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941.
Filippi A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(21):5396-5403
The model reaction between the (R)-1,3-dimethyl-1-cyclohexyl cation (I) and methanol has been investigated under gas-phase radiolytic conditions (750 Torr; 25-120 degrees C) with the aim of evaluating the intrinsic factors that govern the facial selectivity of biased carbocations. The peculiarity of the experimental approach allows the formation of different CH(3) (18)OH.I ionic adducts. Subsequent conversion of these adducts to give the corresponding E/Z covalent products follows different reaction coordinates, which are characterized by their own activation parameters. On the grounds of density functional theory (DFT) results, several [CH(3)OH.I] structures have been located on the relevant potential-energy surface (PES). The experimental results point to a gas-phase facial selectivity, which is mainly governed by entropic factors that arise as a result of the occurrence of different noncovalent ion-molecule "facial adducts" (FA). The formation of FAs may also play an important role in both the reaction dynamics and the positional selectivity. The present results cannot be interpreted by any of the models based on solution-phase experiments. 相似文献
942.
Anna Rakowska Robert Filipek Krzysztof Sikorski Marek Danielewski Renata Bachorczyk 《Mikrochimica acta》2004,145(1-4):183-186
The interdiffusion process in thin and thick (500nm–60µm) Au–Ni layers deposited on different substrates is studied using the EDS technique. In-depth X-ray analysis based on the Pouchou and Pichoir method is applied for obtaining the concentration profiles in nanometre scale multi-layers. A theoretical analysis using the Darken method is employed for modelling interdiffusion in the Au–Ni system. Computer simulations, where intrinsic diffusivities of the Au and Ni are functions of composition, are presented and compared with experimental results. 相似文献
943.
A very simple, selective and sensitive spectrophotometric method for the determination of dodecyl benzene sulfonate (DBS) has been investigated. In this method, the ion pair of DBS and methylene blue was adsorbed onto plasticized poly(vinylchloride) membrane as a solid phase extraction medium. The absorbance of the blue membrane was measured at 660nm against a blank membrane using a spectrophotometer. The influence of type and amount of plasticizer, pH, concentration of methylene blue, temperature of sample solution and time of extraction on the absorbance of membrane was studied. Linear calibration was obtained over a concentration range of 0.1 to 6.5µgmL–1 of DBS. The method was applied to the determination of DBS in different commercial detergents. The precision and accuracy of the method is comparable with the conventional standard method. This method is cost-effective, and the use of organic solvent such as chloroform is completely eliminated. 相似文献
944.
Improvement of cyclosporin A determination in whole blood by reversed-phase high-performance liquid chromatography 总被引:1,自引:0,他引:1
A chromatographic method was developed for the determination of cyclosporin A in human whole blood using reversed-phase HPLC at room temperature. Most previous reports carried out this liquid chromatographic separation at temperatures above 70 degrees C. The present procedure greatly improves the detection limit by controlling peak broadening effects, as well as the lifetime of the column at room temperature. Under optimal conditions and using ketoconazole as an internal standard, the calibration graph was linear in the range of 16-1000 microg/L with a relative standard deviation of 3.72% at 150 microg/L and 2.45% at 300 microg/L (n = 11) of cyclosporin A. The detection limit was of 5.0 microg/L cyclosporin A. By this procedure, cyclosporin A pharmacokinetic parameters in healthy Chinese subjects were studied. The developed method could be applied to the quantification of cyclosporin A in human blood samples and allows the study of its pharmacokinetics in routine laboratories. 相似文献
945.
Dimethylcarbonate (DMC) is a valuable methylating reagent that can replace methyl halides and dimethylsulfate in the methylation of a variety of nucleophiles. It couples tunable reactivity and unprecedented selectivity towards mono-C- and mono-N-methylation. In addition, it is a prototype example of a green reagent, because it is nontoxic, is made by a clean process, is biodegradable, and reacts in the presence of a catalytic amount of base, thereby avoiding the formation of undesirable inorganic salts as by-products. Depending on the reaction conditions, DMC can be reacted under plug-flow, CSTR, or batch conditions. Other remarkable reactions are those where DMC behaves as an oxidant. The reactivity of other carbonates is reported as well. 相似文献
946.
