Caffeine (1,3,7-trimethylxanthine), theobromine (3,7-dimethylxanthine) and theophylline (1,3-dimethylxanthine) are the most important naturally occurring methylxanthines. Caffeine is a constituent of coffee and other beverage and included in many medicines. Theobromine and theophylline are formed as metabolites of caffeine in humans, and are also present in tea, cocoa and chocolate products.
In order to improve the chromatographic resolution (Rs) with a good analysis time, experimental designs were applied for multivariate optimisation of the experimental conditions of an isocratic reversed-phase high-performance liquid chromatographic (RP-HPLC) method used for the simultaneous determination of caffeine, theobromine and theophylline. The optimisation process was carried out in two steps using full three-level factorial designs. The factors optimised were: flow rate and mobile phase composition. Optimal conditions for the separation of the three methylxanthines were obtained using a mixture of water/ethanol/acetic acid (75:24:1%, v/v/v) as mobile phase and a flow rate of 1.0 mL min−1. The RP-HPLC/UV method was validated in terms of limit of detection (LOD), limit of quantitation (LOQ), linearity, recovery and the precision, calculated as relative standard deviation (R.S.D.). In these conditions, the LOD was 0.10 μg L−1 for caffeine, 0.07 μg L−1 for theobromine and 0.06 μg L−1 for theophylline. The proposed method is fast, requires no extraction step or derivatization and was suitable for quantification of these methylxanthines in coffee, tea and human urine samples. 相似文献
Summary The standard molar enthalpy of formation of methyl methylthiomethyl sulfoxide, CH3(CH3SCH2)SO, at T=298.15 K in the liquid state was determined to be -199.4±1.5 kJ mol-1 by means of oxygen rotating-bomb combustion calorimetry. 相似文献
The enthalpy change of formation of the reaction of hydrous dysprosium chloride with ammonium pyrrolidinedithiocarbamate (APDC) and 1,10-phenanthroline (o-phen?H2O) in absolute ethanol at 298.15 K has been determined as (-16.12 ± 0.05) kJ?mol-1 by a microcalormeter. Thermodynamic parameters (the activation enthalpy, the activation entropy and the activation free energy), rate constant and kinetics parameters (the apparent activation energy, the pre-exponential constant and the reaction order) of the reaction have also been calculated. The enthalpy change of the solid-phase reaction at 298.15 K has been obtained as (53.59 ± 0.29) kJ?molt-1 by a thermochemistry cycle. The values of the enthalpy change of formation both in liquid-phase and solid-phase reaction indicated that the complex could only be synthesized in liquid-phase reaction. 相似文献
The ferroin-catalyzed Belousov-Zhabotinsky(BZ) reaction,the oxidation of malonic acid by acidic bromate,is the most commonly investigated chemical system for understanding spatial pattern forma-tion. Various oscillatory behaviors were found from such as mixed-mode and simple period-doubling oscillations and chaos on both Pt electrode and Br-ISE at high flow rates to mixed-mode oscillations on Br-ISE only at low flow rates. The complex dynamic behaviors were qualitatively reproduced with a two-cycle coupling model proposed initially by Gy?rgyi and Field. This investigation offered a proper medium for studying pattern formation under complex temporal dynamics. In addition,it also shows that complex oscillations and chaos in the BZ reaction can be extended to other bromate-driven nonlinear reaction systems with different metal catalysts. 相似文献
Fluorescence and spectral hole burning properties of Eu3+ ions were studied in nanocrystals-precipitated SnO2-SiO2 glasses. The glasses were prepared to contain various amount of Eu2O3 using the sol-gel method, in which SnO2 nanocrystals were precipitated by heating in air. In the glasses containing Eu2O3 less than 1%, the Eu3+ ions were preferentially doped in the SnO2 nanocrystals and their fluorescence intensities were enhanced by the energy transfer due to the recombination of electrons
and holes excited in SnO2 crystals. The SnO2 nanocrystals-precipitated glasses exhibited the persistent spectral holes with the depth of ∼25% of the total fluorescence
intensities of the Eu3+ ions. With the increasing Eu2O3 concentration, the amount of SnO2 nanocrystals decreased and the Sn4+ ions formed the random glass structure together with the silica network. This structure change induced the fluorescence intensities
and the hole depth to decrease. 相似文献
A kind of novel mesoporous, electrochemical active material, amorphous MnO2 has been synthesized by an improved reduction reaction and using supramolecular as template. The synthesized sample was characterized
physically by thermogravimetric analysis, X-ray diffraction, transmission electron microscope (TEM), and Brunauer–Emmett–Teller
