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排序方式: 共有439条查询结果,搜索用时 15 毫秒
1.
The performance of single-drop microextraction (SDME), coupled with gas chromatography/mass spectrometry, was assessed for the determination of tributyltin compounds in water and solid samples. Experimental parameters impacting the performance of SDME, such as microextraction solvent and sampling and stirring time, were investigated. Analytical results obtained by SDME were compared with those generated by conventional solid phase microextraction (SPME) and liquid-liquid extraction (LLE) for the determination of TBT in PACS-2 sediment certified reference material (CRM). 相似文献
2.
A. Sionkowska M. Wisniewski G.F. Poggi C.A. Maxwell 《Polymer Degradation and Stability》2006,91(12):3026-3032
The thermal and mechanical properties of collagen/chitosan blends before and after UV irradiation have been investigated using thermal analysis and mechanical (Instron) techniques. Comparisons were made with the thermal and mechanical properties of both collagen and chitosan films. Air-dried collagen, chitosan and collagen/chitosan films were exposed to UV irradiation (wavelength 254 nm) for different time intervals. Thermal properties of collagen/chitosan blends depend on the composition of the blend and are not significantly altered by UV irradiation.Mechanical properties such as ultimate tensile strength and ultimate percentage of elongation were much better for collagen films than for collagen/chitosan films. The results have shown that the mechanical properties of the blends were greatly affected by the duration of UV irradiation. Ultimate tensile strength and ultimate percentage elongation decreased after UV irradiation of the blend. Increasing UV irradiation leads to an increase in Young's modulus of the collagen/chitosan blend. 相似文献
3.
The complexes CoH(PF3)4?n (PPh3)n (n = 1–3) have been prepared by low from the reaction between CoH(PF3)(PPh3)3 and butadiene. The hydrido complexes are active catalysts for the isomerisation of 1-octene to 2-octene under hydrogen or nitrogen. 相似文献
4.
A series of neutral porphyrin-containing catenanes has been synthesised, consisting of a zinc porphyrin strapped by a polyethylene glycol chain containing four or six ethylenoxy-units and incorporating a central naphthoquinol unit, interlinked with a naphthalene diimide macrocycle. The napthalene diimide precursor units exhibit only weak binding with the strapped porphyrins (Ka between 8 and 0.02 M(-1)), but good yields of the catenanes were obtained by Glaser coupling of the alkynyl napthalene diimide precursors in the presence of the porphyrins. Structures and solution conformations were determined by mass spectral and detailed 1H NMR studies. For the longer strapped porphyrins, the diimide macrocycle rotates around the central naphthoquinol unit at 420-450 times per second, while rotation is virtually prevented in the tighter strapped derivatives. A second dynamic process occurring in both sets of catenanes and described as 'yawing' leads to inequivalence in the naphthalene moieties. UV-Visible spectra indicate charge transfer interactions and electronic communication between the two components of the catenane. 相似文献
5.
Garcia Ruano JL Alcudia A del Prado M Barros D Maestro MC Fernandez I 《The Journal of organic chemistry》2000,65(10):2856-2862
The addition of the lithium anions derived from (R)- and (S)-methyl and -ethyl p-tolyl sulfoxides to (S)-N-benzylidene-p-toluenesulfinamide provides an easy access route to enantiomerically pure beta-(N-sulfinyl)amino sulfoxides. Stereoselectivity can be achieved when the configurations at the sulfur atoms of the two reagents are opposite (matched pair), thus resulting in only one diastereoisomer, even for the case in which two new chiral centers are created. The N-sulfinyl group primarily controls the configuration of the carbon bonded to the nitrogen, whereas the configuration of the alpha-sulfinyl carbanion seems to be responsible for the level of asymmetric induction, as well as for the configuration of the new stereogenic C-SO carbon in the reactions with ethyl p-tolyl sulfoxides. An efficient method for transforming the obtained beta-(N-sulfinyl)amino sulfoxides into optically pure beta-amino alcohols, based on the stereoselective non-oxidative Pummerer reaction, is also reported. 相似文献
6.
7.
A new method of electrochemical analysis has been devised in which the metallic ions present in an electrolyte are discharged into a mercury cathode. The amalgam electrode so formed is then combined with a standard reference electrode in a suitable dissolution medium and the cell thus produced is allowed to supply current against an opposing voltage set up on a potentiometer. By decreasing the potentiometer voltage in stages a curve of current against potential can be constructed for the dissolution of the amalgam anode; in this curve each metal gives rise to a characteristic wave from the height of which the amount of metal originally present can be derived. Although the method is not so universally applicable as the conventional polarographic technique with the dropping mercury cathode, it possesses advantages in sensitivity and simplicity in suitable cases and may form a useful complementary method of analysis. 相似文献
8.
ALI Rahmani Christian B. Eckardt Richard G. Brereton James R. Maxwell 《Photochemistry and photobiology》1993,57(S1):1048-1052
Abstract— This paper reports a new method for monitoring the allomerization reactions of chlorophyll a and pheo-phytin a. Complex mixtures are generated from illuminating pure compounds and monitored using both diode array high-performance liquid chromatography (DAD-HPLC) and liquid chromatography-mass spectrometry (LC-MS). LC-MS allows molecular weight and fragment ion analysis of significant HPLC peaks. Five products of the degradation of chlorophyll a can be simultaneously detected in a mixture, namely the monohydroxy allomer, the methoxylactone allomer, pheophytin a and the two corresponding allomers of the pheophytin. It is demonstrated that more than one pathway must be involved in the in vitro photodegradation of chlorophyll a as shown by the simultaneous existence of several intermediates. 相似文献
9.
Mogon Patel Anthony C. Swain Robert S. Maxwell 《Polymer Degradation and Stability》2006,91(3):548-554
Poly(m-carboranyl-siloxane) elastomers containing a mixture of di-methyl- and methylphenyl-silyl units were synthesised using the ferric chloride catalysed condensation reaction between di-chloro-diorganosilane and bis(di-methylmethoxysilyl)-m-carborane. These elastomeric materials were originally developed to have greater stability to extreme thermal environments and retain tailorable physical and chemical properties relative to comparable non-carborane containing elastomers. Prepared samples were aged either by heating in air at elevated temperatures or by gamma irradiation from a 60Co source. Multinuclear (1H, 13C and 11B) solid and solution state nuclear magnetic resonance (NMR) was used to assess degradation. This included measurements of segmental chain dynamics using a solid-echo pulse sequence reflecting changes in crosslink density and assessing changes to the carborane fragment by 11B and 1H Magic Angle Spinning (MAS) methods. Thermogravimetric measurements were also performed to assess thermal stability. Gamma radiation (to a dose of 1 MGy) was found to induce only a small degree of elastomer hardening as evidenced by a reduction in segmental chain dynamics. The carborane cage however, remained intact at these dose levels. Thermal degradation was observed to lead to oxidative crosslinking, the degree of which is dependent on temperature. At temperatures below 350 °C, only small changes in segmental dynamics were observed commensurate with only minor weight loss at this temperature. At temperatures above 350 °C, the degradation of the elastomer increased dramatically with decreased segmental dynamics and presumed partial oxidation of the carborane cage. The integrity of the m-carborane cage and the segmental dynamics were found to be significantly reduced at temperatures above 580 °C, in line with the known cage rearrangement temperature for icosahedral carboranes. 相似文献
10.