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排序方式: 共有235条查询结果,搜索用时 15 毫秒
91.
Klaus G. Heumann Hans -Peter Schiefer 《Fresenius' Journal of Analytical Chemistry》1979,298(5):358-362
Summary Using bromocresol green as a visual endpoint indicator solutions of the cryptands [2.2.1] and [2.2.2] can be determined by titration with HCl in the concentration range of 10–1–10–3 M with relative standard deviations of 0.2–0.4 %. If both cryptand and cryptate are present in one solution, then the sum of them can be determined titrimetrically in the same way. It is also possible to determine the contents of [2.2.1] and [2.2.2] by using a calcium or barium standard solution and the unknown cryptand solution as a titrant. By evaluation of the pH-curve thus obtained, the endpoint of the titration can be determined. In the concentration range of 10–1–10–2 M relative standard deviations of about 0.5% are obtained. [2.2.1] cryptand solutions are determined by using the standard addition technique of Gran with a calcium chloride solution. An ion-selective electrode is used as a probe for calcium ions. If this method is applied, the relative standard deviation in the range of 10–2–10–4 M solutions is approximately 3%. With this technique cryptand can be analysed even when cryptate is present in the solution.We wish to thank the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for their financial support. 相似文献
92.
A method amenable to the gram scale synthesis of (R)-H 4-BINOL, a derivative of (R)-BINOL and ligand of interest in asymmetric catalysis, is described. The key step is the net partial hydrogenation of (R)-BINOL made possible by prior bis-etherification of the parent BINOL. 相似文献
93.
We consider generalized linear transient advection-diffusion problems for differential forms on a bounded domain in ? d . We provide comprehensive a priori convergence estimates for their spatiotemporal discretization by means of a first-order in time semi-Lagrangian approach combined with a discontinuous Galerkin method. Under rather weak assumptions on the velocity underlying the advection we establish an asymptotic L 2-estimate of order $O(\tau+h^{r}+h^{r+1}\tau^{-\frac{1}{2}}+\tau^{\frac{1}{2}})$ , where h is the spatial meshwidth, τ denotes the time step, and r is the polynomial degree of the forms used as trial functions. This estimate can be improved considerably in a variety of special settings. 相似文献
94.
Direct μ-flow injection isotope dilution ICP-MS for the determination of heavy metals in oil samples
Bettmer J Heilmann J Kutscher DJ Sanz-Medel A Heumann KG 《Analytical and bioanalytical chemistry》2012,402(1):269-275
The determination of trace elements in oil samples and their products is of high interest as their presence significantly
affects refinery processes and the environment by possible impact of their combustion products. In this context, inductively
coupled plasma mass spectrometry (ICP-MS) plays an important role due to its outstanding analytical properties in the quantification
of trace elements. In this work, we present the accurate and precise determination of selected heavy metals in oil samples
by making use of the combination of μ-flow direct injection and isotope dilution ICP-MS (ICP-IDMS). Spike solutions of 62Ni, 97Mo, 117Sn and 206Pb were prepared in an organic solvent, mixed directly with the diluted oil samples and tested to be fit for purpose for the
intended ID approach. The analysis of real samples revealed strong matrix effects affecting the ICP-MS sensitivity, but not
the isotope ratio measurements, so that accurate results are obtained by ICP-IDMS. Typical relative standard deviations were
about 15% for peak area and peak height measurements, whereas the isotope ratios were not significantly affected (RSD < 2%).
The developed method was validated by the analysis of a metallo-organic multi-element standard (SCP-21, typically applied
as a calibration standard) and the standard reference material SRM1084a (wear metals in lubricating oil). The obtained results
were in excellent agreement with the certified values (recoveries between 98% and 102%), so the proposed methodology of combining
μ-flow direct injection and ICP-IDMS can be regarded as a new tool for the matrix-independent, multi-element and reliable
determination of trace elements in oil and related organic liquids. 相似文献
95.
