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991.
Barro R Ares S Garcia-Jares C Llompart M Cela R 《Analytical and bioanalytical chemistry》2005,381(1):255-260
A combination of sorbent enrichment and ultrasound-assisted solvent extraction has been used to determine polychlorinated biphenyls in air. Analytes were sampled by pumping a known volume of air through a porous polymer (Tenax TA). The enriched adsorbent was transferred into a glass vial, and ultrasound-assisted extraction of the analytes was then performed in n-hexane. Quantification was carried out by using gas chromatography coupled to tandem mass spectrometry. Breakthrough volume of the sampling step was studied, indicating that 10 m3 of air could be processed without losses of the most volatile compounds. Good recoveries (75–96%) were obtained, and limits of detection at the sub ng m–3 were achieved for all the analytes. The proposed method is very simple and fast, avoiding the use of large solvent volumes and time-consuming preconcentration steps. 相似文献
992.
α-, β-, and γ-cyclodextrins (CDs) marketed by five different companies were characterized from the thermal and structural
point of view. Three αCD samples showed two-step DSC dehydration profiles and their XRD patterns were characteristic for αCD⋅6H2O form I, whereas one brand with an apparent three-step DSC dehydration behaviour was a mixture of αCD⋅6H2O form I and anhydrous αCD. The differences in the DSC profiles after dehydration and EGA onset decomposition temperatures
recorded for the five βCD brands were attributed to different manufacturing and purification processes. The five γCDs brands
showed a common thermal behaviour and very similar XRD patterns. The patterns did not match the idealized pattern of γCD⋅14.1H2O, indicating the occurrence of two different hydrated crystal structures.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
993.
Karel Strnský Tom Jursík Antonín Vítek Jií Skoepa 《Journal of separation science》1992,15(11):730-740
A method has been developed for calculating more accurate and reproducible equivalent chain length (ECL) values of 33 straight-chain fatty acids (FA), with various positions and numbers of cis double bonds, chromatographed on DB-Wax and DB-1 columns. The dependence of ECL values on the position and the number of double bonds is described, as is a method of utilizing these relationships for the characterization of FA by gas chromatography. 相似文献
994.
Mojumdar S. C. Kozánková J. Chocholoušek J. Majling J. Nemecek V. 《Journal of Thermal Analysis and Calorimetry》2004,78(1):145-152
The conditions to fabricate the bulk porous specimens have been studied on account of sodium borosilicate (NBS) glasses. Glass
composition, heat treatment at phase separation and TiO2 addition have been considered in this study. Original glass samples of composition in mol%: sample A: 9.19 Na2O - 23.58 B2O3 - 67.23 SiO2, sample B: 9.29 Na2O - 3.17 TiO2 - 23.82 B2O3 - 63.72 SiO2 were prepared by melting reagent grade chemicals (Na2CO3, HBO3, SiO2 and AgNO3) in platinum crucibles at 1480°C for 1 h in air. The melts were poured onto stainless steel plates and were annealed at 500°C
for 0.5 h after cooling. Thus, obtained samples were phase separated at 700°C for 2, 15, 25 and 50 h to study their microstructure
by scanning electron microscope (SEM). Besides the direct study of the microstructure by SEM, information on glass structural
changes of samples are provided by measuring in situ changes by the optical transmittance thermal analysis. The isothermal
measurements were carried out at 700, 720 and 740°C. The temperature of phase separation, the leaching and nucleator addition
(TiO2), significantly influence the microstructure of the resulting leached product. TiO2 additive seems to suppress crystallization of cristobalite: especially at the extended above heat treatment phase separation
runs. The phase-separated domains of glasses containing above 80 moles of SiO2 are so small that it is very hard to observe them by SEM. The glass composition in our case was selected in a way to have
relatively large phase separated areas easily observed by SEM at magnification 20 000·. The influence of TiO2 is not too pronounced. It seems to suppress the cristobalite crystallization, especially of longer heating runs. The image
analysis of leached glasses shows the prevailing content of the skeletal phase in a comparison to pores. The TiO2 content diminishes the content of the skeletal phase.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
995.
测定核黄素的化学发光新方法 总被引:4,自引:0,他引:4
研究了核黄素的化学发光性质,建立了一种测定核黄素的化学发光新方法,并应用于实际样品中核黄素的测定,与文献相比,本方法无需避光,而且分析体系和样品处理简单;干扰小,牛血清白蛋白、抗坏血酸、天冬氨酸等8种生物有机物质和 Cu(Ⅱ)、Zn(Ⅱ)、Fe(Ⅲ)等15种常见金属离子的允许量较大;核黄素溶液浓度在0. 10~50 mg/L范围内,与化学发光强度成正比,方法线性范围宽;测定1.0mg/L核黄素溶液12次,求得相对标准偏差为2.9%,方法精密度高;方法的检测限为0.075 mg/L,灵敏度高。 相似文献
996.
