排序方式: 共有73条查询结果,搜索用时 474 毫秒
21.
22.
23.
Spyros Nikas Nestor A. Rodios Anastasios Varvoglis Aristidis Terzis Catherine R. Raptopoulou 《Journal of heterocyclic chemistry》1996,33(3):997-999
The reaction between dimedone anion and trimethylsilylethynyl phenyliodonium triflate leads to the formation of 4,4-dimethyl-6-(1,5-bistrimethylsilylpentane-1,4-diyne-3-ylidene)-δ-valerolactone, whose crystal structure has been determined. 相似文献
24.
A novel on-line sequential injection (SI) dispersive liquid-liquid microextraction (DLLME) system coupled to electrothermal atomic absorption spectrometry (ETAAS) was developed for metal preconcentration in micro-scale, eliminating the laborious and time consuming procedure of phase separation with centrifugation. The potentials of the system were demonstrated for trace lead and cadmium determination in water samples. An appropriate disperser solution which contains the extraction solvent (xylene) and the chelating agent (ammonium pyrrolidine dithiocarbamate) in methanol is mixed on-line with the sample solution (aqueous phase), resulting thus, a cloudy solution, which is consisted of fine droplets of xylene, dispersed throughout the aqueous phase. Three procedures are taking place simultaneously: cloudy solution creation, analyte complex formation and extraction from aqueous phase into the fine droplets of xylene. Subsequently the droplets were retained on the hydrophobic surface of PTFE-turnings into the column. A part of 30 μL of the eluent (methyl isobutyl ketone) was injected into furnace graphite for analyte atomization and quantification. The sampling frequency was 10 h−1, and the obtained enrichment factor was 80 for lead and 34 for cadmium. The detection limit was 10 ng L−1 and 2 ng L−1, while the precision expressed as relative standard deviation (RSD) was 3.8% (at 0.5 μg L−1) and 4.1% (at 0.03 μg L−1) for lead and cadmium respectively. The proposed method was evaluated by analyzing certified reference materials and was applied to the analysis of natural waters. 相似文献
25.
Polychlorotrifluoroethylene (PCTFE) in the form of beads was applied, as packing material for flow injection on-line column preconcentration and separation systems coupled with flame atomic absorption spectrometry (FAAS). Its performance characteristics were evaluated for trace copper determination in environmental samples. The on-line formed complex of metal with diethyldithiophosphate (DDPA) was sorbed on the PCTFE surface. Isobutyl methyl ketone (IBMK) at a flow rate of 2.8 mL min−1 was used to elute the analyte complex directly into the nebulizer-burner system of spectrophotometer. The proposed sorbent material reveal, excellent chemical and mechanical resistance, fast adsorption kinetics permitting the use of high sample flow rates up to 15 mL min−1 without loss of retention efficiency. For copper determination, with 90 s preconcentration time the sample frequency was 30 h−1, the enhancement factor was 250, which could be further improved by increasing the loading (preconcentration) time. The detection limit (3s) was cL = 0.07 μg L−1, and the precision (R.S.D.) was 1.8%, at the 2.0 μg L−1 Cu(II) level. For lead determination, the detection limit was cL = 2.7 μg L−1, and the precision (R.S.D.) 2.2%, at the 40.0 μg L−1 Pb(II) level. The accuracy of the developed method was evaluated by analyzing certified reference materials and by recovery measurements on spiked natural water samples. 相似文献
26.
Harry Joe Haijun Li Aristidis K. Nikoloulopoulos 《Journal of multivariate analysis》2010,101(1):159-270
Tail dependence and conditional tail dependence functions describe, respectively, the tail probabilities and conditional tail probabilities of a copula at various relative scales. The properties as well as the interplay of these two functions are established based upon their homogeneous structures. The extremal dependence of a copula, as described by its extreme value copulas, is shown to be completely determined by its tail dependence functions. For a vine copula built from a set of bivariate copulas, its tail dependence function can be expressed recursively by the tail dependence and conditional tail dependence functions of lower-dimensional margins. The effect of tail dependence of bivariate linking copulas on that of a vine copula is also investigated. 相似文献
27.
A new time-based flow injection on-line solid phase extraction method for chromium(VI) and lead determination using flame atomic absorption spectrometry was developed. The use of hydrophobic poly-chlorotrifluoroethylene (PCTFE)-beads as absorbent in on-line preconcentration system was evaluated. Effective formation of ammonium pyrrolidine dithiocarbamate complexes and subsequently retention in PCTFE packed column, was achieved in pH range 1.0-1.6 and 1.5-3.2 for Cr(VI) and Pb(II) ions, respectively. The sorbed analyte was efficiently eluted with isobutyl-methyl-ketone for on-line FAAS determination. The proposed packing material exhibited excellent chemical and mechanical resistance, fast kinetics for adsorption of Cr(VI) and Pb(II) permitting the use of high sample flow rates at least up to 15 mL min−1 without loss of retention efficiency. For a preconcentration time of 90 s, the sample frequency was 30 h−1, the enhancement factor was 94 and 220, the detection limit was 0.4 and 1.2 μg L−1, while the precision (R.S.D.) was 1.8% (at 5 μg L−1) and 2.1% (at 30 μg L−1) for chromium(VI) and lead, respectively. The applicability and the accuracy of the developed method were estimated by the analysis spiked water samples and certified reference material NIST-CRM 1643d (Trace elements in water) and NIST-SRM 2109 (chromium(VI) speciation in water). 相似文献
28.
29.
Y.O. Ciftci X.F. Wang P. Vasilopoulos 《Physica E: Low-dimensional Systems and Nanostructures》2005,30(1-2):74-80
We study spin transport of holes through microstructures modulated periodically by diluted magnetic semiconductor (DMS) sections, stubless or stubbed. The stubs are symmetric or asymmetric and the magnetizations of consecutive DMS sections are parallel or antiparallel. The transmission coefficients of holes with spin up (T+) or down (T-) are drastically different since the spins feel different potential profiles in the DMS sections. As a result, nearly square-wave patterns, or wide plateaus and oscillations, can be obtained for the transmission and the spin polarization as functions of the incident energy or of various parameters of the periodically repeated unit. Results for simple and composite units with and without deviations from perfect periodicity are reported. Some of the structures considered exhibit a strong spin-filtering behavior. 相似文献
30.
Jie Chen Prof. Xue‐Feng Wang Prof. Panagiotis Vasilopoulos An‐Bang Chen Prof. Jian‐Chun Wu 《Chemphyschem》2014,15(13):2701-2706
A non‐equilibrium Green’s function technique combined with density functional theory is used to study the spin‐dependent electronic band structure and transport properties of zigzag silicene nanoribbons (ZSiNRs) doped with aluminum (Al) or phosphorus (P) atoms. The presence of a single Al or P atom induces quasibound states in ZSiNRs that can be observed as new dips in the electron conductance. The Al atom acts as an acceptor whereas the P atom acts as a donor if it is placed at the center of the ribbon. This behavior is reversed if the dopant is placed on the edges. Accordingly, an acceptor–donor transition is observed in ZSiNRs upon changing the dopant’s position. Similar results are obtained if two silicon atoms are replaced by two impurities (Al or P atoms) but the conductance is generally modified due to the impurity–impurity interaction. If the doping breaks the twofold rotational symmetry about the central line, the transport becomes spin‐dependent. 相似文献