(BET) surface area measurement, respectively. Electrochemical characterization was performed using cyclic voltammetry and
chronopotentiometry in 2 mol/l KOH aqueous solution electrolyte. The results of BET and TEM analysis indicated that supramolecular
template plays an important role in the process of big specific surface area mesoporous material forming. After sintering
at 200 °C, the sample still remained an amorphous structure, and its specific capacitance reached 298.7 F/g and presented
a very stable capacitance after 500 cycles. In addition, the electrochemical process, such as ion transfer and electrical
condition, was also investigated with electrochemical impedance spectroscopy. 相似文献
The quantum yields for singlet oxygen generation of several squarylium cyanine dyes derived from benzothiazole, benzoselenazole and quinoline, displaying absorption within the so-called “phototherapeutic window” (600–1000 nm), were determined, envisioning their potential usefulness for photodynamic therapy (PDT). The determination was performed by a direct method measuring the luminescence decay of the dyes in the near infrared. Considering the absorption and the quantum yields displayed by some of the dyes, these seemed to be potential candidates as sensitizers for PDT. 相似文献
A heterogeneous, multi-layer mass transfer model is proposed for prediction of the effect of multi-layer packing of catalyst
particles adhered to the gas-liquid interface. The behavior of the mass transfer rate with respect to the multi-layer packing,
to the particle size and mass transfer coefficient without particles is discussed. It is shown that enhancement can be considerably
increased by multi-layer packing compared to that of mono-layer packing, depending on the values of particle size and mass
transfer coefficient. The predicted mass transfer rates using the proposed model was verified with experimental data taken
from the literature. The model presented should be superior to that of published in the literature. 相似文献
The present study aims the identification and quantification of trace elements in samples of honey from the Azores and the
Portuguese mainland. Elemental concentrations were determined for Al, Br, Cl, Cs, Cu, Eu, Fe, K, Mg, Mn, Na, Rb, Sb, and Zn.
Some of these elements are essential dietary nutrients for humans. The essential elements, K and to a lesser extent Na and
Cl were the most abundant in the honey samples. However, overall, the elemental content was very low, inferior to the recommended
allowances for daily dietary intake (DDI), with the exception of Na whose concentration exceeded that of the recommended DDI.
The results of the present investigation and data reported for other countries compare well. The honeys correlate much better
when they are from Azores or from Portugal mainland, however, some good correlations were found between honeys from the islands
and the mainland. The color of the honey, which depends of the flower-source, could have a role in the differences and similarities
between the different honeys as suggested by the cluster analysis of the data. The correlation between honey, soil, tree bark
and lichens, all collected in Azores, was poor. 相似文献
A system for separation of zinc traces from large amounts of cadmium is proposed in this paper. It is based on the solid-phase extraction of the zinc in the form of thiocyanate complexes by the polyurethane foam. The following parameters were studied: effect of pH and of the thiocyanate concentration on the zinc extraction, shaking time required for quantitative extraction, amount of PU foam necessary for complete extraction, conditions for the separation of zinc from cadmium, influence of other cations and anions on the zinc sorption by PU foam, and required conditions for back extraction of zinc from the PU foam. The results show that zinc traces can be separated from large amounts of cadmium at pH 3.0±0.50, with the range of thiocyanate concentration from 0.15 to 0.20 mol l−1, and the shaking time of 5 min. The back extraction of zinc can be done by shaking it with water for 10 min. Calcium, barium, strontium, magnesium, aluminum, nickel and iron(II) are efficiently separated. Iron(III), copper(II) and cobalt(II) are extracted simultaneously with zinc, but the iron reduction with ascorbic acid and the use of citrate to mask copper(II) and cobalt(II) increase the selectivity of the zinc extraction. The anions nitrate, chloride, sulfate, acetate, thiosulphate, tartarate, oxalate, fluoride, citrate, and carbonate do not affect the zinc extraction. Phosphate and EDTA must be absent. The method proposed was applied to determine zinc in cadmium salts using 4-(2-pyridylazo)-resorcinol (PAR) as a spectrophotometric reagent. The result achieved did not show significant difference in the accuracy and precision (95% confidence level) with those obtained by ICP–AES analysis. 相似文献