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排序方式: 共有219条查询结果,搜索用时 15 毫秒
1.
Junliang Tian Chen-Lin Chou W. D. Ehmann 《Journal of Radioanalytical and Nuclear Chemistry》1987,110(1):261-274
Instrumental neutron activation analysis was used for the determination of 31 major and trace elements in 32 samples from the Xinji Loess Section, Shaanxi Province, China. Interferences, including those from uranium fission products, were evaluated and corrections applied where necessary. The 39.7-meter deep section comprises of Lishi Loess of the middle Pleistocene (Q2) and Malan Loess of the late Pleistocene (Q3). The section is characterized by the presence of 5 layers of paleosol, and each paleosol is underlain by a precipitation layer. When the elemental abundances are converted to a carbonate-free basis, there is little compositional difference among the carbonate-free fractions of loess, paleosol and precipitation layers. This indicates that dissolution of carbonate minerals by downward-moving surface water was an important process in paleosol formation while other minerals were not severely weathered and elemental fractionation was minimal. The parent materials of the paleosol and precipitation layers closely resemble the loess layers in their elemental abundances, which suggests that all layers in the section have a compositionally similar source. 相似文献
2.
The action of ultra violet rays of 260 nm on aqueous solutions of bases, nucleosides and nucleotides of DNA at 77°K is studied by electron spin resonance. It is shown that the free radicals observed are similar, with a few noteworthy exceptions to those induced by X-rays, under the same conditions of temperature, in the solid state. Contrary to what might be excepted on the basis of the results obtained by X-rays, the variation in the yield in paramagnetic centres in each sequence studied does not seem to be important. 相似文献
3.
ALBERT VAN DE VORST 《Photochemistry and photobiology》1971,13(4):321-336
Abstract— –The photosensitization of amino acids by proflavine is studied using the technique of electron spin resonance spectroscopy. The analysis of the line shape as a function of the incident microwave power (both in the presence and absence of oxygen) and the dependence of the numbers of free radicals on the intensity of the incident light allow one to suggest that two types of radicals are formed. One is formed by a biphotonic process, the nature of the radicals being the same as in the case of ionising radiation, while the other is probably the RO2 radical formed as a result of photodynamic action. 相似文献
4.
BARB FLORIN DAN; IONESCU VLAD; KONING WILLEM DE 《IMA Journal of Mathematical Control and Information》1994,11(4):277-309
In this paper, we deal with the digital output-measurement-feedbackH control problem for Pritchard-Salamon infinite-dimensionalsystems with unbounded input and output operators. A discretePopov-theory-based solution is given in terms of the solvabilityof Kalman-Szegö-Popov-Yakubovitch systems associated withthe equivalent discrete-time time-invariant system obtainedby lifting the T-periodic continuous-time system. 相似文献
5.
L. Tandon E. J. Kasarskis W. D. Ehmann 《Journal of Radioanalytical and Nuclear Chemistry》1995,195(1):13-19
Human kidney and liver tissues were studied for generalized elemental imbalances in amyotrophic lateral sclerosis (ALS) by instrumental neutron activation analysis (INAA). Iron was significantly increased (p<0.05) in ALS kidneys and Co and Fe (marginal, p<0.10) were increased in ALS liver compared with their respective controls. Mercury values were almost two-fold higher for ALS kidney and 17% higher for ALS liver as compared with their respective controls. However, the Hg data exhibited large variations and ALS-control differences were not significant. Data from the present study are discussed with reference to the role of metallothioneins (MT) in ALS, and a possible linkage between a free radical mediated mechanism and degeneration of cells in ALS is also explored. 相似文献
6.
