
M.M. Cavalcanti V.N. Domingos Cavalcanti R. Fukuoka J.A. Soriano 《Journal of Mathematical Analysis and Applications》2009,351(2):661-225
Let (M,g) be an n-dimensional (n?2) compact Riemannian manifold with or without boundary where g denotes a Riemannian metric of class C∞. This paper is concerned with the study of the wave equation on (M,g) with locally distributed damping, described by
45.
Liliana Fátima Bezerra de Lira Jose Geraldo Andrade Pacheco Eduardo Homem de Siqueira Cavalcanti Maria Fernanda Pimentel 《Microchemical Journal》2010,96(1):126-34
This paper evaluates analytical methods based on near infrared (NIR) and middle infrared (MIR) spectroscopy and multivariate calibration to monitor the stability of biodiesel. There was a focus on three parameters: oxidative stability index, acid number and water content. Ethylic and methylic biodiesel from different feedstocks were used in experiments of accelerated aging, in order to take into account the wide variety of oilseeds and feedstocks available in Brazil. Partial least squares (PLS) and multiple linear regression (MLR) models were developed. Different pre-processing techniques and spectral variable/regions selection algorithms were evaluated. For MLR models, the successive projection algorithm (SPA) was employed. Interval PLS (iPLS) and selection of variables taking into account the significant regression coefficients were used for PLS models. Results showed that both near and middle infrared regions, and all variable selection methods tested were efficient for predicting these three important quality parameters of B100, the root mean squares error of prediction (RMSEP) values being comparable to the reproducibility of the corresponding standard method for each property investigated. 相似文献
46.
Daniela de Araújo Viana Carolina de Albuquerque Lima Rejane Pereira Neves Cristina Souza Mota Keila Aparecida Moreira José Luiz de Lima-Filho Maria Taciana Holanda Cavalcanti Attilio Converti Ana Lúcia Figueiredo Porto 《Applied biochemistry and biotechnology》2010,162(3):830-842
Cow raw milk from dairy cooperatives was examined for its microbial composition. Among the isolates identified, 17.6% were
yeasts. The most frequent genus was Candida, although members belonging to the genera Brettanomyces, Dekkera, and Geotricum were also identified. Although qualitative and quantitative tests for extracellular proteolytic activity were positive for
all the species isolated, Candida buinensis showed the highest response (23.5 U/mg); therefore, it was selected for subsequent investigation. The results of fermentations
carried out at variable temperature, pH, and soybean flour concentration, according to a 23 full factorial design, demonstrated that this yeast ensured the highest production of extracellular proteases (573 U/mL)
when cultivated at 35 °C, pH 6.5, and using soybean flour concentrations in the range 0.1–0.5% (w/v). The cell-free supernatants showed the highest activity at 25 °C and pH 7.0, and satisfactory stability in the ranges 25–30 °C
and pH 7–9. The first-order rate constants of protease inactivation in the cell-free supernatants were calculated at different
temperatures from semi-log plots of the residual activity versus time and then used in Arrhenius and Eyring plots to estimate
the main thermodynamic parameters of thermoinactivation (E* = 40.0 kJ/mol; ΔH* = 37.3 kJ/mol; ΔS* = −197.5 J/mol K; ΔG* = 101 kJ/mol). 相似文献
47.
D.C.J. Rezende F.O. Borges G.H. Cavalcanti M. Raineri M. Gallardo J. Reyna Almandos A.G. Trigueiros 《Journal of Quantitative Spectroscopy & Radiative Transfer》2010,111(12-13):2000-2006
The spectrum of four-times-ionized krypton (Kr V) has been observed in the 230–4900 Å wavelength range, resulting in 91 new classified lines. We were able to identify 21 new energy levels belonging to the 4s24p5d, 4s24p5s, 4s24p6s, 4s24p5p and 4s4p24d configurations. Relativistic Hartree–Fock calculations were used to predict energy levels and transitions. 相似文献
48.
M.M. Cavalcanti V.N. Domingos Cavalcanti P. Martinez 《Nonlinear Analysis: Theory, Methods & Applications》2008
The linear viscoelastic equation is considered. We prove uniform decay rates of the energy by assuming a nonlinear feedback acting on the boundary, without imposing any restrictive growth assumption on the damping term and strongly weakening the usual assumptions on the relaxation function. Our estimate depends both on the behavior of the damping term near zero and on the behavior of the relaxation function at infinity. 相似文献
49.
M. M. Cavalcanti V. N. Domingos Cavalcanti J. Ferreira 《Mathematical Methods in the Applied Sciences》2001,24(14):1043-1053
This paper is concerned with the non‐linear viscoelastic equation We prove global existence of weak solutions. Furthermore, uniform decay rates of the energy are obtained assuming a strong damping Δut acting in the domain and provided the relaxation function decays exponentially. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
50.
Gil R. Cavalcanti Marco Gualtieri 《Bulletin of the Brazilian Mathematical Society》2011,42(4):537-557
We show that the blow-up of a generalized Kähler 4-manifold in a nondegenerate complex point admits a generalized Kähler metric. As with the blow-up of complex surfaces, this metric may be chosen to coincide with the original outside a tubular neighbourhood of the exceptional divisor. To accomplish this, we develop a blow-up operation for bi-Hermitian manifolds. 相似文献