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排序方式: 共有840条查询结果,搜索用时 15 毫秒
831.
832.
A novel approach for the selection of the operational parameters (linear velocity, column length) for a comprehensive 2D-LC system is discussed. Starting point for the calculations is a given second dimension ((2)D) separation and a desired peak capacity for the 2D system. Using the theory developed here the optimum settings for the first dimension ((1)D) column can be derived. Theory clearly indicates that the choice of the (1)D conditions is basically limited to just one set of column lengths and linear velocities. The new method is tested on a comprehensive two-dimensional liquid chromatography system which uses size-exclusion chromatography (SEC) followed by reversed phase liquid chromatography (RPLC). A novel LC/LC interface, using a six-port valve rather than storage loops, joins the two chromatographic dimensions. From a theoretical comparison of continuous low flow and stop-flow operation the latter method was found to be an attractive mode of interfacing. The common idea that stop-flow operation results in additional band broadening is shown to be incorrect. The new interface design operated in the stop-flow mode permits the use of conventional analytical diameter HPLC columns, 7.8mm for SEC and 4.6mm for RPLC. The reversed phase chromatography utilizes a monolithic C-18 modified silica column, which produces fast and efficient analyses. As test samples complex mixtures of peptides were analyzed. 相似文献
833.
Boric acid as a mobile phase additive for high performance liquid chromatography separation of ribose, arabinose and ribulose 总被引:1,自引:0,他引:1
A new high performance liquid chromatographic (HPLC) method is described for the analysis of ribose, arabinose and ribulose mixtures obtained from (bio)chemical isomerization processes. These processes gain importance since the molecules can be used for the synthesis of antiviral therapeutics. The HPLC method uses boric acid as a mobile phase additive to enhance the separation on an Aminex HPX-87K column. By complexing with boric acid, the carbohydrates become negatively charged, thus elute faster from the column by means of ion exlusion and are separated because the complexation capacity with boric acid differs from one carbohydrate to another. Excellent separation between ribose, ribulose and arabinose was achieved with concentrations between 0.1 and 10 gL(-1) of discrete sugar. 相似文献
834.
Wim Fremout Jana Sanyova Steven Saverwyns Peter Vandenabeele Luc Moens 《Analytical and bioanalytical chemistry》2009,393(8):1991-1999
Proteins in works of art are generally determined by the relative amounts of amino acids. This method, however, implies a
loss of information on the protein structure and its modifications. Consequently, we propose a method based on the analysis
of trypsin digests using high-performance liquid chromatography (HPLC) UV diode array detection (DAD) for painting binder
studies. All reaction steps are done in the same vial; no extraction methods or sample transfer is needed, reducing the risk
of sample losses. A collection of pure binders (collagen, ovalbumin, yolk and casein) as well as homemade and historical paint
samples have been investigated with this method. Chromatograms of unknowns at 214 nm and 280 nm are compared with those of
the reference samples as a fingerprint. There is a good agreement between many peptides, but others seem to have been lost
or their retention time shifted due to small compositional changes because of ageing and degradation of the paint. The results
are comparable with the results of other techniques used for binder identification on the same samples, with the additional
advantage of differentiation between egg yolk and glair. 相似文献
835.
Brent Daelemans Samuel Eyley Carlos Marquez Vincent Lemmens Dirk E. De Vos Wim Thielemans Wim Dehaen Steven De Feyter 《Chemical science》2022,13(31):9035
Adsorptive separation is a promising lower-energy alternative for traditional industrial separation processes. While carbon-based materials have a long history in adsorptive removal of organic contaminants from solution or gas mixtures, separation using an adsorption/desorption protocol is rarely considered. The main drawbacks are the limited control in bulk adsorption experiments, as often all organic molecules are adsorbed, and lack of desorption methods to retrieve the adsorbed molecules. Using high-resolution on-surface characterization with scanning tunneling microscopy (STM), an increased understanding of the on-surface adsorption behavior under different conditions was obtained. The insight obtained from the nanoscale experiments was used to develop a highly selective separation method using adsorption and desorption on graphite, which was tested for the separation of quinonoid zwitterions. These experiments on adsorptive separation using self-assembly on graphite show its potential and demonstrate the advantage of combining surface characterization techniques with bulk experiments to exploit different possible applications of carbon-based materials.Insights from high-resolution on-surface characterization techniques are used to improve the control over adsorption and desorption on graphite in bulk adsorptive separation processes. 相似文献
836.