Ph. Quevauviller 《Journal of chromatography. A》1996,750(1-2):25-33
The awareness of a need for an improved control of environmental contamination levels has led to the development of new hyphenated techniques for the determination of a wide variety of chemical species (e.g., organotins, methyl-mercury, alkyl-lead compounds etc.). These techniques generally involve many analytical steps such as extraction, derivatisation, separation and detection which have to be carried out in such a way that the speciation is not changed during the abalytical process. The need for evaluating the method's performance has led the BCR programme of the European Commission (now Standards, Measurements and Testing programme) to conduct series of interlaboratory studies during the last decade. These projects followed a step-by-step approach for the evaluation of different steps of the analytical methods used, e.g., simple solutions to test the detection, cleaned extract to evaluate the separation, spiked samples to test the extraction and natural samples to evaluate the whole analytical procedures. These collaborative projects allowed most of the sources of errors related to either a technique or a laboratory to be detected and removed. This paper gives an account of discussions of possible errors occurring in speciation analysis and presents examples of technical scrutiny of hyphenated techniques using chromatography as applied to the determination of tributyltin, methyl-mercury and trimethyllead. 相似文献
997.
V. A. Dorokhoy L. G. Vorontsova M. G. Kurella L. I. Parfenova 《Russian Chemical Bulletin》1996,45(11):2592-2594
A bis-diphenylboron chelate has been synthesized from 3-(diaminomethylene)pentane-2,4-dione through the action of (butylthio)diphenylborane and its molecular and crystal structures have been determined. The two boron -containing rings have the same six-membered skeleton but differ substantially in conformation and geometric parameters.Translated fromlzvestiva Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2733–2735, November, 1996, 相似文献
998.
Jorge Moreda-Pieiro Carmen Moscoso-Prez María Pieiro-Iglesias Purificacin Lpez-Mahía Soledad Muniategui-Lorenzo Esther Fernndez-Fernndez Darío Prada-Rodríguez 《Talanta》2007,71(5):1834-1841
A novel, rapid and simple method by hydride generation-electrothermal atomic absorption spectrometry (HG-ETAAS) after direct As, Bi, Sb and Sn hydrides generation from untreated filters of atmospheric particulate matter (PM10 and PM2.5) was optimised. PM10 and PM2.5 were not subjected to any pre-treatment: circular portions between 0.28 and 6.28 cm2 were directly placed into the reaction vessel of a batch mode generation system. A 28 × 3/64 Plackett–Burman design was used as a multivariate strategy for the evaluation of the effects of several variables affecting the hydride generation, trapping and atomisation efficiencies. Trapping temperature was the most statistically significant variable for As, Bi and Sn. Atomisation temperature was also statistically significant for Sb determination. Optimum values of significant variables were selected by using univariate optimisation approaches. An aqueous calibration method was used throughout. The developed method has been found to be precise with relative standard deviations of 6.2, 5.3, 9.1 and 7.5% for 11 determinations in a filter sample containing 0.7, 1.0, 1.4 and 1.7 μg l−1 for As, Bi, Sb and Sn, respectively. Results obtained by direct solid sampling-HG-ETAAS have been found statistically comparable with those obtained after conventional method based on an acid digestion followed to ICP-MS. Absolute detection limits were 37, 15, 30, and 41 ng l−1 for As, Bi, Sb and Sn, respectively. Detection limits referred to the air volume sampled (in the range of 0.020–0.050 ng m−3) were low enough for the determination of several hydride-forming elements from PM10 and PM2.5 samples collected in a non-polluted suburban area of A Coruña (NW Spain). 相似文献
999.
1000.
Five new representatives of the cembrane class of marine natural products have been isolated as minor metabolites from an undescribed species of Eunicea, a sea whip collected near the Colombian Southwestern Caribbean Sea. The structure of the crystalline metabolite 1 was solved by single-crystal X-ray diffraction analysis. Structures could then be proposed for cembradienes 2-5 by comprehensive spectral analyses involving 2D NMR, IR, UV, and high-resolution mass spectrometry, as well as chemical interconversion studies. Compound 1 exhibited weak cytotoxicity in the NCI 3-cell line panel human cancer screening program, whereas compounds 1, 2 and 4 displayed significant antiplasmodial activity against Plasmodium falciparum. 相似文献