全文获取类型
收费全文 | 4318篇 |
免费 | 303篇 |
国内免费 | 287篇 |
专业分类
化学 | 904篇 |
晶体学 | 1篇 |
力学 | 390篇 |
综合类 | 42篇 |
数学 | 2488篇 |
物理学 | 1083篇 |
出版年
2024年 | 13篇 |
2023年 | 68篇 |
2022年 | 67篇 |
2021年 | 92篇 |
2020年 | 107篇 |
2019年 | 124篇 |
2018年 | 103篇 |
2017年 | 118篇 |
2016年 | 121篇 |
2015年 | 105篇 |
2014年 | 172篇 |
2013年 | 311篇 |
2012年 | 197篇 |
2011年 | 198篇 |
2010年 | 158篇 |
2009年 | 239篇 |
2008年 | 242篇 |
2007年 | 244篇 |
2006年 | 204篇 |
2005年 | 213篇 |
2004年 | 209篇 |
2003年 | 196篇 |
2002年 | 167篇 |
2001年 | 131篇 |
2000年 | 98篇 |
1999年 | 114篇 |
1998年 | 106篇 |
1997年 | 95篇 |
1996年 | 86篇 |
1995年 | 65篇 |
1994年 | 60篇 |
1993年 | 44篇 |
1992年 | 43篇 |
1991年 | 42篇 |
1990年 | 37篇 |
1989年 | 39篇 |
1988年 | 32篇 |
1987年 | 16篇 |
1986年 | 19篇 |
1985年 | 26篇 |
1984年 | 29篇 |
1983年 | 22篇 |
1982年 | 26篇 |
1981年 | 15篇 |
1980年 | 9篇 |
1979年 | 29篇 |
1978年 | 12篇 |
1977年 | 14篇 |
1976年 | 10篇 |
1974年 | 6篇 |
排序方式: 共有4908条查询结果,搜索用时 15 毫秒
71.
72.
Reduced inorganic phosphorus in the natural environment: significance, speciation and determination 总被引:1,自引:0,他引:1
It is commonly assumed that phosphorus occurs almost exclusively in the environment as fully oxidized phosphate (primarily H2PO4− and HPO42−, where the oxidation state of phosphorus is +V). Recent developments in the field of microbiology and research on the origin of life have suggested a possibly significant role for reduced, inorganic forms of phosphorus in bacterial metabolism and as evolutionary precursors of biological phosphate compounds. Reduced inorganic forms of phosphorus include phosphorus acid (H3PO3, P(+III)), hypophosphorus acid (H3PO2, P(+I)) and various forms of phosphides (P(−III)). Reduced phosphorus has been detected in anaerobic sediments, sewage treatment facilities and in industrial and agricultural processes.Microbiological evidence suggests a significant role for reduced phosphorus species in metabolic processes and raises interesting questions regarding the biogeochemistry of this nutrient in the environment. However, the paucity of data on the presence and cycling of reduced phosphorus compounds in the environment requires attention in order to elucidate the role of these compounds in natural systems. This paper discusses the significance of reduced phosphorus in the natural environment, its speciation and methods of detection. 相似文献
73.
Surface plasmon resonance (SPR) has been successfully applied for the simple, rapid, and label-free assay of various biomolecules. This assay evaluates a novel wavelength modulation SPR biosensor for the detection of tetanus toxin. The wavelength modulation SPR biosensor is designed based on fixing the incident angle of light and measuring the reflected intensities in the resonance wavelength range spanning 400-800 nm simultaneously. Tetanus toxin (TeNT), one of the most potent toxins known, is synthesized as a 150 kDa single polypeptide chain. The SPR biosensor has been shown to be capable of directly detecting concentration of tetanus toxin as low as 0.028 Lf ml−1. Under selected experimental conditions, the SPR biosensor has a good reproducibility, sensitivity and reversibility. The results illustrate how wavelength modulation SPR biosensor can be used to detect biomolecular interactions. 相似文献
74.
