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941.
942.
O. E. Ayala D. Lardizábal A. Reyes M. I. Rosales J. A. Matutes A. González Arias 《Journal of Thermal Analysis and Calorimetry》2000,59(3):943-949
Expressions for calculating the cation vacancy contents of MnZn ferrites from thermogravimetric curves are presented together
with some experimental data. In a single-phase MnZn ferrite synthesized by conventional ceramic procedures, the O2 evolution accompanying ferrite formation follows the formal equation.
Mn2+
σα Znσβ Fe3+
2σ(1–γ) [V ]σ/4(1–2γ) O4 =σ'/σ Mn2+
σ(α–2ϕ) Znσβ Fe2+
2σθ Mn3+
2σϕ Fe3+
2σ(1–γ–θ) [V ]σ/4(1–2γ–3ϕ) O4 +σ'φ/2O2 (g)
where α and β denote the MnO and ZnO mole fractions in the primary mixture γ=α+β, θ and ϕ depend on the quantities of Fe2+ and Mn3+ formed, respectively, φ=θ–ϕ and σ'/σ is a function of the former parameters. Even though the relative amounts of Fe2+ /Fe3+ and Mn2+ /Mn3+ remain uncertain, the vacancy content [V ] of the ferrite can be determined because it depends on φ alone, which is related to the change in mass of the sample as
the synthesis takes place through the equation
φ=(1.5–γ) μβ /μO2 (1–m
f /m
i )
Here, m
i and m
f are the masses of the sample before and after O2 evolution, μB is the formula mass of the ferrite and μO2 is the O2 molar mass. Practically vacancy-free single-phase MnZn ferrite samples were obtained by sintering in air at 1250°C and cooling
in pure N2 .
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
943.
M Sáez A Gómez-Parra E González-Mazo 《Fresenius' Journal of Analytical Chemistry》2001,371(4):486-490
Sorption experiments using different homologues of linear alkylbenzene sulfonate (LAS) and sulfophenylcarboxylic acid (SPC) on several marine microalgae have been carried out. The steady state seems to be reached in the first 4 hours. Longer exposure times lead to biodegradation of the compound and, therefore, to an overestimation of the bioconcentration factor. Sorption coefficients are higher for Nannochloropsis gaditana, for example, 1,293 Lkg(-1) for C11-LAS and 525 Lkg(-1) for C11-SPC versus 727 Lkg(-1) for C11-LAS and 28 Lkg(-1) for C11-SPC for Dunaliella salina. For both algae an increase in the sorption coefficient is observed when the polarity of the compound decreases (C5-SPC相似文献
944.
A physical chemistry experiment is described that involves the determination of some spectroscopic parameters of carbon dioxide, a molecule that obeys Bose-Einstein statistics. The main advantage of this experiment is that the spectra are easily recorded, not requiring a gas cell, because the sensitivity and resolution of conventional FTIR spectrometers is good enough to record spectra with a high signal-to-noise ratio and good resolution of the fine rotational structure. From the rotational lines of the antisymmetric stretching band, the moments of inertia and the bond lengths of CO2 in the fundamental and the first-excited state can be accurately obtained. The particular case that carbon dioxide represents helps students understand the restrictions that symmetry and statistics impose on some molecules and the consequences that they have on the absorption of radiation. 相似文献
945.
M. C. Toro Gordillo E. Pinilla Gil M. A. Rodríguez González A. Murciego Murciego P. Ostapczuk 《Analytical and bioanalytical chemistry》2001,370(4):434-437
A potentiometric stripping analysis (PSA) method has been developed and checked for the fast and reliable determination of
antimony in vegetation samples of Cistus ladanifer from a mining area in Badajoz, Southwest Spain. The method, modified from previous PSA methods for Sb in environmental samples,
is based on dry ashing of the homogenized leaves, dissolution in hydrochloric acid, and PSA analysis on a mercury film plated
on to a glassy carbon disk electrode. The influence of experimental variables such as the deposition potential, the deposition
time, the signal stability and the calibration parameters, has been investigated. The method has been compared with an independent
technique (instrumental neutron activation analysis) by analysis of standards and reference materials and comparison of the
results. As a result of automation of the PSA equipment, the proposed method enables unattended analysis of 20 digested samples
in a total time of 2 h, thus providing a useful tool for Sb monitoring of a large number of samples.
Received: 13 December 2000 / 14 February 2001 / Accepted: 26 February 2001 相似文献
946.
M González J L Segura C Seoane N Martín J Garín J Orduna R Alcalá B Villacampa V Hernández J T López Navarrete 《The Journal of organic chemistry》2001,66(26):8872-8882
Novel pi-conjugated donor-acceptor chromophores, based on the strong electron-donating tetrathiafulvalene moiety and different electron-withdrawing acceptors, exhibit large second-order optical nonlinearities. The effect of increasing the length of the polyenic spacer and the influence of the nature of the acceptor moiety on the NLO properties have been studied by using the electric field-induced second-harmonic generation (EFISH) technique as well as by semiempirical and ab initio theoretical calculations. A charge-transfer band has been observed in the absorption spectra of these D-pi-A compounds that undergoes an hypsochromic shift when increasing the number of vinylenic spacer units connecting both donor and acceptor moieties. The degree of the intramolecular charge transfer from the donor to the acceptor has also been analyzed by means of Raman spectroscopy. 相似文献
947.
