全文获取类型
收费全文 | 32637篇 |
免费 | 269篇 |
国内免费 | 311篇 |
专业分类
化学 | 12930篇 |
晶体学 | 303篇 |
力学 | 1518篇 |
综合类 | 12篇 |
数学 | 9353篇 |
物理学 | 9101篇 |
出版年
2023年 | 47篇 |
2022年 | 245篇 |
2021年 | 249篇 |
2020年 | 185篇 |
2019年 | 176篇 |
2018年 | 1344篇 |
2017年 | 1573篇 |
2016年 | 895篇 |
2015年 | 682篇 |
2014年 | 664篇 |
2013年 | 996篇 |
2012年 | 3394篇 |
2011年 | 2574篇 |
2010年 | 2016篇 |
2009年 | 1708篇 |
2008年 | 729篇 |
2007年 | 749篇 |
2006年 | 704篇 |
2005年 | 4479篇 |
2004年 | 3982篇 |
2003年 | 2331篇 |
2002年 | 518篇 |
2001年 | 313篇 |
2000年 | 116篇 |
1999年 | 200篇 |
1998年 | 127篇 |
1997年 | 94篇 |
1996年 | 68篇 |
1995年 | 52篇 |
1994年 | 67篇 |
1992年 | 187篇 |
1991年 | 164篇 |
1990年 | 153篇 |
1989年 | 119篇 |
1988年 | 113篇 |
1987年 | 72篇 |
1986年 | 46篇 |
1985年 | 59篇 |
1984年 | 51篇 |
1983年 | 41篇 |
1980年 | 50篇 |
1976年 | 90篇 |
1975年 | 41篇 |
1974年 | 44篇 |
1973年 | 59篇 |
1972年 | 43篇 |
1970年 | 41篇 |
1969年 | 46篇 |
1968年 | 43篇 |
1966年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
731.
Sun S Szakal C Winograd N Wucher A 《Journal of the American Society for Mass Spectrometry》2005,16(10):1677-1686
The ion bombardment-induced release of particles from a metal surface is investigated using energetic fullerene cluster ions as projectiles. The total sputter yield as well as partial yields of neutral and charged monomers and clusters leaving the surface are measured and compared with corresponding data obtained with atomic projectile ions of similar impact kinetic energy. It is found that all yields are enhanced by about one order of magnitude under bombardment with the C60+ cluster projectiles compared with Ga+ ions. In contrast, the electronic excitation processes determining the secondary ion formation probability are unaffected. The kinetic energy spectra of sputtered particles exhibit characteristic differences which reflect the largely different nature of the sputtering process for both types of projectiles. In particular, it is found that under C60+ impact (1) the energy spectrum of sputtered atoms peaks at significantly lower kinetic energies than for Ga+ bombardment and (2) the velocity spectra of monomers and dimers are virtually identical, a finding which is in pronounced contrast to all published data obtained for atomic projectiles. The experimental findings are in reasonable agreement with recent molecular dynamics simulations. 相似文献
732.
Adams CM Kjeldsen F Zubarev RA Budnik BA Haselmann KF 《Journal of the American Society for Mass Spectrometry》2004,15(7):1087-1098
First results are reported on the application of ECD in analysis of 2+ and 3+ ions of stereoisomers of Trp-cage (NLYIQWLKDGGPSSGRPPPS), the smallest and fastest-folding protein, which exhibits a tightly folded tertiary structure in solution. The chiral recognition based on the ratios of the abundances of z(18) and z(19) fragments in ECD of 2+ ions was excellent even for a single amino acid (Tyr) D-substitution (R(chiral) = 8.6). The chiral effect decreased with an increase of temperature at the electrospray ion source, as well as at a higher degree of ionization, 3+ ions (R(chiral) = 1.5). A general approach is suggested for charge localization in n+ ions by analysis of ECD mass spectra of (n + 1)+ ions. Application of this approach to 3+ Trp-cage ions revealed the protonation probability order in 2+ ions: Arg(16) > Gln(5) > approximately N-terminus. The ECD results for native form of the 2+ ions favor the preservation of the solution-phase tertiary structure, and chiral recognition through the interaction between the charges and the neutral bond network. Conversely, ECD of 3+ ions supports the dominance of ionic hydrogen bonding which determines a different gas-phase structure than found in solution. Vibrational activation of 2+ ions indicated greater stability of the native form, but the fragmentation patterns did not provide stereoisomer differentiation, thus underlying the special position of ECD among other MS/MS fragmentation techniques. Further ECD studies should yield more structural information as well as quantitative single-amino acid D/L content measurements in proteins. 相似文献
733.