Microorganisms produce toxins against its competitors sometimes, and variable yields are useful to explain the observed oscillatory
behavior in the reactor. In this paper, a model with general quadric yields of competition in the bioreactor of two competitors
for a single nutrient where one of the competitors can produce toxin against its opponent, is proposed. We analyze the asymptotic
behavior of the model in terms of the relevant parameters. The conditions of the three dimensional Hopf bifurcation, and the
existence of limit cycles in the nutrient-organism phase plane are obtained. 相似文献
75.
Detection of single molecules, particles, and rapid redox events is a challenge of electrochemical investigations and requires either an amplification strategy or significant averaging for the electrochemical current to exceed the noise level. We consider the minimum number of electrons required to reach the limit of quantification in these electrochemical measurements. A survey of the literature indicates that the state-of-the-art limit in current detection for different types of measurements (e.g. voltammetry, single-molecule redox cycling, ion channel recordings of single molecules, metal nanoparticle collision, and phase nucleation) is independent of the nature of the measurement and increases linearly with reciprocal response time, Δt?1, over ~5 orders of magnitude (from ~10 to ~106 s?1). We demonstrate that the practical limit of quantification requires cumulative measurement of ~2100 electrons during Δt and is determined by statistics of counting electrons, that is, the shot noise in the current. 相似文献
76.
The possibility to perform gas chromatographic analysis using both a mass detector and a selective nitrogen-phosphorous detector will enable highly sensitive analytical results and an optimal identification reliability. In this work we describe an easily workable technical solution, which has been accomplished in our laboratory and the application of the above system to analytical procedures for the detection and determination of nitrogenous herbicides in drinking water and phosphorylated pesticides in agricultural products is discussed. 相似文献
77.
Selective detection and identification of Se containing compounds--review and recent developments 总被引:5,自引:0,他引:5
The complexity of selenium (Se) chemistry in the environment and in living organisms presents broad analytical challenges. The selective qualitative and quantitative determination of particular species of this element is vital in order to understand selenium's metabolism and significance in biology, toxicology, clinical chemistry and nutrition. This calls for state-of-the-art analytical techniques such as hyphenated methods that are reviewed with particular emphasis on interfaced separation with element-selective detection and identification of the detected selenium compounds. Atomic spectral element specific detection for monitoring chromatographic eluent enabled quantitative determination of selenium species in selenized yeast and qualitative measurement for breath samples. Gas chromatography with atomic emission detection (AED) of ethylated species and fluoroacid ion pair HPLC applied to the analysis of currently produced or archived selenized yeast and Brassica juncea have revealed the presence of a previously unrecognised Se-S amino acid, S-(methylseleno)cysteine. 相似文献
78.
The determination of nanometer thick layers of poly(methyl methacrylate) coated on to the surface of poly(ethylene terephthalate) film has been investigated by high resolution pyrolysis gas chromatography without sample pretreatment or modification of the instrumentation used. A good linear relationship was observed between the quantity of the characteristic pyrolysate and the thickness of the poly(methyl methacrylate) layer; the detection limit was sufficient to enable the quantitation of poly(methyl methacrylate)-to-poly(ethylene terephthalate) film thickness ratios of 1:20000 in composite materials. 相似文献
79.
The Commission Decision of August 12, 2002 on the performance of analytical methods and the interpretation of results was applied to the HPLC method for the analysis of parabens, 2-phenoxyethanol and 1-phenoxypropan-2-ol in cosmetic products. This method is published in the seventh Directive 96/45/EC of the European Commission. Non-compliant concentrations, taking into account the data distribution (CC) and the probability of false negative values (CC) were determined. The repeatability and reproducibility amount to <4% and <7%, respectively. These values were obtained with blanc samples that were fortified in the laboratory. Calibration linearity was confirmed by absence of lack of fit for all seven preservatives. Matrix effects on the determinations of the preservatives in body milk or shampoo are negligible. 相似文献
80.