全文获取类型
收费全文 | 266篇 |
免费 | 15篇 |
国内免费 | 3篇 |
专业分类
化学 | 189篇 |
晶体学 | 1篇 |
力学 | 4篇 |
数学 | 34篇 |
物理学 | 56篇 |
出版年
2023年 | 4篇 |
2021年 | 2篇 |
2020年 | 7篇 |
2019年 | 8篇 |
2018年 | 9篇 |
2017年 | 2篇 |
2016年 | 7篇 |
2015年 | 16篇 |
2014年 | 2篇 |
2013年 | 13篇 |
2012年 | 25篇 |
2011年 | 28篇 |
2010年 | 10篇 |
2009年 | 11篇 |
2008年 | 16篇 |
2007年 | 12篇 |
2006年 | 12篇 |
2005年 | 17篇 |
2004年 | 5篇 |
2003年 | 9篇 |
2002年 | 7篇 |
2001年 | 6篇 |
2000年 | 5篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1997年 | 5篇 |
1995年 | 2篇 |
1993年 | 4篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1935年 | 2篇 |
1933年 | 1篇 |
排序方式: 共有284条查询结果,搜索用时 15 毫秒
91.
In this work, four different metabolite purification approaches are investigated prior to metabolomics of human HT29 colon cancer cells. Namely, methanol deproteinization, ultrafiltration and two SPE methods using C18 and polymer-based cartridges were studied. The extracts were characterized via a metabolomic approach based on the application of CE TOF MS (CE-MS). CE-MS analysis time was less than 20 min per sample and allowed the simultaneous and reproducible analysis of more than 80 metabolites in a single run with a minimum consumption of sample and reagents. Metabolome analysis revealed in some cases important differences among the studied metabolite purification procedures. No significant differences were observed in the metabolite profile using C18 and polymer-based cartridges, or between ultrafiltration and methanol deproteinization. However, important differences were observed in the metabolomic profiles obtained from SPE and methanol deproteinization samples. These results demonstrate the crucial role of the metabolite purification strategy in metabolomics since it can bias (and in some cases mislead) the conclusions achieved by the metabolomic study. 相似文献
92.
Vendelbo SB Johansson M Nielsen JH Chorkendorff I 《Physical chemistry chemical physics : PCCP》2011,13(10):4486-4493
The influence of monoatomic steps and defects on the methanation reaction over ruthenium has been investigated. The experiments are performed on a Ru(0 1 54) ruthenium single crystal, which contains one monoatomic step atom for each 27 terrace atoms. The methanation activity is measured at one bar of hydrogen and CO in a high pressure cell, which enables simultaneous measurements of the local reactivity of the well defined single crystal surface and the global reactivity of the entire crystal and its auxiliary support. By adding sulfur we observe that the measured activity from the well defined stepped front-side of the crystal is poisoned faster than the entire crystal containing more defects. We also observe that additional sputtering of the well-defined front-side increases the reactivity measured on the surface. Based on this, we conclude that the methanation reaction takes place on undercoordinated sites, such as steps and kinks, and that the methanation reaction is extremely structure dependent. Simulations of the flow, temperature, and product distributions in the high pressure cell are furthermore presented as supplementary information. 相似文献
93.
Murphy S Strebel C Vendelbo SB Conradsen C Tison Y Nielsen K Bech L Nielsen RM Johansson M Chorkendorff I Nielsen JH 《Physical chemistry chemical physics : PCCP》2011,13(21):10333-10341
Using model catalysts, we demonstrate that CO desorption from Ru surfaces can be switched from that typical of single crystal surfaces to one more characteristic of supported nanoparticles. First, the CO desorption behaviour from Ru nanoparticles supported on highly oriented pyrolytic graphite was studied. Both mass-selected and thermally evaporated nanoparticles were deposited. TPD spectra from the mass-selected nanoparticles exhibit a desorption peak located around 410 K with a broad shoulder extending from around 480 K to 600 K, while spectra obtained from thermally evaporated nanoparticles exhibit a single broad feature from ~350 K to ~450 K. A room temperature deposited 50 ? thick Ru film displays a characteristic nanoparticle-like spectrum with a broad desorption feature at ~420 K and a shoulder extending from ~450 K to ~600 K. Subsequent annealing of this film at 900 K produced a polycrystalline morphology of flat Ru(001) terraces separated by monatomic steps. The CO desorption spectrum from this surface resembles that obtained on single crystal Ru(001) with two large desorption features located at 390 K and 450 K due to molecular desorption from terrace sites, and a much smaller peak at ~530 K due to desorption of dissociatively adsorbed CO at step sites. In a second experiment, ion sputtering was used to create surface defects on a Ru(0 1 54) single crystal surface. A gradual shift away from the desorption spectrum typical of a Ru(001) surface towards one resembling desorption from supported Ru nanoparticles was observed with increasing sputter time. 相似文献
94.
