排序方式: 共有12条查询结果,搜索用时 15 毫秒
1.
Kenneth B. Tomer Tarekegn Gebreyesus Carl Djerassi 《Journal of mass spectrometry : JMS》1973,7(4):383-393
The mass spectra of C13 labeled o- nitrobenzoic acid and o-nitrobenzaldehyde were studied. Earlier proposed fragmentations involving group migration to a charge-carrying vacant ortho position were verified. A completely unexpected interaction in o- nitroanisole was uncovered by O18labeling. 相似文献
2.
A hollow fiber supported liquid membrane extraction method for the liquid chromatographic determination of dinitrophenolic compounds at ppt levels has been developed. Different variables affecting the extraction process, such as extraction time, shaking speed, acceptor pH, acceptor buffer concentration, salt content and humic acids have been studied. Enrichment factors up to 7000 times were obtained. Validation of the method included calibration experiments and studies of the linearity of the responses in different matrices. Good linearity was obtained in the environmental matrices evaluated. Detection limits range from 6.0 to 8.0 ng/L, and the relative standard deviations do not exceed 7% in terms of repeatability. 相似文献
3.
Synthesis of the putative marine sterols [24R]- and [24S]-23,24-dimethyl-5α-cholest-23(29)-en-3β-ol.
The synthesis and stereochemistry of [24R]- and [24S]-23,24-dimethyl-5α-cholest-23(29)-en-3β-ol is reported; both isomers are different from a natural marine sterol to which the structure had been assigned. 相似文献
4.
David A. Jaeger Kenneth B. Tomer Paul D. Woodgate Tarekegn Gebreyesus Carl Djerassi Robert H. Shapiro 《Journal of mass spectrometry : JMS》1974,9(5):551-553
Corrected relative migratory aptitudes for aryl groups in the electron-impact-induced rearrangement of aryl phenyl carbonates have been determined. 相似文献
5.
Polyaniline-ZnO−NiO Nanocomposite Based Non-enzymatic Electrochemical Sensor for Malathion Detection
Samuel Chufamo Jikamo Tesfaye Haile Habtemariam Tarekegn Heliso Dolla 《Electroanalysis》2023,35(4):e202200317
An electrochemical sensor based on polyaniline-ZnO−NiO (PANI-ZnO−NiO) nanocomposite was developed for the non-enzymatic detection of malathion. The structure, surface morphology, and optical properties of the as-prepared nanocomposite were studied by XRD, FTIR, SEM, and UV-Vis spectroscopy. The electrochemical behavior of the nanocomposite based sensor was first evaluated through cyclic voltammetry (CV). Under optimum conditions, differential pulse voltammetry (DPV) was further utilized for malathion detection, which proved the PANI-ZnO−NiO/GCE electrode as an effective electrochemical sensor. The developed electrochemical sensor showed a low detection limit of 1.0×10−8 M with a wider linear range of 10 to 70 nM for malathion. 相似文献
6.
7.
Scherer M Lücke B Gebreyesus G Topic O Deuretzbacher F Ertmer W Santos L Arlt JJ Klempt C 《Physical review letters》2010,105(13):135302
Parametric amplification of quantum fluctuations constitutes a fundamental mechanism for spontaneous symmetry breaking. In our experiments, a spinor condensate acts as a parametric amplifier of spin modes, resulting in a twofold spontaneous breaking of spatial and spin symmetry in the amplified clouds. Our experiments permit a precise analysis of the amplification in specific spatial Bessel-like modes, allowing for the detailed understanding of the double symmetry breaking. On resonances that create vortex-antivortex superpositions, we show that the cylindrical spatial symmetry is spontaneously broken, but phase squeezing prevents spin-symmetry breaking. If, however, nondegenerate spin modes contribute to the amplification, quantum interferences lead to spin-dependent density profiles and hence spontaneously formed patterns in the longitudinal magnetization. 相似文献
8.
Tarekegn Berhanu Negussie Megersa Theodros Solomon 《International journal of environmental analytical chemistry》2013,93(13):933-945
A new design of equilibrium hollow fibre liquid phase microextraction (HF-LPME) was developed for the determination of three freely dissolved organophosphorus pesticides (OPPs), i.e. diazinon (O,O-diethyl-O-2-isopropyl-4-methyl-6-pyrimidyl thiophosphate), chlorpyrifos (O,O-diethyl-O-[3,5,6-trichloro-2-pyridyl] phosphorothioate), and fenthion (O,O-dimethyl-O-4-methylthio-m-tolyl phosphorothioate) as model compounds. In this new design a 1.2–1.4 cm length of a hollow fibre (HF), inserted to the end of 20 cm copper wire and impregnated with organic solvent, was used to extract the freely dissolved concentration of OPPs in various water samples. The limits of detection (LOD) in reagent water using gas chromatography-mass spectrometry in the selected ion monitoring (SIM) mode was in the range of 15–80 ng L?1. The relative standard deviations of the analysis (inter- and intra-day) were 8.7–30%. The method was applied to the extraction of spiked lake and ground water samples. The ground water sample was spiked at 0.1 and 0.2 µg L?1 concentrations of the analytes under study and the average extraction efficiency at the two concentrations was below 1% showing the non-depletive nature of the extraction, meaning that the freely dissolved concentrations are measured as opposed to total concentrations. Good linearity was obtained for all of the analytes in both reagent water and lake water samples with correlation coefficients, R 2, ranging from 0.991 to 0.996, in the concentration ranges of 25–400 ng L?1. The method was found to be very simple and inexpensive, with the possibility of running hundreds of samples in parallel with very minimal expenses for the determination of freely dissolved OPPs. 相似文献
9.
A new sesquiterpene hydrocarbon, precapnelladiene (I) which does not obey the isoprene rule, was isolated from the nonpolar fractions of the soft coral Capnella imbricata. Its structure was established by chemical as well as spectral evidence, with additional aid of the CONGEN computer program. Precapnelladiene has a fused 5- and 8-membered ring system and may be a biogenetic precursor of the unique tricyclic capnellane skeleton typical of sesquiterpenes isolated from the same marine animal. 相似文献
10.
O. Topic M. Scherer G. Gebreyesus T. Henninger P. Hyllus C. Klempt W. Ertmer L. Santos J. J. Arlt 《Laser Physics》2010,20(5):1156-1162
Bose-Einstein condensates of atoms with non-zero spin are known to constitute an ideal system to investigate fundamental properties
of magnetic superfluids. More recently it was realized that they also provide the fascinating opportunity to investigate the
macroscopic amplification of quantum and classical fluctuations. This is strikingly manifested in a sample initially prepared
in the m
F
= 0 state, where spin-changing collisions triggered by quantum fluctuations may lead to the creation of correlated pairs
in m
F
= ±1. We show that the pair creation efficiency is strongly influenced by the interplay between the external trapping potential
and the Zeeman effect. It thus reflects the confinement-induced magnetic field dependence of elementary spin excitations of
the condensate. Remarkably, pair production in our experiments is therefore characterized by a multi-resonant dependence on
the magnetic field. Pair creation at these resonances acts as strong parametric matter-wave amplifier. Depending on the resonance
condition, this amplification can be extremely sensitive or insensitive to the presence of seed atoms. We show that pair creation
at a resonance which is insensitive to the presence of seed atoms is triggered purely by quantum fluctuations and thus the
system acts as a matter-wave amplifier for the vacuum state. 相似文献