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排序方式: 共有318条查询结果,搜索用时 15 毫秒
71.
A novel, simple and inexpensive approach to absorptive extraction of organic compounds from environmental samples is presented. It consists of a polydimethylsiloxane rod used as an extraction media, enriched with analytes during shaking, then thermally desorbed and analyzed by GC-MS. Its performance was illustrated and evaluated for the enrichment of sub- to ng/l of selected chlorinated compounds (chlorobenzenes and polychlorinated biphenyls) in water samples. The new approach was compared to the stir bar sorptive extraction performance. A natural ground water sample from Bitterfeld, Germany, was also extracted using both methods, showing good agreement. The proposed approach presented good linearity, high sensitivity, good blank levels and recoveries comparable to stir bars, together with advantages such as simplicity, lower cost and higher feasibility. 相似文献
72.
Fernández de la Pradilla R Manzano P Montero C Priego J Martínez-Ripoll M Martínez-Cruz LA 《The Journal of organic chemistry》2003,68(20):7755-7767
A novel route to enantiopure densely functionalized epoxy sulfinyl tetrahydrofurans, based on the unexpected and highly stereoselective remote nucleophilic epoxidation of hydroxy 1-sulfinyl butadienes with t-BuOOK, followed by ring closure and subsequent epoxidation of the resulting sulfinyl dihydrofurans, is described. Alternatively, the treatment of these dienes with m-CPBA followed by acid-catalyzed cyclization gives rise to related sulfonyl dihydrofurans in high yields but with low selectivity. The stereochemical outcome of the nucleophilic epoxidation of these substrates has also been studied. 相似文献
73.
74.
Carmen Ramírez M. Rico L. Barral J. Díez S. García-Garabal B. Montero 《Journal of Thermal Analysis and Calorimetry》2007,87(1):69-72
The thermal behaviour of an epoxy resin cured with an amine-POSS was studied using differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. The kinetic of polymerization reaction and the thermal degradation have been analyzed based on an iso-conversional model. The obtained results showed that the activation energies of both processes depend on the degree of conversion. 相似文献
75.
Merino S Domènech O Díez-Pérez I Sanz F Montero MT Hernández-Borrell J 《Colloids and surfaces. B, Biointerfaces》2005,44(2-3):93-98
Atomic force microscopy (AFM) was used to study the influence of a membrane protein, lactose permease of Escherichia coli (LacY), on the surface spreading behavior and the features of self-assembled phospholipids bilayers on mica. The miscibility of phospholipids used, 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), was investigated by surface pressure area isotherm measurements at the air-water interface. A composition with an equimolar proportion of POPC and DMPC was used to form the liposomes. Surface layers formed with DMPC:POPC (0.5:0.5, mol/mol) or LacY reconstituted in proteoliposomes with the same phospholipid composition were imaged by using AFM. When lactose permease was reconstituted in DMPC:POPC (0.5:0.5, mol/mol), self-assembled structures that remained firmly adsorbed onto the mica surface were observed. These sheets had an irregular shape and their upper layer was more corrugated than that obtained for the phospholipid matrix. 相似文献
76.
Journal of Algebraic Combinatorics - Given any irreducible Coxeter group C of hyperbolic type with nonlinear diagram and rank at least 4, whose maximal parabolic subgroups are finite, we construct... 相似文献
77.
Raine Garrido Arteaga Felix Cardoso San Jorge María del C. Rodríguez Montero Vicente Vérez Bencomo Herman Vélez Castro 《Magnetic resonance in chemistry : MRC》2012,50(8):525-528
Quimi‐Hib is a conjugate vaccine against Haemophilus influenza type b (Hib) where the Hib antigen is the only one produced by chemical synthesis. NMR has become the alternative of choice for the identity of intermediates during the chemical synthesis of Hib antigen. We explore a rapid quantitative proton magnetic resonance (qHNMR) assay for the determination of N,N‐dimethylformamide (DMF) as a residual in one of the critical intermediates. The proposed assay has been shown to be accurate, precise for intermediate precision conditions (relative standard deviation <3% for spectrometer‐to‐spectrometer variations), specific (no detected interferences), and rugged (percentage difference <3% for day‐to‐day and spectrometer‐to‐spectrometer variations). The quantitative NMR assay can replace the common chromatographic methods for monitoring the DMF contents in one crucial step of the synthetic scheme. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
78.
Luis A. Montero Jos Molina Jürgen Fabian 《International journal of quantum chemistry》2000,79(1):8-16
Multiple minima of water cluster hypersurfaces are explored to find thermodynamic properties by means of the corresponding partition functions of their canonical distributions. The combination of semiempirical quantum chemical procedures for calculating the cluster energies in local minima of supermolecules and the statistical thermodynamics approach for both the evaluation of macroscopic association energies and the possible reduction by average of absolute errors intrinsic to the parametrized Hamiltonian are shown to provide an appropriate model for comparison between experimental and theoretical results. The method can explicitly take into account environmental effects due to intermolecular interaction. Water trimer and tetramer association energies of ?10.9 and ?14.1 kJ/mol obtained from virial coefficient calculations compare very well to the values of ?10.5 and ?16.4 kJ/mol, respectively, found for the theoretical association energies in this paper. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 79: 8–16, 2000 相似文献
79.
van Pinxteren MS Montero L Jäsch S Paschke H Popp P 《Analytical and bioanalytical chemistry》2009,393(2):767-775
A new approach for headspace sorptive extraction is presented and demonstrated for the determination of 12 chlorobenzenes
in water samples. It consists of a silicone tube (15-mm length) arranged around a stainless steel rod. This device is fixed
on a septum cap and exposed to the headspace of 50 mL of a salt-saturated water sample. After extraction (60-min optimized
extraction time), thermodesorption is carried out by direct insertion of the silicone tube into the thermodesorption-gas chromatography-mass
spectrometry system. Desorption of the analytes is performed at 250 °C for 5 min with a gas flow of 50 mL/min. Repeatability
(relative standard deviation 5–10%), extraction yields (9–46%), enrichment factors (129–657), and detection limits (0.002–0.012 μg/L)
were determined and four real water samples were analyzed with the headspace tube extraction. The results were verified by
standard addition. A comparison of headspace tube extraction with other headspace enrichment techniques underlined the high
extraction capacity of the proposed method. A big advantage of tube extraction is the low cost of the silicone material. The
tubes can be discarded after single use, avoiding carryover problems and cross-contamination.
Figure Scheme of the HS-tube extraction and thermodesorption system 相似文献
80.
Doménech O Merino-Montero S Montero MT Hernández-Borrell J 《Colloids and surfaces. B, Biointerfaces》2006,47(1):102-106
In this work, using atomic force microscopy (AFM), we have studied the influence of the temperature on the properties of the surface planar bilayers (SPBs) formed with: (i) the total lipid extract of Escherichia coli; (ii) 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPC) (1:1, mol/mol); and, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanol-amine (POPE) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) (3:1, mol/mol). According to the height profile analysis we performed, the height of the SPBs of DMPC:POPC were temperature dependent. Separated domains were observed in the SPBs of the POPE:POPG mixture and the E. coli lipid extract. The implication of those domains for the correct insertion of membrane proteins into proteoliposomes is discussed. 相似文献