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941.
It is difficult to quantify NH4+ by ion chromatography in the presence of high concentrations of Na+ due to peak overlap. The Dionex IonPac CS15 column, which contains phosphonate, carboxylate, and 18-crown-6 functional groups, was originally developed to overcome this problem. We have found that the addition of 18-crown-6 to the eluent promotes improved peak resolution between Na+ and NH4+ even at concentrations as high as 60,000 to 1 using this column. Its use also improves the separation of alkali and alkaline earth metal and amine cations. Mobile phase 18-crown-6 increased the retention times of CH3NH3+, NH4+, and K+, and decreased the retention time of Sr2+. The retention times of Li+, Na+, Mg2+, Ca2+, (CH3)2NH2+, and (CH3)3NH+ were not affected. This method makes possible the direct analysis of ammonia from nitrogenase, the enzyme responsible for biological nitrogen fixation. The resolution of the NH4+ peak from the Na+ and Mg2+ peaks improved from zero resolution to values of 6.19 and 5.65, respectively. This technique considerably reduces the analysis time of NH4+ in the presence of high concentrations of Mg2+ and Na+ over traditional indophenol measurements. 相似文献
942.
1 INTRODUCTION The assembly of coordination networks is a field of increasing interest[1~4]. Research along this line is induced by the idea that coordination networks have potential technological applications such as optoele- ctronic devices and microporous materials for shape and size separation and catalysis[5~7]. In this paper, there has been current interest in using polycarboxy- late as anion linking groups to support stable poly- meric coordination open frameworks with transition m… 相似文献
943.
GAO Sheng li ** CHEN San ping HU Rong zu SHI Qi zhen Shaanxi Key Laboratory of Physico Inorganic Chemistry Department of Chemistry Northwest University Xi′an P.R. China 《高等学校化学研究》2003,19(2)
IntroductionNickel is an essential trace biological element.L-α- Amino acids are the structural units of pro-teins.L- α- Histidine is one of the eight species ofamino acids which have to be absorbed from foodbecause they are not synthesized by organism.Thus,the investigation on the complexation ofnickel and L -α- histidine is of considerable practicaland fundamental importance.For the nickel com-plexes of amino acids,more extensive work hasbeen carried out[1— 3 ] . However,the thermochem… 相似文献
944.
Thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) studies were carried out on gamma radiation synthesized polydiallyldimethylammonium chloride (PDADMAC). The polymer was found to undergo thermal degradation in two stages. The first stage showed a weight loss of 33% and the second stage showed a weight loss of 67%. The DSC thermogram shows two endothermic peaks corresponding to the two stages in the TG thermogram and the experimental enthalpy change associated with the first and second stages were 650 J g−1 and 129.5 J g−1, respectively. The nth-order kinetic parameters (order of the reaction, activation energy and the pre-exponential factor) were determined from a single dynamic DSC or thermogravimetric (TG) thermogram by the method of least square. Theoretical TG/differential thermogravimetric (DTG) and DSC thermograms derived from the calculated kinetic parameters were in good agreement with the experimental ones at the heating rate employed. However, the kinetic parameters determined using TG and DSC were different. This leads to the conclusion that the degradation mechanism could be complicated and may consists of a number of parallel or consecutive reactions. The glass transition temperature (Tg) of the polymer was found to be around 150 °C depending on the test method employed. 相似文献
945.
An efficient synthesis of 6-alkyl-3-cyano-2(1H)-pyridinethiones by the reactions of the sodium salts of -ketoaldehydes with cyanothioacetamide was developed. Pyridinethiones undergo selectiveS-alkylation with haloacetonitriles and haloacetophenones followed by cyclization to the corresponding thieno[2,3-b]pyridines.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 727–731, April, 1995.This study was financially supported by the Russian Foundation for Basic Research (Project No. 94-03-08-823). 相似文献
946.
Dissociative and nondissociative electron attachment in the electron impact energy range 0–14 eV are reported for SOF2 SOF4, SO2F2, SF4, SO2, and SiF4 compounds which can be formed by electrical discharges in SF6. The electron energy dependences of the mass-identified negative ions were determined in a time-of-flight mass spectrometer. The ions studied include F– and SOF
2
–*
from SOF2; SOF
3
–
and F– from SOF4; SO2F
2
–*
, SO2F–, F
2
–
, and F– from SO2F2; SF
4
–*
and F– from SF4; O–, SO–, and S– from SO2; and SiF
3
–
and F– from SiF4. Thermochemical data have been determined from the threshold energies of some of the fragment negative ions. Lifetimes of the anions SOF
2
–*
, SO2F
2
–*
, and SF
4
–*
are also reported. 相似文献
947.
