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971.
The acylation of 2-methoxynaphthalene with acetic anhydride was carried out over zeolite beta catalysts having various Si/Al
ratios. It was found that the reaction performance is strongly dependent on the Si/Al ratio of the catalyst. The catalytic
reaction features were elucidated in terms of the acidic properties of the catalysts.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
972.
(Z)-γ-Cyano-β-perfluoroalkyl-β,γ-unsaturated esters have been synthesized by the reaction of perfluoroacylated phosphonates with organozinc reagent in 50-71% (three steps) yields. 相似文献
973.
The present short review deals with electroanalytical aspects of electrochemical response of ascorbic acid (Vitamin C) at conducting and electrogenerated polymer modified electrodes. Two main topics are considered: (i) electrocatalytic oxidation of ascorbate at conducting polymer modified electrodes, leading to electroanalytical techniques for ascorbate assay, and (ii) retardation of ascorbate penetration through a layer of electrogenerated polymers, leading to permselective coatings and their diverse uses, especially for biosensing devices. 相似文献
974.
Determination of selenium, zinc and cadmium in antidandruff shampoos by atomic spectrometry after microwave assisted sample digestion 总被引:1,自引:0,他引:1
Microwave assisted pre-treatments for atomic spectrometric determination (inductive coupled plasma-optical emission spectrometry, ICP-OES or flame atomic absorption spectrometry, FAAS) of metallic elements, usually present in antidandruff shampoos, are proposed. They are based on the digestion of the sample with HNO3 into a closed reactor, which is irradiated at 800 W for a few minutes. Selenium was determined by ICP-OES. The limit of detection was 0.11 mg l−1; the relative standard deviation (R.S.D.) for the selenium content in the samples was in the 0.6–3.6% range. The results obtained were in agreement with the label contents and the recovery of the proposed method was in the 100–106% range. Zinc and cadmium were determined by FAAS. The limit of detection for zinc determination was 0.078 mg l−1; the R.S.D. for zinc contents was in the 0.8–8.6% range. A limit of detection of 0.09 mg l−1 was obtained for cadmium determination; the R.S.D. for cadmium contents was in the 0.7–2.7% range. The determinations were performed after two different sample mineralization pre-treatments — dry ashing (in an electric furnace) and wet mineralization (in a microwave oven). Both methodologies provided comparable results for zinc and cadmium determination in shampoos. The proposed microwave assisted digestion procedures allow a precise and accurate determination of selenium, zinc and cadmium in commercial antidandruff shampoos, and the sample pre-treatment is less time-consuming than the classic methods. 相似文献
975.
《Arabian Journal of Chemistry》2022,15(1):103472
As a representative of traditionally fermented Chinese medicine, Massa Medicata Fermentata (MMF) shows the functions of invigorating the spleen and stomach and promoting digestion, which plays an important role in the treatment of gastrointestinal diseases. The fermentation mechanism and the key factors that affect the quality of MMF have not been revealed yet, which has become an urgent issue that limits its clinical application. This article aims to systematically and comprehensively reveal the transformation of physical properties and the dynamic trend of chemical components including substrate components, volatile components, and lactic acid as anaerobic fermentation product during MMF fermentation. Along with obvious hyphae growth observed for MMF, the weight of MMF decreased, and the moisture and temperature increased. Through the quantified 14 components from substrate, ferulic acid increased from 45.53 ± 6.94 to 141.89 ± 78.40 μg/g, while glycosides and phenolic acids declined except caffeic acid. Also, within the 66 volatile components analyzed, alcohols and acids increased, while aldehydes and ketones decreased. Lactic acid was not detected in the fermentation substrate, but an apparent increase in lactic acid content was observed along with the increased fermentation days, resulting in 2.54 ± 0.15 mg/g on day 8. Based on the tested components, the fermentation process of MMF was discriminated into three distinct stages by principal component analysis, and an optimal fermentation time of four days was proposed. The results of this study will be of great significance to clarify the characteristics of fermentation and conduce to improving quality standards of MMF. 相似文献
976.