Li W Shavel A Guzman R Rubio-Garcia J Flox C Fan J Cadavid D Ibáñez M Arbiol J Morante JR Cabot A 《Chemical communications (Cambridge, England)》2011,47(37):10332-10334
An oriented attachment and growth mechanism allows an accurate control of the size and morphology of Cu(2-x)S nanocrystals, from spheres and disks to tetradecahedrons and dodecahedrons. The synthesis conditions and the growth mechanism are detailed here. 相似文献
95.
Although the causes of cancer lie in mutations or epigenic changes at the genetic level, their molecular manifestation is the dysfunction of biochemical pathways at the protein level. The 518 protein kinases encoded by the human genome play a central role in various diseases, a fact that has encouraged extensive investigations on their biological function and three dimensional structures. Selenium (Se) is an important nutritional trace element involved in different physiological functions with antioxidative, antitumoral and chemopreventive properties. The mechanisms of action for selenocompounds as anticancer agents are not fully understood, but kinase modulation seems to be a possible pathway. Various organosulfur compounds have shown antitumoral and kinase inhibition effects but, in many cases, the replacement of sulfur by selenium improves the antitumoral effect of compounds. Although Se atom possesses a larger atomic volume and nucleophilic character than sulfur, Se can also formed interactions with aminoacids of the catalytic centers of proteins. So, we propose a novel chemical library that includes organoselenium compounds as kinase modulators. In this study thirteen selenocompounds have been evaluated at a concentration of 3 or 10 μM in a 24 kinase panel using a Caliper LabChip 3000 Drug Discover Platform. Several receptor (EGFR, IGFR1, FGFR1…) and non-receptor (Abl) kinases have been selected, as well as serine/threonine/lipid kinases (AurA, Akt, CDKs, MAPKs…) implicated in main cancer pathways: cell cycle regulation, signal transduction, angiogenesis regulation among them. The obtained results showed that two compounds presented inhibition values higher than 50% in at least four kinases and seven derivatives selectively inhibited one or two kinases. Furthermore, three compounds selectively activated IGF-1R kinase with values ranging from -98% to -211%. In conclusion, we propose that the replacement of sulfur by selenium seems to be a potential and useful strategy in the search of novel chemical compound libraries against cancer as kinase modulators. 相似文献
96.
Botero-Coy AM Marín JM Ibáñez M Sancho JV Hernández F 《Analytical and bioanalytical chemistry》2012,402(7):2287-2300
Monitoring pesticide residues in tropical fruits is of great interest for many countries, e.g., from South America, that base
an important part of their economy on the exportation of these products. In this work, a LC-MS/MS multi-residue method using
a triple quadrupole analyzer has been developed for around 30 pesticides in seven Colombian tropical fruits of high commercial
value for domestic and international markets (uchuva, tamarillo, granadilla, gulupa, maracuya, papaya, and pithaya). After
sample extraction with acetonitrile, an aliquot of the extract was diluted with water and directly injected into the HPLC-MS/MS
system (electrospray interface) without any cleanup step. The formation of sodium adducts—of poor fragmentation—was minimized
using 0.1% formic acid in the mobile phase, which favored the formation of the protonated molecule. However, the addition
of ammonium acetate made the formation of the ammonium adducts in some particular cases possible, avoiding the presence of
the sodium adducts. The highest sensitivity was observed in positive electrospray ionization for the wide majority of pesticides,
with a few exceptions for acidic compounds that gave better response in the negative mode (e.g., 2,4-D, fluazinan). Thus,
simultaneous acquisition on the positive/negative mode was applied. Two MS/MS transitions were acquired for each compound
to ensure a reliable quantification and identification of the compounds detected in samples, although for malathion a third
transition was acquired due to the presence of interfering isobaric compounds in the sample extracts. A detailed study of
matrix effects was made by a comparison of standards in solvent and in matrix. Both ionization suppression and ionization
enhancement were observed depending on the analyte/matrix combination tested. Correction of matrix effects was made by the
application of calibration in matrix. Three matrices were selected (uchuva, maracuya, gulupa) to perform matrix calibration
in the analysis of all seven fruit varieties studied. The method was validated by recovery experiments in samples spiked at
two levels (0.05 and 0.5 mg/kg). The data were satisfactory for the wide majority of analyte/matrix combinations, with most
recoveries between 70% and 110% and the RSD below 15%. Several samples collected from the market were finally analyzed. Positive
findings were confirmed by evaluating the experimental Q/q ratios and retention times, and comparing them with those of reference standards. 相似文献
97.