S Pérez-Magari?o I Revilla M L González-SanJosé S Beltrán 《Journal of chromatography. A》1999,847(1-2):75-81
Different options of liquid chromatography-mass spectrometry were used to establish the most suitable ion source and conditions to analyse or detect some low-molecular mass phenols, flavan-3-ols, and apply such conditions to a complex sample (wine). Data presented in this work confirm the great utility of atmospheric pressure-ionisation electrospray mass spectrometry coupled to HPLC for analysis of phenolic compounds, under negative mode in the case of low-molecular mass phenols, and under both positive and negative modes in flavan-3-ol compounds. A fragmentor voltage of 60 V could be the most suitable for analysing the compounds under study. 相似文献
948.
Real dispersion of isolated fumed silica nanoparticles in highly filled PMMA prepared by high energy ball milling 总被引:1,自引:0,他引:1
Castrillo PD Olmos D Amador DR González-Benito J 《Journal of colloid and interface science》2007,308(2):318-324
Fumed silica nanoparticles with 14 nm of diameter were blended with poly(methylmethacrylate), PMMA, by means of a high energy ball milling process. AFM analysis revealed how this process of blending allows obtaining a very homogeneous dispersion of the nanoparticles within the PMMA. Furthermore, it was observed that the properties of the composite are highly dependent on the active milling time: (i) SEM inspection showed that the particle size of the silica-PMMA nanocomposite decreases and (ii) DSC analysis demonstrated that the Tg also decreases due to a reduction in the molecular weight of the PMMA caused by chain scission during the high energy blending process. Two Tg's were obtained in the case of the nanocomposite when milling times were higher than 6 h, one of them being even higher than that of the PMMA without being subjected to the HEBM process. This result was assigned to surface-induced molecular weight segregation near the nanoparticles surface. It has been demonstrated the possibility of preparing transparent nanocomposites with excellent moldability. 相似文献
949.
Pesticide residue analysis in cereal-based baby foods using multi-walled carbon nanotubes dispersive solid-phase extraction 总被引:2,自引:0,他引:2
González-Curbelo MA Asensio-Ramos M Herrera-Herrera AV Hernández-Borges J 《Analytical and bioanalytical chemistry》2012,404(1):183-196
In the present study, a new analytical method has been developed for the simultaneous quantification of 15 organophosphorus pesticides, including some of their metabolites, (disulfoton-sulfoxide, ethoprophos, cadusafos, dimethoate, terbufos, disulfoton, chlorpyrifos-methyl, malaoxon, fenitrothion, pirimiphos-methyl, malathion, chlorpyrifos, terbufos-sulfone, disulfoton-sulfone and fensulfothion) in three different types of commercial cereal-based baby foods. Dispersive solid-phase extraction (dSPE) with multi-walled carbon nanotubes (MWCNTs) was used together with gas chromatography with nitrogen phosphorus detection. Most favorable conditions involved a previous ultrasound-assisted extraction of the sample with acetonitrile containing formic acid. After evaporation of the extract and redissolution in water, a dSPE procedure was carried out with MWCNTs. The whole method was validated in terms of repeatability, linearity, precision and accuracy and matrix effect was also evaluated. Absolute recoveries were in the range 64-105 % with relative standard deviation values below 7.6 %. Limits of quantification achieved ranged from 0.31 to 5.50 μg/kg, which were lower than the European Union maximum residue limits for pesticide residues in cereal-based baby foods. 相似文献
950.
Camila Rodrigues de Carvalho Mariana de Lourdes Almeida Vieira Charles L. Cantrell David E. Wedge Tânia M.A. Alves Carlos Leomar Zani 《Natural product research》2016,30(4):478-481
Endophytic fungi represent ubiquitous microbial organisms able to live in the tissues of different plants around the world and represent a prolific source of bioactive metabolites. In the present study, the endophytic fungus Aspergillus calidoustus was isolated from the medicinal plant Acanthospermum australe (Asteraceae), and identified using molecular, physiological and morphological methods. A methylene chloride crude extract of A. calidoustus has been produced and subjected to antifungal bioassay-directed fractionation which resulted in the isolation of the two bioactive compounds: ophiobolin K and 6-epi-ophiobolin K. These pure compounds displayed antifungal activity against fungal plant pathogens, protozoal activity against Trypanosoma cruzi, and cytotoxic activity against human tumoral cell lines. The results show that A. calidoustus was able to produce the antifungal and cytotoxic metabolites ophiobolin K and 6-epi-ophiobolin K, which may help the fungus to colonise and occupy the substratum as well as survive in natural environments. 相似文献