全文获取类型
收费全文 | 1176篇 |
免费 | 47篇 |
国内免费 | 5篇 |
专业分类
化学 | 955篇 |
力学 | 20篇 |
数学 | 86篇 |
物理学 | 167篇 |
出版年
2024年 | 3篇 |
2023年 | 17篇 |
2022年 | 59篇 |
2021年 | 83篇 |
2020年 | 32篇 |
2019年 | 37篇 |
2018年 | 17篇 |
2017年 | 15篇 |
2016年 | 41篇 |
2015年 | 43篇 |
2014年 | 42篇 |
2013年 | 53篇 |
2012年 | 78篇 |
2011年 | 92篇 |
2010年 | 59篇 |
2009年 | 53篇 |
2008年 | 78篇 |
2007年 | 85篇 |
2006年 | 51篇 |
2005年 | 66篇 |
2004年 | 50篇 |
2003年 | 31篇 |
2002年 | 31篇 |
2001年 | 10篇 |
2000年 | 6篇 |
1999年 | 5篇 |
1998年 | 3篇 |
1997年 | 4篇 |
1996年 | 6篇 |
1995年 | 1篇 |
1994年 | 10篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 8篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 7篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1977年 | 2篇 |
1976年 | 5篇 |
1975年 | 6篇 |
1974年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有1228条查询结果,搜索用时 0 毫秒
81.
Cavaliere C Foglia P Guarino C Nazzari M Samperi R Laganà A 《Rapid communications in mass spectrometry : RCM》2007,21(4):550-556
A liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) method for measurement of aflatoxins B1, B2, G1, and G2 in maize is described. Aflatoxins (AFs) were extracted from 1 g samples by using tri-portions of acetonitrile/water (80:20, v/v) (10 + 7 + 7 mL), and 2/5 of the extract diluted to 500 mL by water was cleaned up with a 100 mg Carbograph-4 cartridge. After the addition of the internal standard AFM1, the final extract was analyzed by LC/ESI-MS/MS in positive ion mode using multiple reaction monitoring with a triple-quadrupole instrument. A C(18) column thermostatted at 45 degrees C with a mobile phase gradient of acetonitrile/water with 2 mmol/L ammonium formate was used. Although the matrix suppression effect was negligible, quantitation was achieved by an external calibration procedure using matrix-matched standard solutions to improve accuracy. Sample recoveries at four spiking levels ranged from 81 to 101% (relative standard deviation (RSD) =12%), and the method quantification limit ranged from 0.1 to 0.6 microg/kg. This method is sensitive, accurate and selective, and can thus be applied as a confirmatory procedure in establishing non-compliance with EU permitted maximum levels (2 microg/kg for aflatoxin B1 and 4 microg/kg for total AFs in corn products). This work also includes an application to samples obtained from retail markets and from experimental trial fields. Of the 48 samples, 15 were found to be contaminated with AFs and, of these, 5 did not comply with European Union legislation (AFB1 =2 microg/kg). 相似文献
82.
Zoccatelli G Dalla Pellegrina C Mosconi S Consolini M Veneri G Chignola R Peruffo A Rizzi C 《Electrophoresis》2007,28(3):460-466
Wheat proteinaceous alpha-amylase inhibitors (alpha-AIs) are increasingly investigated for their agronomical role as natural defence molecules of plants against the attack of insects and pests, but also for their effects on human health. The wheat genomes code for several bioactive alpha-AIs that share sequence homology, but differ in their specificity against alpha-amylases from different species and for their aggregation states. Wheat alpha-AIs are traditionally classified as belonging to the three classes of tetrameric, homodimeric and monomeric forms, each class being constituted by a number of polypeptides that display different electrophoretic mobilities. Here we describe a proteomic approach for the identification of bioactive alpha-AIs from wheat and, in particular, a 3-D technique that allows to best identify and characterize the dimeric fraction. The technique takes advantage of the thermal resistance of alpha-AIs (resistant to T > 70 degrees C) and consists in the separation of protein mixtures by 2-D polyacrylamide/starch electrophoresis under nondissociating PAGE (ND-PAGE, first dimension) and dissociating (urea-PAGE or U-PAGE second dimension) conditions, followed by in-gel spontaneous reaggregation of protein complexes and identification of the alpha-amylase inhibitory activity (antizymogram, third dimension) using enzymes from human salivary glands and from the larvae of Tenebrio molitor coleopter (yellow mealworm). Dimeric alpha-AIs from Triticum aestivum (bread wheat) were observed to exist as heterodimers. The formation of heterodimeric complexes was also confirmed by in vitro reaggregation assays carried out on RP-HPLC purified wheat dimeric alpha-AIs, and their bioactivity assayed by antizymogram analysis. The present 3-D analytical technique can be exploited for fast, full-fledged identification and characterization of wheat alpha-AIs. 相似文献
83.
84.
Dionigi C Stoliar P Porzio W Destri S Cavallini M Bilotti I Brillante A Biscarini F 《Langmuir : the ACS journal of surfaces and colloids》2007,23(4):2030-2036
We demonstrate field effect transistors based on organic semiconductor molecules dispersed in a self-organized polystyrene (PS) latex bead matrix. An aqueous colloidal composite made of PS and tetrahexylsexithiophene (H4T6) is deposited with a micropipet into the channel of a bottom-contact field effect transistor. The beads self-organize into a network whose characteristic distances are governed by their packing. The semiconductor molecules crystallize in the interstitial voids, leading to the growth of large interconnected domains. Depending on the bead size and the ratio between H4T6 and PS, the fraction of the different phases in the polymorph can be controlled. In the transistors where the H4T6 metastable "red phase" is the largest, the device response and the charge mobility are comparable to those of sexithienyl thin films grown by high-vacuum sublimation. 相似文献
85.