M Ibáñez E Gracia-Lor JV Sancho F Hernández 《Journal of mass spectrometry : JMS》2012,47(8):1040-1046
Pharmaceuticals are emerging contaminants of increasing concern because of their presence in the aquatic environment and potential to reach drinking-water sources. After human and/or veterinary consumption, pharmaceuticals can be excreted in unchanged form, as the parent compound, and/or as free or conjugated metabolites. Determination of most pharmaceuticals and metabolites in the environment is commonly made by liquid chromatography (LC) coupled to mass spectrometry (MS). LC coupled to tandem MS is the technique of choice nowadays in this field. The acquisition of two selected reaction monitoring (SRM) transitions together with the retention time is the most widely accepted criterion for a safe quantification and confirmation assay. However, scarce attention is normally paid to the selectivity of the selected transitions as well as to the chromatographic separation. In this work, the importance of full spectrum acquisition high-resolution MS data using a hybrid quadrupole time-of-flight analyser and/or a suitable chromatographic separation (to reduce the possibility of co-eluting interferences) is highlighted when investigating pharmaceutical metabolites that share common fragment ions. For this purpose, the analytical challenge associated to the determination of metabolites of the widely used analgesic dipyrone (also known as metamizol) in urban wastewater is discussed. Examples are given on the possibilities of reporting false positives of dypirone metabolites by LC-MS/MS under SRM mode due to a wrong assignment of identity of the compounds detected. Copyright ? 2012 John Wiley & Sons, Ltd. 相似文献
98.
A kinematic analysis of the scattering of helium atoms from oxygen adsorbed on Nickel (001) has been performed. For a substrate temperature of 375 K and an oxygen coverage of θ ~ 0.21 a metastable mixture of p(2×2) without long range order and some c(2×2) is obtained. Annealing at 475 K results in a well ordered p(2×2) structure with long range order. An order-disorder transition is identified at ~ 535 K. 相似文献
99.
Eduardo Beltrán María Ibáñez Juan Vicente Sancho Félix Hernández 《Rapid communications in mass spectrometry : RCM》2009,23(12):1801-1809
A rapid multianalyte‐multiclass method with little sample manipulation has been developed for the simultaneous determination of eleven mycotoxins in different food commodities by using ultra‐high‐pressure liquid chromatography coupled to triple quadrupole mass spectrometry (UHPLC/MS/MS). Toxins were extracted from the samples with acetonitrile/water (80:20, v/v) 0.1% HCOOH and, after a two‐fold dilution with water, directly injected into the system. Thanks to the fast high‐resolution separation of UHPLC, the eleven mycotoxins were separated by gradient elution in only 4 min. The method has been validated in three food matrices (maize kernels, dry pasta (wheat), and eight‐multicereal babyfood (wheat, maize, rice, oat, barley, rye, sorghum, millet)) at four different concentration levels. Satisfactory recoveries were obtained (70–110%) and precision (expressed as relative standard deviation) was typically below 15% with very few exceptions. Quantification of samples was carried out with matrix‐matched standards calibration. The lowest concentration successfully validated in sample was as low as 0.5 µg/kg for aflatoxins and ochratoxin A in babyfood, and 20 µg/kg for the rest of the selected mycotoxins in all matrices tested. Deoxynivalenol could be only validated at 200 µg/kg, due the poor sensitivity for this mycotoxin analysis. With only two exceptions (HT‐2 and deoxynivalenol), the limits of detection (LODs), estimated for a signal‐to‐noise ratio of 3 from the chromatograms of samples spiked at the lowest level validated, varied between 0.1 and 1 µg/kg in the three food matrices tested. The method was applied to the analysis of different kinds of samples. Positive findings were confirmed by acquiring two transitions (Q quantification, q confirmation) and evaluating the Q/q ratio. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
100.
M. Ibáñez de Berganza E. E. Ferrero S. A. Cannas V. Loreto A. Petri 《The European physical journal. Special topics》2007,143(1):273-275
When the two dimensional q-color Potts model in the square lattice is quenched at zero temperature with Glauber dynamics,
the energy decreases in time following an Allen-Cahn power law, and the system converges to a phase with energy higher than
the ground state energy after an arbitrary large time when q>4. At low but finite temperature, it cesses to obey the power-law
regime and orders after a very long time, which increases with q, and before which it performs a domain growth process which
tends to be slower as q increases. We briefly present and comment numerical results on the ordering at nonzero temperature. 相似文献