Yangming Lin Qing Lu Feihong Song Linhui Yu Anna K. Mechler Robert Schlgl Saskia Heumann 《Angewandte Chemie (International ed. in English)》2019,58(26):8917-8921
The abundance of available surface chemical information and edge structures of carbon materials have attracted tremendous interest in catalysis. For the oxygen evolution reaction (OER), the edge effects of carbon materials have rarely been studied in detail because of the complexity of various coexisting edge configurations and the controversy between carbon corrosion and carbon catalysis. Herein, the exact roles of common carbon active edge sites in the OER were interrogated using polycyclic aromatic hydrocarbons (PAHs) with designated configurations (zigzag and armchair) as model probe molecules, with a focus on structure–function relationships. Zigzag configurations of PAHs showed high activity for the OER while also showing a good stability at a reasonable potential. They show a TOF value of 0.276 s?1 in 0.1 m KOH. The catalytic activity of carbon edge sites was further effectively regulated by extending the π conjugation structure at a molecular level. 相似文献
96.
97.
Zirconium trace analyses play an important role for polyolefins produced by modern catalytic processes with zirconium metallocenes. A reliable and fast routine testing method by inductively coupled plasma isotope dilution mass spectrometry (ICP-IDMS) was therefore developed, which allows the determination of zirconium in polymers down to the low ng/g level. With respect to its precision, accuracy, and time-consumption this method is suitable for routine testing of production processes. A spike solution, enriched in the stable isotope 91Zr, was prepared and used for the isotope dilution procedure, which has the advantage of being an internal “one point” calibration method. The polyolefin samples were dissolved by microwave assisted digestion with a mixture of concentrated HNO3/HF. 相似文献
98.
Isotope-labelling exchange experiments were carried out to investigate the kinetic stability of Cr(III) complexes with humic substances (HS). To compare the results with those of an ion, not expected to form kinetically stable HS complexes with respect to its electron configuration, Cu(II) was investigated under the same conditions. HS solutions of different origin were therefore spiked with 53Cr(III) or 65Cu(II) after saturation of HS with chromium and copper of natural isotopic composition. In fractions of metal/HS complexes with different molecular weight, obtained by ultrafiltration and HPLC/ICP-MS using size exclusion chromatography (SEC), respectively, the isotope ratios of chromium and copper were determined by ICP and thermal ionisation mass spectrometry. Distinct differences in the isotopic composition of chromium were found in the permeate of the ultrafiltration compared with the corresponding unseparated solution, which indicates kinetically stable Cr(III)/HS complexes. On the other hand, the copper isotopic composition was identical in the permeate and the unseparated solution, which shows that a total exchange of Cu2+ ions took place between free and HS complexed copper ions. The SEC/ ICP-MS experiments also resulted in a different isotopic distribution of chromium in the chromatographically separated complexes whereas the copper complexes, separated by SEC, showed identical isotopic composition. The kinetic stability of Cr(III)/HS complexes could be explained by the d3 electron configuration of Cr3+ ions, a fact which is well known from classical Cr(III) complexes, and influences substantially the mobility of this heavy metal in the environment. 相似文献
99.
B. Wiedemann G. Rädlinger H. Ch. Alt K. G. Heumann K. Bethge 《Analytical and bioanalytical chemistry》1999,364(8):772-776
The residual or doped element concentration [E] in GaAs measured by SSMS is only accurate with respect to the relative sensitivity
coefficient RSCE. For a trace element concentration, the RSCE = [E]SSMS/[E]TRUE is set to unity, if no reference material or method is available to approximate the concentration to the true value. For
boron a relative sensitivity coefficient of RSCB = 0.94 ± 0.08 was obtained using TI-IDMS as a reference method. RSCN = 1 is used for nitrogen determinations. A boron and nitrogen detection limit of 4.4 × 1013 cm–3 is achieved. SSMS was used as reference method to calibrate the FTIR factor fE = [E] / Iα due to the integrated local vibrational mode absorption Iα of atomic boron and nitrogen in GaAs. A factor of fB = (12.0 × 2.7) × 1016 cm–1 (517 cm–1) and fN = (7.4 ± 0.1) × 1015 cm–1 (472 cm–1) was obtained for a boron and nine nitrogen containing GaAs samples at 77 K and 10 K, respectively.
Received: 15 December 1998 / Revised: 8 April 1999 / Accepted: 13 April 1999 相似文献