734.
Muhammad I. Khan Abad Khan Zafar Iqbal Lateef Ahmad Yasar Shah 《Chromatographia》2010,71(7-8):577-586
A versatile isocratic reversed-phase liquid chromatographic/ultraviolet–visible detection method for simultaneous determination of all-trans-retinol and α-tocopherol in human serum was developed and validated after optimization of various chromatographic conditions and other experimental parameters. Analytes were separated on a Kromasil 100 RP18 (150 × 4.6 mm, 5 μm) analytical column protected by a Perkin Elmer RP18 (30 × 4.6 mm, 10 μm) guard cartridge. The mobile phase, methanol–water (96:04 v/v) was pumped at a flow rate of 2.2 mL min?1 and the column eluents were monitored at the wavelength of 292 nm using retinyl acetate (1.0 μg mL?1) as the internal standard for both analytes. Sample preparation was based on protein precipitation and stabilization with 2,6-bis(1,1-dimethylethyl)-4-methylphenol/ethanol and a two step extraction process using n-hexane followed by dichloromethane as extraction solvents. Sample size was kept 20 μL and separation of analytes was achieved in less than 7 min. The present method demonstrated acceptable values for specificity/selectivity, linearity within the expected concentration range, recovery, precision, sensitivity, stability of solutions, robustness, and system suitability specifications and tests. The method was used for monitoring all-trans-retinol and α-tocopherol concentrations in human serum samples and could also be applied to other sample matrices such as brain slices and cosmetic products if attention is paid to the extraction procedure. 相似文献
735.
Investigation of the oxidation reaction of electron rich alkoxy substituted β-aryl β-hydroxyphosphonates to corresponding β-ketophosphonates, which may be utilized in syntheses of lignans with various oxidizing agents (PCC, PDC, SIBX, CAN, Oxone®, KMNO4/SiO2, KMnO4/MS 4 Å, KMnO4/CuSO4, KMnO4/CuSO4/Al2O3, MnO2, CrO3/SiO2, H2O2/salen) is described. The effect of oxidants and reaction conditions on the reaction efficiency and yield was also investigated. 相似文献
736.
The γ-selective allylation of catalytic and stoichiometric alkylzinc-cuprates have been kinetically studied. The reactivity profiles generated by allylation reactions of n-butylzinc chloride catalyzed by CuX compounds (X = I, Br, Cl, CN, SCN) and also catalyzed by n-butylzinc-copper reagents and di n-butylzinc-copper reagents were evaluated. Reactivity profiles for allylation of stoichiometric n-butylzinc-copper reagents and di n-butylzinc-copper reagents were also prepared. All CuX compounds have been screened for the preparation of Grignard reagent derived n-butylzinc-copper reagents and di n-butylzinc-copper reagents.The evaluation of the profiles indicates that the active catalyst might be RCu(X)ZnCl and also to some degree, R2CuZnCl · ZnClX, which both could favor formation of γ-product. All data supports the reductive elimination of σ-allyl Cu (III) complex formed at vinylic terminal to give γ-allylated product with a quite slow isomerization to σ-allyl Cu (III) complex formed at allylic terminal to give α-allylated product. In the allylation mechanism of zinc cuprates, the role of counter ion, ZnCl+ has been discussed. 相似文献
737.
A simple isocratic reversed-phase high-performance liquid chromatographic method (RP-HPLC) was developed for the simultaneous determination of buprenorphine hydrochloride, naloxone hydrochloride dihydrate and its major impurity, noroxymorphone, in pharmaceutical tablets. The chromatographic separation was achieved with 10 mmol L−1 potassium phosphate buffer adjusted to pH 6.0 with orthophosphoric acid and acetonitrile (17:83, v/v) as mobile phase, a C-18 column, Perfectsil Target ODS3 (150 mm × 4.6 mm i.d., 5 μm) kept at 35 °C and UV detection at 210 nm. The compounds were eluted isocratically at a flow rate of 1.0 mL min−1. The average retention times for naloxone, noroxymorphone and buprenorphine were 2.4, 3.8 and 8.1 min, respectively. The method was validated according to the ICH guidelines. The validation characteristics included accuracy, precision, linearity, range, specificity, limit of quantitation and robustness. The calibration curves were linear (r > 0.996) over the concentration range 0.22-220 μg mL−1 for buprenorphine hydrochloride and 0.1-100 μg mL−1 for naloxone hydrochloride dihydrate and noroxymorphone. The recoveries for all three compounds were above 96%. No spectral or chromatographic interferences from the tablet excipients were found. This method is rapid and simple, does not require any sample preparation and is suitable for routine quality control analyses. 相似文献
738.