Frazzoli C Dragone R Mantovani A Massimi C Campanella L 《Analytical and bioanalytical chemistry》2007,389(7-8):2185-2194
Toxicological implications of exposure to bioavailable platinum group metals, here Pd, Pt, and Rh, are still to be clarified.
This study obtained by a biosensor-based method preliminary information on potential effects on cellular metabolism as well
as on possible tolerance mechanisms. Aerobic respiration was taken as the toxicological end point to perform tandem tests,
namely functional toxicity test and tolerance test. Cells were suspended in the absence of essential constituents for growth.
The dose–response curves obtained by exposure (2 h) to the metals (nanogram per gram range) suggested the same mechanisms
of action, with Rh showing the greatest curve steepness and the lowest EC50 value. Conservative (95% lower confidence interval) EC10 values were 187, 85 and 51 ng g−1 for Pt, Pd, and Rh respectively. Tolerance patterns were tested during the same runs. The full tolerance obtained after 12 h
of exposure to each metal suggested mitochondrial inhibition of aerobic respiration as a target effect. The hazard rating
of the metals in the tolerance test changed in the Rh EC50 range, where Rh showed the lowest toxicity. The observed tolerance might suggest a protective mechanism such as metallothionein
induction at concentrations around the EC50 values. The performance of the bioassay was satisfactory, in terms of the limit of detection, repeatability, reproducibility,
roboustness, sensibility, and stability; the method’s critical uncertainty sources were identified for improvements.
Figure Respirometric curved 相似文献
86.
87.
Stefania?Gaudino Chiara?Galas Maria?Belli Sabrina?Barbizzi Paolo?de?Zorzi Radojko?Ja?imovi? Zvonka?Jeran Alessandra?Pati Umberto?SansoneEmail author 《Accreditation and quality assurance》2007,12(2):84-93
The measurement of trace-element concentration in soil, sediment and waste, is generally a combination of a digestion procedure
for dissolution of elements and a subsequent measurement of the dissolved elements. “Partial” and “total” digestion methods
can be used in environmental monitoring activities. To compare measurement results obtained by different methods, it is crucial
to determine and to maintain control of the bias of the results obtained by these methods. In this paper, ICP-MS results obtained
after matrix digestion with modified aqua regia (HCl+HNO3+H2O2) method and two “total” digestion methods (microwave aqua regia+HF and HNO3+HF) are compared with those obtained by instrumental neutron activation analysis, a non-destructive analytical method for
the determination of the total mass concentrations of inorganic components in environmental matrices. The comparison was carried
out on eight agricultural soil samples collected in one test area and measured by k0-INAA and ICP-MS to determine As, Co, Cr, Sb and Zn mass concentration. The bias of results for As, Cd, Co, Cr, Cu, Ni, Pb,
Sb and Zn of the three digestion methods were assessed using selected measurement standards. This paper highlights that the
digestion procedure is an integral part of the measurement and can affect the measurement result in environmental analysis. 相似文献
88.
Most known perylene diimides are lipophilic, with few exceptions of hydrophilic derivatives. Even in the latter case, the compounds have limited water solubility and show a strong tendency to self-aggregation. In this paper we present the synthesis of four new perylene derivatives with three and four basic side chains, obtained by functionalizing the bay-area of perylene. These molecules show great solubility in aqueous media as hydrochlorides and their tendency to self-aggregate is remarkably reduced with respect to the previously synthesized two-chained perylene diimides. Their different spectroscopic properties in various solvents and conditions are reported and discussed. 相似文献
89.
90.
Wu J Zaleski TJ Valenzano C Khosla C Cane DE 《Journal of the American Chemical Society》2005,127(49):17393-17404
Picromycin/methymycin synthase (PICS) is a modular polyketide synthase (PKS) that is responsible for the biosynthesis of both 10-deoxymethynolide (1) and narbonolide (2), the parent 12- and 14-membered aglycone precursors of the macrolide antibiotics methymycin and picromycin, respectively. PICS module 2 is a dehydratase (DH)-containing module that catalyzes the formation of the unsaturated triketide intermediate using malonyl-CoA as the chain extension substrate. Recombinant PICS module 2+TE, with the PICS thioesterase domain appended to the C-terminus to allow release of polyketide products, was expressed in Escherichia coli. Purified PICS module 2+TE converted malonyl-CoA and 4, the N-acetylcysteamine thioester of (2S,3R)-2-methyl-3-hydroxypentanoic acid, to a 1:2 mixture of the triketide acid (4S,5R)-4-methyl-5-hydroxy-2-heptenoic acid (5) and (3S,4S,5R)-3,5-dihydroxy-4-methyl-n-heptanoic acid-delta-lactone (10) with a combined kcat of 0.6 min(-1). The triketide lactone 10 is formed by thioesterase-catalyzed cyclization of the corresponding d-3-hydroxyacyl-SACP intermediate, a reaction which competes with dehydration catalyzed by the dehydratase domain. PICS module 2+TE showed a strong preference for the syn-diketide-SNAC 4, with a 20-fold greater kcat/K(m) than the anti-(2S,3S)-diketide-SNAC 14, and a 40-fold advantage over the syn-(2R,3S)-diketide-SNAC 13. PICS module 2(DH(0))+TE, with an inactivated DH domain, produced exclusively 10, while three PICS module 2(KR(0))+TE mutants, with inactivated KR domains, produced exclusively or predominantly the unreduced triketide ketolactone, (4S,5R)-3-oxo-4-methyl-5-hydroxy-n-heptanoic acid-delta-lactone (7). These studies establish for the first time the structure and stereochemistry of the intermediates of a polyketide chain elongation cycle catalyzed by a DH-containing module, while confirming the importance of key active site residues in both KR and DH domains. 相似文献