全文获取类型
收费全文 | 3077篇 |
免费 | 311篇 |
出版年
2021年 | 43篇 |
2020年 | 28篇 |
2019年 | 43篇 |
2018年 | 45篇 |
2017年 | 42篇 |
2016年 | 71篇 |
2015年 | 138篇 |
2014年 | 139篇 |
2013年 | 165篇 |
2012年 | 189篇 |
2011年 | 175篇 |
2010年 | 84篇 |
2009年 | 110篇 |
2008年 | 121篇 |
2007年 | 135篇 |
2006年 | 116篇 |
2005年 | 115篇 |
2004年 | 118篇 |
2003年 | 108篇 |
2002年 | 144篇 |
2001年 | 72篇 |
2000年 | 76篇 |
1999年 | 68篇 |
1998年 | 40篇 |
1997年 | 43篇 |
1996年 | 37篇 |
1995年 | 34篇 |
1994年 | 30篇 |
1993年 | 21篇 |
1992年 | 55篇 |
1991年 | 52篇 |
1990年 | 42篇 |
1989年 | 45篇 |
1988年 | 32篇 |
1987年 | 40篇 |
1986年 | 30篇 |
1985年 | 28篇 |
1984年 | 32篇 |
1983年 | 33篇 |
1982年 | 23篇 |
1981年 | 21篇 |
1979年 | 27篇 |
1978年 | 19篇 |
1977年 | 22篇 |
1976年 | 22篇 |
1975年 | 32篇 |
1974年 | 27篇 |
1973年 | 25篇 |
1972年 | 19篇 |
1969年 | 20篇 |
排序方式: 共有3388条查询结果,搜索用时 875 毫秒
1.
2.
Bruce R. Dorrbecker Susan H. Mercik Paul A. Kramer 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1984,336(1)
A high-performance liquid chromatographic procedure is reported for reproducibly and sensitively quantitating caffeine and its N-demethylated metabolite paraxanthine in micro-samples. A 5-μm reversed-phase radial compression column and 214-nm fixed wavelength ultraviolet detector were used to attain a sensitivity sufficient to quantitate these compounds at concentratios as low as 80 ng/ml using only 25 μl of sample. The assay is applicable to microliter samples of whole blood, serum, plasma, saliva, amniotic, cerebro-spinal and gastric fluids such as might be obtained in studies involving small animals or neonates. The utility of the assay is illustrated with caffeine and paraxanthine levels measured in several maternal and fetal fluids following constant-rate intravenous infusion of caffeine into a rabbit throughout pregnancy. 相似文献
3.
The non-recombining nature of the Y-chromosome determines the non-independence of alleles between loci. The evolution of short tandem repeat (STR) loci in the Y-chromosome is the result of different factors such as differential mutation rates, mutation modes, gene conversion, selection and demographic processes. The degree of correlation between loci is dependent on the magnitude of these processes. The simulation of data is a routine tool used for testing hypotheses in population and evolutionary studies. The most basic parameters hitherto used in lineage haplotype simulations are the allele frequency distributions and mutation rates, assuming either full independence or linkage between loci. In this study we introduce use of the Spearman correlation coefficient to estimate the degree of dependence between non-recombining loci. Then, both the interdependence between loci and the allele frequency distributions at multi-allelic loci are incorporated in an algorithm for simulating haplotypes. We illustrate the method using published and unpublished Y-chromosome STR data. 相似文献
4.
5.
6.
7.
Dielectric spectroscopy is a non-invasive contact technique that permits the in vivo measurement of the specific electrical properties of biological tissue induced by an external electrical field. Permittivity, relaxation time and specific conductivity as a function of corneal hydration (wet weight/dry weight) and temperature were measured in 10 porcine corneas. Variation of tissue hydration has a minor influence on the signal, with a significant variation of the signal being detectable only for relatively dry tissue. A much greater influence was found for temperature, in particular on relaxation times. Dielectric spectroscopy provides us with an opportunity to detect structural, in particular temperature-induced, changes in living tissue. In the frequency range investigated, hydration has only a small influence on the dielectric properties of the tissue. 相似文献
8.
An electric circuit for plant protoplast manipulation is described. The circuit used readily available materials and was designed for use in teaching. This integrated circuit can be placed in a single small box with controls for the aligning voltage, the aligning frequency, the pulse voltage, and the pulse timing. The circuit can be supplied by any suitable source of dc power and can be easily altered for individual requirements. The circuit, as presented here, can be assembled for less than $250. 相似文献
9.
10.
A disbalance between beta-adrenergic and muscarinic responses caused by hydrogen peroxide in rat airways in vitro 总被引:3,自引:0,他引:3
K Kramer C J Doelman H Timmerman A Bast 《Biochemical and biophysical research communications》1987,145(1):357-362
The effect of hydrogen peroxide on adrenergic and muscarinic responses of rat airway smooth muscle was studied. The trachea muscle and the lung parenchymal strip were contracted with methacholine and relaxed with (-)-isoprenaline. Recording of three (-)-isoprenaline curves on the trachea muscle and the lung parenchymal strip was followed by treatment for 30 min with hydrogen peroxide (H2O2) (1mM) after which a new dose response curve for (-)-isoprenaline was constructed. Using the trachea muscle this treatment with H2O2 resulted in a decrease of 61% of the maximum contraction by methacholine compared with the control and a complete inhibition of the relaxation by (-)-isoprenaline. In the lung parenchymal strip preparation we found, after the same treatment no reduction of the contraction by methacholine and 61% reduction of the relaxation by (-)-isoprenaline, compared with the control. The results demonstrate that the adrenergic response in rat airways is more susceptible to hydrogen peroxide than the muscarinic response. 相似文献