Dr. Merten Grupe Pit Boden Patrick Di Martino-Fumo Dr. Xin Gui Cecilia Bruschi Roumany Israil Marcel Schmitt Dr. Martin Nieger Prof. Dr. Markus Gerhards Prof. Dr. Wim Klopper Dr. Christoph Riehn Dr. Claudia Bizzarri Prof. Dr. Rolf Diller 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(61):15252-15271
Chemical and spectroscopic characterization of the mononuclear photosensitizers [(DPEPhos)Cu(I)(MPyrT)]0/+ ( CuL , CuLH ) and their dinuclear analogues ( Cu2L’ , Cu2L'H2 ), backed by (TD)DFT and high-level GW-Bethe-Salpeter equation calculations, exemplifies the complex influence of charge, nuclearity and structural flexibility on UV-induced photophysical pathways. Ultrafast transient absorption and step-scan FTIR spectroscopy reveal flattening distortion in the triplet state of CuLH as controlled by charge, which also appears to have a large impact on the symmetry of the long-lived triplet states in Cu2L’ and Cu2L'H2 . Time-resolved luminescence spectroscopy (solid state), supported by transient photodissociation spectroscopy (gas phase), confirm a lifetime of some tens of μs for the respective triplet states, as well as the energetics of thermally activated delayed luminescence, both being essential parameters for application of these materials based on earth-abundant copper in photocatalysis and luminescent devices. 相似文献
837.
Franziska A. Klapper Dr. Christine Kiel Dr. Peter Bellstedt Prof. Dr. Wim Vyverman Prof. Dr. Georg Pohnert 《Angewandte Chemie (International ed. in English)》2023,62(43):e202307165
Diatoms are abundant unicellular microalgae, responsible for ≈20 % of global photosynthetic CO2 fixation. Nevertheless, we know little about fundamental aspects of their biology, such as their sexual reproduction. Pheromone-mediated chemical communication is crucial for successful mating. An attraction pheromone was identified in the diatom Seminavis robusta, but metabolites priming cells for sex and synchronizing search and mating behavior remained elusive. These sex-inducing pheromones (SIP) induce cell cycle arrest and trigger the production of the attraction pheromone. Here we describe the challenging structure elucidation of an S. robusta SIP. Guided by metabolomics, a candidate metabolite was identified and elucidated by labeling experiments, NMR, ESI MSn analyses, and chemical transformations. The use of negative ion mode MS was essential to decipher the unprecedented hydroxyproline and β-sulfated aspartate-containing cyclic heptapeptide that acts in femtomolar concentrations. 相似文献
838.
Michael Wübbenhorst Ernout M. Van Koten John C. Jansen Wim Mijs Jan van Turnhout 《Macromolecular rapid communications》1997,18(2):139-147
The molecular dynamics of amorphous and liquid-crystalline (LC) side-chain polycarbonates was studied by dielectric spectroscopy at frequencies from 10−2 to 106 Hz and at temperatures from −160 to 180°C. ‘Model’ compounds containing no mesogenic side-groups showed two relaxations, which originate from the carbonate group (α, βm-relaxation). By contrast, in LC-polycarbonates bearing a mesogenic nitrostilbene side group around and above the glass transition temperature Tg up to three relaxation modes were distinguished (α-, λ1-, λ2-process); below Tg four secondary relaxations (γ-, βm-, βs-, βsc-relaxation) were observed. The γ-relaxation was found only in compounds possessing an aliphatic spacer linked to the backbone by an ether bond. Apart from βm-, two additional β-processes were identified as relaxations associated with the mesogenic unit in the glassy (βs) or in the crystalline state (βsc). 相似文献
839.
Peter A. Cuypers George M. Willems H. Coenraad Hemker Wim Th. Hermens 《Macromolecular Symposia》1988,17(1):155-159
A model for protein adsorption kinetics is presented. This model includes diffusion limited adsorption, adsorption and desorption rate constants which are dependent on the surface concentration and an interaction term for the mutual influence of the adsorbed protein molecules. It is shown that, in first approximation, the values of the adsorption and desorption rate constants are exponential functions of the surface concentration. Assuming an adequate interaction term it is possible to show with this model for the adsorption kinetics of a mixture of proteins that the ratio of the adsorbed proteins is strongly dependent on the overall surface concentration even if the ratio of the bulk concentrations of these proteins is kept constant. Differences in interaction terms for the different proteins offer a possible explanation for the peculiar behaviour of plasma protein adsorption on a surface at different dilutions of the plasma, the so called “Vroman effect”. 相似文献
840.