A procedure for estimation of measurement uncertainty of routine pH measurement (pH meter with two-point calibration, with
or without automatic temperature compensation, combination glass electrode) based on the ISO method is presented. It is based
on a mathematical model of pH measurement that involves nine input parameters. Altogether 14 components of uncertainty are
identified and quantified. No single uncertainty estimate can be ascribed to a pH measurement procedure: the uncertainty of
pH strongly depends on changes in experimental details and on the pH value itself. The uncertainty is the lowest near the
isopotential point and in the center of the calibration line and can increase by a factor of 2 (depending on the details of
the measurement procedure) when moving from around pH 7 to around pH 2 or 11. Therefore it is necessary to estimate the uncertainty
separately for each measurement. For routine pH measurement the uncertainty cannot be significantly reduced by using more
accurate standard solutions than ±0.02 pH units – the uncertainty improvement is small. A major problem in estimating the
uncertainty of pH is the residual junction potential, which is almost impossible to take rigorously into account in the framework
of a routine pH measurement.1
Received: 11 August 2001 Accepted: 22 February 2002 相似文献
977.
Sanjeev K. Garg 《Tetrahedron letters》2005,46(10):1721-1724
Copper(II) tetrafluoroborate has been found to be a new and highly efficient catalyst for Michael addition of thiols to α,β-unsaturated carbonyl compounds under solvent-free conditions and in H2O at room temperature. The reactions are very fast and are completed in 2 min to 1 h affording high yields. The rate of thiol addition was dependent on the steric hindrance at the β-carbon of the α,β-unsaturated carbonyl substrate. In the case of chalcones, the reactions are best carried out in MeOH as solvent. 相似文献
978.
Michael A. Carpenter Susana Rios Wilson Crichton 《Journal of solid state chemistry》2007,180(1):360-369
Lattice parameters of a synthetic powder sample of Ca0.35Sr0.65TiO3 perovskite have been determined by the method of Le Bail refinement, using synchrotron X-ray diffraction patterns collected at pressures up to 15.5 GPa with a membrane-driven diamond anvil cell. At ambient conditions, diffraction data were consistent with the I4/mcm structure reported previously in the literature for the same composition. Diffraction data collected at high pressures were consistent with tetragonal (or, at least, pseudo-tetragonal) lattice geometry, and no evidence was found for the development of any of the orthorhombic structures identified in other studies of (Ca, Sr)TiO3 perovskites. Additional weak reflections, which could not be accounted for by the normal I4/mcm perovskite structure, were detected in diffraction patterns collected at pressures of 0.9-2.5 GPa, and above ∼13.5 GPa, however. Small anomalies in the evolution of unit cell volume and tetragonal strain were observed near 3 GPa, coinciding approximately with breaks in slope with increasing pressure of bulk and shear moduli for a sample with the same composition which had previously been reported. The anomalies could be due either to new tetragonal↔tetragonal/pseudo-tetragonal phase transitions or to subtle changes in compression mechanism of the tetragonal perovskite structure. 相似文献
979.
高效液相色谱-电喷雾电离三级四极杆质谱检测动物源食品中氯霉素、甲砜霉素、氟苯尼考残留量 总被引:6,自引:1,他引:6
建立了动物源产品中氯霉素、甲砜霉素、氟苯尼考残留量的高效液相色谱-电喷雾电离三级四极杆质谱(LC-ESI-MS/MS)的方法。前处理方法包括添加同位素内标氯霉素-d5,碱化乙酸乙酯提取,C18小柱净化。该方法采用负离子,多反应监测氯霉素四对离子(321.0/151.9,321.0/256.6,321.0/194.2,321.0/175.4),甲砜霉素两对离子(354.1/185.0,354.1/290.0),氟苯尼考两对离子(356.0/335.9,356.0/185.1)和同位素内标氯霉素-d5(326.0/157.1)。该方法线性范围为0.1~1.6μg/kg;对不同基质样品的加标回收率为80%~112.5%;相对标准偏差小于11%;方法的测定低限为0.1μg/kg。 相似文献
980.
V. G. Berezkin E. G. Sumina S. N. Shtykov V. Z. Atayan D. A. Zagniboroda G. A. Nekhoroshev 《Chromatographia》2006,64(1-2):105-108
A new variant of thin-layer chromatography (TLC), based on a gradual change of mobile phase acidity during elution, is proposed.
The pH change occurs in the mobile phase moving along the TLC plate as a result of its contact with an acidic or a basic gas
phase that replaces the initial mobile phase vapor in the TLC chamber. The potential of this approach has been demonstrated
by using carbon dioxide and ammonia gases to improve the resolution of benzoic acids and aromatic amines on polyamide TLC
plates. 相似文献