Miniaturization continues to be one of the leading trends in analytical chemistry and one that brings advantages that can be particularly beneficial in biochemical research. Use of a miniaturized scale enables efficient analysis in a short time and requires very small amounts of samples, solvents, and reagents. This can result in a remarkable decrease in costs of enzyme kinetics studies, especially when expensive or rare enzymes and/or substrates are involved. Free zone electrophoresis is without a doubt the most common microscale separation technique for capillary and on-chip enzyme assays. Progress and applications in this field are reviewed frequently whereas other modes of separation, although successfully applied, receive only marginal interest in such publications. This review summarizes applications of less common modes of separation in capillary or chip formats, namely micellar electrokinetic chromatography, liquid chromatography, gel electrophoresis, isoelectric focusing, and isotachophoresis. Because these techniques are based on separation mechanisms different from those of free zone electrophoresis, they can be, and have been, successfully used in cases where zone electrophoresis fails. Advantages and drawbacks of these alternative separation techniques are discussed, as also are the difficulties encountered most often and solutions proposed by different research groups. 相似文献
739.
Meier SM Tsybin YO Dyson PJ Keppler BK Hartinger CG 《Analytical and bioanalytical chemistry》2012,402(8):2655-2662
The interaction between oxaliplatin and the model protein ubiquitin (Ub) was investigated in a top–down approach by means
of high-resolution electrospray ionization mass spectrometry (ESI-MS) using diverse tandem mass spectrometric (MS/MS) techniques,
including collision-induced dissociation (CID), higher-energy C-trap dissociation (HCD), and electron transfer dissociation
(ETD). To the best of our knowledge, this is the first time that metallodrug–protein adducts were analyzed for the metal-binding
site by ETD-MS/MS, which outperformed both CID and HCD in terms of number of identified metallated peptide fragments in the
mass spectra and the localization of the binding sites. Only ETD allowed the simultaneous and exact determination of Met1
and His68 residues as binding partners for oxaliplatin. CID-MS/MS experiments were carried out on orbitrap and ion cyclotron
resonance (ICR)-FT mass spectrometers and both instruments yielded similar results with respect to number of metallated fragments
and the localization of the binding sites. A comparison of the protein secondary structure with the intensities of peptide
fragments generated by collisional activation of the [Ub + Pt-(chxn)] adduct [chxn = (1R,2R)-cyclohexanediamine] revealed a correlation with cleavages in solution phase random coil areas, indicating that the N-terminal β-hairpin and α-helix structures are retained in the gas phase. 相似文献
740.
Robert ?ysek 《Tetrahedron》2007,63(28):6558-6572
Starting from (±)-7-oxanorbornenone ((±)-14), (±)-(1RS,2RS,3SR,6SR)-6-azidocyclohex-4-en-1,2,3-triol ((±)-24) and (±)-(1RS,2RS,3SR,6RS)-6-azidocyclohex-4-en-1,2,3-triol ((±)-26) were obtained. Epoxidation of the latter cyclohexene derivative gave two epoxides (±)-30 and (±)-31 that were converted into (±)-conduramine F-1 epoxides (±)-10 and (±)-11 and N-substituted derivatives (±)-12 and (±)-13. Compound (±)-(1RS,2SR,3RS,4SR,5RS,6SR)-5-({[4-(trifluoromethyl)phenyl]methyl}amino)-7-oxabicyclo[4.1.0]heptane-2,3,4-triol ((±)-12c) is a good, non-competitive inhibitor of β-xylosidase from Aspergillus niger (Ki=2.2 μM), and (±)-(1RS,2RS,3SR,4RS,5SR,6SR)-5-{[(biphenyl-4-yl)methyl]amino}-7-oxabicyclo[4.1.0]heptane-2,3,4-triol ((±)-13d) is a good inhibitor of α-glucosidase from brewer's yeast (Ki=2.8 μM, non-competitive). 相似文献