Lancelot N Elbayed K Raya J Piotto M Briand JP Formaggio F Toniolo C Bianco A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(6):1317-1323
A tetra- and a hepta-homopeptide from the C(alpha)-tetrasubstituted Aib (alpha-aminoisobutyric acid) residue were covalently linked to the POEPOP resin by the fragment-condensation approach. The conformational preferences of the two model peptides were determined for the first time on a solid support by means of high-resolution magic angle spinning NMR spectroscopy. The results obtained indicate that the Aib homopeptides adopt a regular 3(10)-helical structure even when they are covalently bound to a polymeric matrix, and thus confirm the remarkable conformational stability of the peptides rich in this amino acid. An ATR-FTIR spectroscopic investigation, performed in parallel, also confirmed that these polymer-bound peptides do indeed adopt a helical conformation. The results of this study open the possibility to exploit the peptide-resin conjugates based on C(alpha)-tetrasubstituted alpha-amino acids as helpful, structurally organized templates in molecular recognition studies or as catalysts in asymmetric synthesis. 相似文献
947.
Summary The separation of hydroxycinnamic acids has been investigated by high-pressure liquid chromatography using a micro C18 bonded phase. Irradiation oftrans-ferulic and sinapic acids readily gave a mixture ofcis-trans isomers which can be also separated with the system described. 相似文献
948.
Summary The theory of the evaporation of the liquid stationary phase is elaborated and experimentally verified. On the basis of this theory the role played by the losses in the amount of liquid phase present is quantitatively determined. General techniques are examined which minimize the losses; these techniques are based on saturating the incoming carrier gas with liquid phase vapours and raising the pressure of the carrier gas in the column, e.g. by connecting a capillary to the column outlet in order to offer resistance to the gas flow. The application of these techniques ensures stable performance of the gas chromatographic columns using a volatile liquid phase. 相似文献
949.
以3-脲丙基三甲氧基硅烷为偶联剂,制备了一种新型极性脲丙基-C30(TPU-C30)反相色谱固定相。采用扫描电子显微镜、元素分析、红外光谱和热分析等对该固定相进行表征。结果表明,TPU-C30固定相已成功制备,连续制备3次固定相,其元素含量的相对偏差均小于5%,说明该合成工艺重复性良好。以不同极性、位置异构、碱性化合物为溶质探针,以传统的C18色谱柱与C30色谱柱为参比,对制备的固定相的色谱性能进行了研究。研究结果表明,TPU-C30固定相具有不同于传统C18柱、C30柱的选择性和更优的择形性,明显改善了碱性物质的峰形,其具有广阔的应用空间。 相似文献
950.
M. Jochum R. Bakry I. Wartusch C. W. Huck H. Engelhardt G. K. Bonn 《Chromatographia》2002,56(5-6):263-268
Summary This paper reports on the use of a polymer-based, strong anion-exchange stationary phase for rapid, selective and sensitive
analysis of physiological important mono-, di- and oligosaccharides by high-performance anion-exchange chromatography (HPAEC)
with pulsed amperometric detection (PAD) under alkaline conditions. The adsorbent was obtained by direct nitration of 3 and
5 μm, spherical non-porous highly cross-linked, styrene-divinylbenzene copolymer (PS-DVB) beads, followed by reduction of
superficially introduced nitro groups with nascent hydrogen and quaternization of the resultant amino groups with iodomethane.
Extended exposure to high pressure and strong alkaline conditions did not have any untoward effect on mechanical stability
and chromatographic performance. A comparison of the 3 and 5 μm beads showed, that the synthesized 3 μm highly cross-linked
PS-DVB particles are the preferred phases for the separation of monosaccharides and the 5 μm particles are preferable for
the separation of oligosaccharides. To demonstrate the suitability for the analysis of complex samples, the optimized and
validated system was used for the determination of glucose, fructose and sucrose in apple juice and other soft drinks such
as Coca Cola. Finally, analysis within a few minutes without sample pretreatment down to a lower limit of detection of 0.174–0.504
μg mL−1 at a linearity with R2>0.994 and a repooducibility higher than 98% further confirmed the efficiency of these polymeric sorbents. 相似文献