On Hexagonal Perovskites with Cationic Vacancies. XXXI. Systems BaO? Re2O7? M O5 with MV = Nb, Ta In the systems BaO? Re2O7? MO5 three quaternary oxides are formed, which belong to the perovskite stacking polytypes with cationic vacancies: Ba8Re7/2M□3O24 (MV = Nb, Ta; rhombohedral 24 L type; sequence (hhhhchhc)3; space group R3 m), Ba4Re9/8Ta13/8□5/4O12 (rhombohedral 12 L type; sequence (hhcc)3; space group R3 m) and the phases Ba5BaRe3/2?xM □O15?x□x (MV = Nb, Ta; variants of a hexagonal 5 L type). 相似文献
7.
Fifteen elements at trace levels have been determined by neutron activation analysis in the biological standard kale distributed
byBowen. La, Br, As, Se, Sc, Ag, Zn, Co, Cr, Sb, Eu, Fe, and Zr have been determined by a nondestructive technique using a high-resolution
Ge(Li) detector. Two more elements, Au and Hg, have been determined after radiochemical separation. The nondestructive procedure
is shown to yield data in generally good agreement with those obtained by destructive techniques. Potential sources of error
in the nondestructive technique are discussed. 相似文献
8.
In the system BaFe1?xRuxO3?y three phases, separated by immiscibility gaps, are present: an Fe-rich phase (x = 0-0.75) with hexagonal BaTiO3 structure (6H; sequence (hcc)2), a Ru-rich phase (x = 0.9) of hexagonal 4H type (sequence (hc)2), and the pure Ru compound BaRuO3 with rhombohedral 9R structure (sequence (hhc)3). By vibrational spectroscopic investigations in the 6H phase a transition from n-type semiconduction (Fe-rich oxygen-deficient compounds) to good, metal-like conduction (Ru-rich compounds with complete oxygen lattice) can be detected. The 4H and 9R stacking polytypes are good, metal-like conductors. 相似文献
9.
ELISABETH TAFFIN DE GIVENCHY FREDERIC GUITTARD FREDERIC BRACON AIME CAMBON 《Liquid crystals》2013,40(9):1371-1377
In order to determine the influence of 'sulphur-containing' spacers on the formation of mesophases in low molecular mass compounds, we have examined the mesomorphic behaviour of molecules which consists of a 4-biphenyl unit linked to an unbranched fluorinated chain via a short spacer including at least a sulphur atom. The synthesis of these compounds has been carried out from 2-F-butylethyl iodide or from the 2-F-alkylethyl mercaptans in the case of the F-hexyl and F-octyl tails. The mesomorphic properties have been characterized by polarized light microscopy and by differential thermal analysis showing the peculiar contribution of each of the spacers. The influence of the fluorinated chain and the shape of the connector on the stability of the mesophases has been investigated. The compounds with a thioether or a hemithioacetal spacer showed no mesomorphic properties, whereas the structures with a thioester spacer showed a very interesting enantiotropic behaviour of the smectic A type over a wide temperature range. Furthermore from the series exhibiting liquid crystalline behaviour, increasing the number of fluoromethylene units simultaneously increases both the melting and the clearing temperature. These mesomorphic properties within the fluorinated series are compared with those of their monocatenar hydrocarbon homologues. 相似文献
10.
We attempt to resolve the discrepancy between the Buckingham—Longuet-Higgins (BLH) and Imrie—Raab (IR) theories of linear birefringence induced in a gas of dipolar molecules by an electric field gradient. To this end we present a new calculation of the effect, based on forward scattering of a light beam incident on a thin lamina of gas molecules. We work to electric quadrupole—magnetic dipole order. The beam undergoes a time delay which is proportional to the thickness of the lamina and the electric field gradient, and can therefore be interpreted in terms of a contribution to the refractive index of the gas due to the field gradient. The birefringence is obtained by considering appropriate polarizations of the incident beam. To avoid the occurrence of a divergent quantity, such as appears in the BLH theory, it is essential to take account of the finite beam width. Calculations are performed using both primitive (traced) and traceless molecular quadrupole moments; as required on basic grounds, these results are equivalent. They are also identical to the BLH result. By contrast, the IR result is physically unacceptable because it is not invariant with respect to the use of traced and traceless moments. The source of error in the IR theory remains unclear. 相似文献