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941.
Push-pull 2-alkylidene-4-oxothiazolidine vinyl bromides undergo efficient C(5) functionalization through DMSO-assisted carbon-bromine cleavage, followed by a bromine transfer-substitution (or elimination) sequence. A mechanism for this novel transformation is proposed. 相似文献
942.
为增强有机锗化合物的生物活性,合成了一系列有机锗氨基酸酯类倍半氧化物,并重点研究了它们的快原子轰击质谱(FABMS)。研究结果表明:该系列化合物的各碎片离子峰与其组成及结构均有很好的对应关系及很强的规律性 相似文献
943.
944.
Ziegler-Natta催化剂催化乙烯气相聚合 总被引:1,自引:0,他引:1
本文综述了乙烯气相聚合催化剂研究状况,列举了一些制备气相聚合催化剂有代表性的方法。同时对预聚合的作用及气相聚合与淤浆聚合表观动力学行为的不同性也作了简要介绍。 相似文献
945.
A SPME-HPLC-post-column fluorescent derivatization method for the direct determination of saxitoxin (STX), the most potent paralytic shellfish poisoning (PSP) toxin, in water has been developed. Commercially available SPME devices with 50 microm Carbowax templated resin (CW/TPR) coating was found to be able to pre-concentrate STX from aqueous media. A special pre-conditioning treatment of soaking the SPME coating in 0.1 M NaOH solution significantly improved the extraction efficiency. The optimal pH for the SPME process is 8.1 and the equilibration time is 40 min. The partition coefficient, K, of the distribution of STX between the SPME coating and the aqueous media was measured to be 2.99 +/- 0.04 x 10(3). Extracted toxin on the SPME stationary phase was difficult to be desorbed by the HPLC mobile phase under dynamic desorption mode. A static ion-pairing desorption technique using a desorption solvent mixture of 20 mM sodium 1-heptanesulfonate in 30% aqueous acetonitrile acidified with 50 mM sulfuric acid was developed to overcome this problem. The method detection limit and repeatability achieved by this SPME-HPLC method were 0.11 ng ml(-1) and 3.7%, respectively, with a sample volume of just 5 ml of water. This analytical method is adequate for the monitoring of the PSP toxin in fresh/drinking waters. However, serious interference was observed when this technique was applied to saline water samples. This is probably due to competition of sodium ions with the cationic STX for absorption into the SPME stationary phase. 相似文献
946.
Zeisler R 《Analytical and bioanalytical chemistry》2004,378(5):1277-1283
NIST maintains a portfolio of more than 1300 standard reference materials (SRM), more than a third of these relating to measurements in the biological and environmental fields. As part of the continuous renewal and replacement efforts, a set of new marine sediments has been recently developed covering organic and inorganic determinations. This paper describes the steps taken in sample preparation, homogeneity assay, and analytical characterization and certification with specific emphasis on SRM 2702 inorganics in marine sediment. Neutron activation analysis showed the SRM to be highly homogeneous, opening the possibility for use with solid sampling techniques. The certificate provides certified mass fraction values for 25 elements, reference values for eight elements, and information values for 11 elements, covering most of the priority pollutants with small uncertainties of only several percent relative. The values were obtained by combining results from different laboratories and techniques using a Bayesian statistical model. An intercomparison carried out in field laboratories with the material before certification illustrates a high commutability of this SRM.Electronic Supplementary Material Supplementary material is available in the online version of this article at 相似文献
947.
Resonance interaction in the deformation (1400 to 1500 cm?1) region of the Raman spectrum of n-C23 H48 and n-C44H90 at high pressures are reported. Implications of these results concerning the interpretation of the corresponding bands of the Raman spectrum of crystalline polyethylene are discussed. 相似文献
948.
949.
W. Schr?n A. Detcheva B. Dre?ler K. Danzer 《Fresenius' Journal of Analytical Chemistry》1998,361(2):106-109
Direct solid sampling graphite furnace AAS (SS-GF-AAS) provides an advantageous alternative to the conventional AAS which
requires wet digestion of the samples. This method is suitable for trace element determination in calcium fluoride and other
fluoride-containing samples. Matrix effects were studied by using calibration standards, certified reference materials with
different matrices and by means of three-dimensional calibration. 3D calibration is suitable for selection of calibration
samples for more reliable analyses. Cu, Pb, Cd, Zn, and Fe were determined by means of calibration with variation of sample
weight using certified reference materials and suitable fluoride-containing calibration samples.
Received: 25 July 1997 / Revised: 15 December 1997 / Accepted: 20 December 1997 相似文献
950.
Jo‐Nan Chen Fang‐Ming Hsu Hui‐Chun Wang Chung‐Wen Wu Pao‐Swu Cheng Wen‐Liang Tsai 《中国化学会会志》2006,53(4):931-938
Four series of compounds 11?50 containing terminal alicyclic rings such as cyclohexylmethyl, cyclopentylmethyl, cyclobutylmethyl, and cyclopropylmethyl rings were synthesized and their liquid crystal behavior studied. The ring size and the length of flexible alkoxy chain influence the phase formation in different ways. While the smaller ring and the shorter alkoxy chain tend to favor the formation of the N phase, the larger ring and the longer alkoxy chain tend to favor the formation of the SmC phase. All the compounds except 11 and 21 exhibit SmA phases. The widest temperature range of the N, SmA, and SmC phases are found in the compounds 41 , 46 , and 20 , respectively, which are 75 °C for 41 , 115 °C for 46 , and 100 °C for 20 . 相似文献