《Tetrahedron letters》2005,46(16):2771-2774
Treatment of 3β-acetoxy-5-bromo-6β,19-epoxy-5α-androstan-17-one with Ac2O and BF3·OEt2, produced the cleavage of the epoxy moiety and migration of the bromine atom to afford 3β,19-diacetoxy-6α-bromo-5-hydroxy-5β-androst-17-one in high yield. 相似文献
948.
Korolev A. M. Eremenko L. T. Meshikhina L. V. Eremenko I. L. Aleksandrov G. G. Konovalova N. P. Lodygina V. P. 《Russian Chemical Bulletin》2003,52(8):1859-1863
New procedures were developed for the synthesis of 3,3-bis(hydroxymethyl)oxetane dinitrate (1) by O-nitration of the corresponding glycol (3) or its mononitrate (6), which were prepared by the reactions of 2,2-bis(hydroxymethyl)propane-1,3-diol (pentaerythritol) (2) mono- (4) and dinitrates (5), respectively, with alkali. A new method was devised for the synthesis of compounds 4 and 5 by the reaction of tetraol 2 with concentrated HNO3 in dichloroethane. The structures of compounds 1 and 6 were established by X-ray diffraction analysis. 相似文献
949.
Anand Pal Singh P. Ramos Cabrer E. Alvarez-Parrilla F. Meijide J. Vázquez Tato 《Journal of inclusion phenomena and macrocyclic chemistry》1999,35(1-2):335-348
In order to study its guest binding and the inclusion phenomena, 6-deoxy-6-(aminoethyl)amino--cyclodextrin (CDN) was synthesised and its binding properties examined. The complexation phenomena of sodium cholate (NaC) and sodium deoxycholate (NaDC) with CDN has been monitored by the NMR method using 13C chemical shift data. The method of continuous variation Job's method has been used to determine the stoichiometry of these supramolecular complexes. The Job's plot confirms the 1 : 1 supramolecular complex for NaC: CDN and the 1 : 2 supramolecular complex for NaDC: CDN. The interaction of NaC and NaDC with CDN has been obtained through two-dimensional Rotational Nuclear Overhauser Effect Spectroscopy (ROESY) NMR. Equilibrium constants were also obtained from 13C chemical shift data (C-1, C-3 & C-4) at different pH values (7, 9, & 11). 相似文献
950.
M. Chavanpatil F. D. Dawre D. S. Shakleya P. R. Vavia 《Journal of inclusion phenomena and macrocyclic chemistry》2002,44(1-4):145-149
The purpose of the present work was to investigate the effect ofcomplexation of rofecoxib with -cyclodextrin on its dissolutioncharacteristics and bioavailability. Inclusion complexes of rofecoxibwith -cyclodextrin were made by freeze-drying technique. Phasesolubility studies were conducted as suggested by Higuchi and Connors.The samples were characterized by performing dissolution studies, X-rayDiffraction studies and Differential Scanning Calorimetry. The complexeswere compressed into tablets and compared in-vitro with various marketedformulations. A single dose study on healthy human volunteers was performedin comparison with a marketed formulation of rofecoxib (without-cyclodextrin) to investigate the relative bioavailability.Phase solubility studies confirmed the formation of a 1 : 1complex in solution of rofecoxib with -cyclodextrin. Tablets ofsolid inclusion complexes of rofecoxib with -cyclodextrin preparedby freeze drying technique showed enhanced dissolution rate in distilledwater in comparison with all the marketed formulations analyzed. This isattributed to the increased solubility and wettability along with decreasedcrystallinity caused by complex formation, which is confirmed, by XRD and DSCstudies. The bioequivalence studies performed showed statistically significantenhancement in bioavailability as compared to the marketed formulation.Apparently, tablets containing complexes of rofecoxib with -cyclodextrinshows faster onset of action due to improved solubility, enhanced dissolutionand faster absorption of the molecule. The results of this investigation withrofecoxib in -cyclodextrin lend ample credence to its better oralbioavailability on complexation. 相似文献