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991.
992.
López-García I Merino Meroño B Campillo N Hernández-Córdoba M 《Analytical and bioanalytical chemistry》2002,373(1-2):98-102
Two manifolds were assessed for the purpose of determining both the total chromium content and that present as a soluble form in industrial effluents by flow injection flame atomic absorption spectrometry (FI-FAAS). To determine the chromium content in the soluble fraction the samples were used without additional treatment, a 0.45 microm filter being included in the FI system. To determine the total chromium content, the samples were acidified with nitric acid 20% (v/v) and heated for 30 s in a microwave oven (temperatures of about 70 degrees C were reached). The problem posed by the very different concentration range in which total and soluble chromium are present was overcome by using programmed flow rate methodology and by only partially emptying the sample loop. A personal computer controlled both the rotation speed of a peristaltic pump and the volume of sample injected into the system, thus obtaining the dispersion degree required. Using the manifold proposed, the chromium content in the soluble fraction can be determined in the 0.5-20 microg mL(-1) range using a 10 microg mL(-1) single standard for calibration. To determine the total chromium content, a calibration line in the 20-200 microg mL(-1) range was obtained using a single 50 microg mL(-1) chromium standard solution. The reliability of the semi-automatic devices was verified by comparing the results obtained with those found by treating the samples and using both FAAS in a conventional way and a spectrophotometric method using diphenylcarbazide at the 95% confidence level (ANOVA test). The proposed procedures showed a RSD lower than +/-3%. 相似文献
993.
Andrés J Moliner V Safont VS Domingo LR Picher MT 《The Journal of organic chemistry》1996,61(22):7777-7783
As a model of the chemical reactions that take place in the active site of gluthatione reductase, the nature of the molecular mechanism for the hydride transfer step has been characterized by means of accurate quantum chemical characterizations of transition structures. The calculations have been carried out with analytical gradients at AM1 and PM3 semiempirical procedures, ab initio at HF level with 3-21G, 4-31G, 6-31G, and 6-31G basis sets and BP86 and BLYP as density functional methods. The results of this study suggest that the endo relative orientation on the substrate imposed by the active site is optimal in polarizing the C4-Ht bond and situating the system in the neighborhood of the quadratic region of the transition structure associated to the hydride transfer step on potential energy surface. The endo arrangement of the transition structure results in optimal frontier HOMO orbital interaction between NADH and FAD partners. The geometries of the transition structures and the corresponding transition vectors, that contain the fundamental information relating reactive fluctuation patterns, are model independent and weakly dependent on the level of theory used to determine them. A comparison between simple and complex molecular models shows that there is a minimal set of coordinates describing the essentials of hydride transfer step. The analysis of transition vector components suggests that the primary and secondary kinetic isotope effects can be strongly coupled, and this prompted the calculation of deuterium and tritium primary, secondary, and primary and secondary kinetic isotope effects. The results obtained agree well with experimental data and demonstrate this coupling. 相似文献
994.
R. González-Ramírez H. Jiménez-Domínguez O. Solorza-Feria E. Ordóñez-Regil A. Cabral-Prieto S. Bulbulian 《Journal of Radioanalytical and Nuclear Chemistry》1993,174(2):291-298
Conditions for the electrodeposition of59Co and57Co on stainless steel foils have been studied. A set of electrolytic parameters has been determined to get a reasonably good electrodeposited sample. The57Co electrodeposited foil was then introduced in a quartz tube to be annealed in an argon flow. The final product was a 11 Ci Mössbauer source for college experiments which was succesfully tested by obtaining spectra of stainless steel and sodium nitroprusside. 相似文献
995.
M. Pospíšil 《Journal of Thermal Analysis and Calorimetry》1984,29(1):49-59
The reduction of mixed oxides NiO-CeO2 of various composition with hydrogen was studied at 290–470° by means of thermogravimetry. Some physicochemical properties of the system were also investigated and compared with earlier studied analogous samples of different “origin”. The degree of mutual interaction of the two components, the reduction kinetics and the sensitivity to the different sorts of radiation (gamma-rays, fast neutrons and electrons), applied in different doses, are strongly affected by the “pre-history” and by the composition in both compared series of mixed oxides. 相似文献
996.
When excesses of ammonium thiocyanate and tetrabutylammonium chloride are added to an iron(III) solution, a water-insoluble ion-pair is formed. This compound is soluble in chloroform and other organic solvents. The variables affecting extraction with CHCl3 are studied in order to obtain the optimal conditions and an extraction procedure is proposed. The separation of iron as a previous step to its spectrophotometric determination is studied. 相似文献
997.
An extraction-AAS method of determination of Co, Ni, and Mn in metallurgical copper materials containing considerable amounts ofCu, Fe, Pb, Zn, and Al has been developed. Good selectivity of group separation of Co, Ni, and Mn has been achieved by (a) the use of tetrabutylammonium bromide to improve extractability of HTTA complexes by ion-pair formation, and (b) the masking of major elements with sodium thiosulphate and sulphosalicylic acid. The extracts are stable for at least 3 weeks. 相似文献
998.
Marián Kucharík František Šimko Vladimír Danielik Miroslav Boča Roman Vasiljev 《Monatshefte für Chemie / Chemical Monthly》2007,138(12):1211-1215
Summary. The phase diagram of the system Na3AlF6–NaVO3 was determined by means of thermal analysis. The system is a simple binary eutectic one. The eutectic point was estimated
at x(NaVO3) = 0.975 and t
eut = 617°C. The XRD patterns of samples after thermal analysis revealed the presence of cryolite and NaVO3 only supporting the above assumption of a simple eutectic binary system. 相似文献
999.
Luís J. Moreno López Manuel Callejón Mochón Juán C. Jiménez Sánchez Alfonso Guiraúm Pérez 《Mikrochimica acta》1996,124(3-4):187-194
A method for the determination of salicylaldehyde (2 × 10–6–10–4
M) by differential-pulse polarography, based on the in situ formation of its Girard-P derivative in aqueous solution at pH 2.5, is proposed. The relative standard deviation was 1.5% (ten determinations of 4 × 10–5
M level). The applicability of this method was checked in synthetic samples containing salicyl alcohol,o-aminophenol, 2-methylphenol, salicylic acid and 4-aminobenzoic acid. 相似文献
1000.
Alvarez-Lueje A Valenzuela C Squella JA Núñez-Vergara LJ 《Journal of AOAC International》2005,88(6):1631-1636
In this work, a liquid chromatography stability-indicating method was developed and applied to study the hydrolytic behavior of simvastatin in different pH values and temperatures. The selected chromatographic conditions were a C18 column; acetonitrile-28 mM phosphate buffer solution, pH 4 (65 + 35) as the mobile phase; 251 degrees C column temperature; and flow rate 1 mL/min. The developed method exhibited an adequate repeatability and reproducibility (coefficient of variation 0.54 and 0.74%, respectively) and a recovery higher than 98%. Furthermore, the detection and quantification limits were 9.1 x 10(-7) and 2.8 x 10(-6) M, respectively. The degradation of simvastatin fitted to pseudo-first order kinetics. The degradation was pH dependent, being much higher at alkaline pH than at acid pH. Activation energy, kinetic rate constants (k) at different temperatures, the half life (t1/2) and the time for 10% degradation to occur (t90) values are also reported. 相似文献