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91.
A Cristina Henriques Alves M Margarida Pontes Boavida Gonçalves M Manuel Serrano Bernardo B Simões Mendes 《Journal of separation science》2012,35(19):2653-2658
Dispersive liquid-liquid microextraction (DLLME) combined with gas chromatography and mass spectrometry (GC-MS) was applied to the determination of five organophosphorous pesticides (OPPs) in water samples. The analytes included in this study were prophos, diazinon, chlorpyrifos methyl, fenchlorphos, and chlorpyrifos. The use of nonhalogenated solvents (cyclohexane, heptane, and octane) as extraction solvents was investigated using acetone, acetonitrile, or methanol, as dispersion solvents. The combination of less polar dispersion solvents (1-propanol and 2-propanol) and nonhalogenated extraction solvents was also studied in dispersive liquid-liquid microextraction for the first time. Several experimental conditions were tested (nature and volume of extraction solvents, nature and volume of dispersion solvents, salting-out effect) and the corresponding enrichment factors and recoveries were evaluated. The best microextraction condition was obtained using 50 μL of cyclohexane and 0.3 mL of 1-propanol. The detection and quantification limits were in the low ppt range, with values between 3.3-8.0 ng/L and 11.0-26.6 ng/L, respectively. Relative standard deviations were between 6.6 and 13.1% for a fortification level of 500 ng/L. At the same fortification level, the relative recoveries (RR) of Alvito's dam water, Judeu's river water, and well water samples were in the range of 50.3-97.1%. 相似文献
92.
Margarida Moreira dos Santos Margarida João Queiroz Pedro V. Baptista 《Journal of nanoparticle research》2012,14(5):859
A strategy for the development of novel antimicrobials is to combine the stability and pleiotropic effects of inorganic compounds
with the specificity and efficiency of organic compounds, such as antibiotics. Here we report on the use of gold:silver-alloy
(Au:Ag-alloy) nanoparticles, obtained via a single-step citrate co-reduction method, combined to conventional antibiotics
to enhance their antimicrobial effect on bacteria. Addition of the alloy nanoparticles considerably decreased the dose of
antibiotic necessary to show antimicrobial effect, both for bacterial cells growing in rich medium in suspension and for bacterial
cells resting in a physiological buffer on a humid cellulose surface. The observed effect was more pronounced than the sum
of the individual effects of the nanoparticles and antibiotic. We demonstrate the enhancement effect of Au:Ag-alloy nanoparticles
with a size distribution of 32.5 ± 7.5 nm mean diameter on the antimicrobial effect of (i) kanamycin on Escherichia coli (Gram-negative bacterium), and (ii) a β-lactam antibiotic on both a sensitive and resistant strain of Staphylococcus aureus (Gram-positive bacterium). Together, these results may pave the way for the combined use of nanoparticle–antibiotic conjugates
towards decreasing antibiotic resistance currently observed for certain bacteria and conventional antibiotics. 相似文献
93.
M.S. Baptista E.J. Ngamga Paulo R.F. Pinto Margarida Brito 《Physics letters. A》2010,374(9):1135-1140
Observing how long a dynamical system takes to return to some state is one of the most simple ways to model and quantify its dynamics from data series. This work proposes two formulas to estimate the KS entropy and a lower bound of it, a sort of Shannon's entropy per unit of time, from the recurrence times of chaotic systems. One formula provides the KS entropy and is more theoretically oriented since one has to measure also the low probable very long returns. The other provides a lower bound for the KS entropy and is more experimentally oriented since one has to measure only the high probable short returns. These formulas are a consequence of the fact that the series of returns do contain the same information of the trajectory that generated it. That suggests that recurrence times might be valuable when making models of complex systems. 相似文献
94.
95.
A conjecture of Miles Reid states that the relative canonical algebra for a pencil of curves of genus greater than one is
always generated in degrees at most three (1-2-3 conjecture). We give some explicit counter-examples to the conjecture.
Received: 10 December 1998/ Revised version: 15 April 1999 相似文献
96.
Antnio P. Gonalves Laura C. J. Pereira Paulo A. S. Silva Margarida Godinho Manuel Almeida Moshe Kuznietz 《Journal of magnetism and magnetic materials》2000,210(1-3)
Polycrystalline (U0.50Dy0.50)Ni2B2C solid solution was prepared and found by X-ray diffraction to crystallize in BCT LuNi2B2C-type structure (space group I4/mmm) of the end compounds UNi2B2C and DyNi2B2C. AC susceptibility and magnetization show paramagnetic behavior down to 6.5 K, with the values θ=−5(5) K and μeff=7.7(1) μB, compatible with those of the end compounds, and indicate possible cooperative phenomena at lower temperatures. The observed paramagnetism, at variance with antiferromagnetic ordering in (Pr0.50Dy0.50)Ni2B2C, is attributed to a directional frustration of the magnetic moments on the (U,Dy) site. 相似文献
97.
98.
Sílvia M. G. Rodrigues Margarida Facão Mário F. S. Ferreira 《Fiber and Integrated Optics》2015,34(1-2):76-89
AbstractKagome hollow-core photonic crystal fibers were found to be ideal for the occurrence of ultrafast non-linear optics. This article reports the optimal conditions for the generation of ultraviolet light using a gas filled kagome hollow-core-photonic crystal fiber. It is shown that by changing the pressure of the gas and the input pulse characteristics, the efficiency of conversion and quality of ultraviolet light can be improved, as well as tuning its central frequency. Results suggest that a highly coherent and tunable ultraviolet light source can be constructed, which can find numerous applications. 相似文献
99.
We investigate a Galois connection in poset enriched categories between subcategories and classes of morphisms, given by means of the concept of right-Kan injectivity, and, specially, we study its relationship with a certain kind of subcategories, the KZ-reflective subcategories. A number of well-known properties concerning orthogonality and full reflectivity can be seen as a particular case of the ones of right-Kan injectivity and KZ-reflectivity. On the other hand, many examples of injectivity in poset enriched categories encountered in the literature are closely related to the above connection. We give several examples and show that some known subcategories of the category of T0-topological spaces are right-Kan injective hulls of a finite subcategory. 相似文献
100.
Margarida S. Miranda Joaquim C. G. Esteves da Silva Alvaro Castillo Aaron T. Frank Alexander Greer Jodian A. Brown Brittny C. Davis Joel F. Liebman 《Journal of Physical Organic Chemistry》2013,26(8):613-625
We have performed high level ab initio quantum mechanical calculations for aminoethene and the three isomeric 1,1‐ (Z)‐ or (E)‐1,2‐diaminoethenes as well as their singly and doubly charged cations derived by loss of electrons and/or upon protonation. Gas phase molecular structures were computed at the MP2/6‐311 + G(3df,2p) level. Standard molar enthalpies of formation in the gas phase, at T = 298.15 K, were estimated using the G3 composite method and atomization, isodesmic and homodesmotic reactions. Other energetic parameters were also calculated at the G3 level: proton affinities, basicities and adiabatic ionization enthalpies. Theoretical and experimental data are compared. The reported experimental data refer only to aminoethene wherein the standard molar enthalpy of formation has a considerable uncertainty, although the molecular structure is well established. There are no such data, neither structural nor thermochemical, for any of the three isomeric diaminoethenes. Isoelectronic comparisons are made. For example, the diprotonated diaminoethenes are isoelectronic to isobutene and (Z)‐ and (E)‐butene, while the doubly ionized diaminoethenes are likewise related to trimethylenemethane and 1,3‐butadiene. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献