全文获取类型
收费全文 | 46921篇 |
免费 | 3213篇 |
国内免费 | 4810篇 |
出版年
2023年 | 755篇 |
2022年 | 1026篇 |
2021年 | 1170篇 |
2020年 | 1147篇 |
2019年 | 1804篇 |
2018年 | 1610篇 |
2017年 | 1307篇 |
2016年 | 1180篇 |
2015年 | 1144篇 |
2014年 | 2310篇 |
2013年 | 2903篇 |
2012年 | 1777篇 |
2011年 | 2173篇 |
2010年 | 1641篇 |
2009年 | 2135篇 |
2008年 | 2205篇 |
2007年 | 2429篇 |
2006年 | 2181篇 |
2005年 | 1774篇 |
2004年 | 1505篇 |
2003年 | 1496篇 |
2002年 | 1334篇 |
2001年 | 1146篇 |
2000年 | 953篇 |
1999年 | 822篇 |
1998年 | 793篇 |
1997年 | 752篇 |
1996年 | 738篇 |
1995年 | 730篇 |
1994年 | 721篇 |
1993年 | 631篇 |
1992年 | 637篇 |
1991年 | 628篇 |
1990年 | 478篇 |
1989年 | 497篇 |
1988年 | 441篇 |
1987年 | 437篇 |
1986年 | 415篇 |
1985年 | 662篇 |
1984年 | 890篇 |
1983年 | 721篇 |
1982年 | 752篇 |
1981年 | 601篇 |
1980年 | 611篇 |
1979年 | 546篇 |
1978年 | 432篇 |
1977年 | 403篇 |
1976年 | 372篇 |
1974年 | 244篇 |
1973年 | 256篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Barry M. Gordon 《Biological trace element research》1987,12(1):153-159
Information concerning the chemical state of trace elements in biological systems generally has not been available. Such information
for toxic elements and metals in metalloproteins could prove extremely valuable in the elucidation of their metabolism and
other biological processes. The shielding of core electrons by binding electrons affect the energy required for creating inner-shell
holes. Furthermore, the molecular binding and symmetry of the local environment of an atom affect the absorption spectrum
in the neighborhood of the absorption edge. X-ray absorption near-edge structure (XANES) using synchrotron radiation excitation
can be used to provide chemical speciation information for trace elements at concentrations as low as 10 ppm. The structure
and position of the absorption curve in the region of an edge can yield vital data about the local structure and oxidation
state of the trace element in question. Data are most easily interpreted by comparing the observed edge structure and position
with those of model compounds of the element covering the entire range of possible oxidation states. Examples of such analyses
will be reviewed. 相似文献
142.
A. S. Paschoa M. E. Wrenn N. P. Singh F. W. Bruenger S. C. Miller M. Cholewa K. W. Jones 《Biological trace element research》1987,13(1):275-282
Several geological formations of the Utah-Colorado mining region mined for uranium ore during and after World War II had been
mined earlier for vanadium. Therefore, most miners and millers from that region were exposed to those metals’ ores or tailings
at one time or another. Preliminary investigation to determine uranium and vanadium retained in the lungs of a former uranium
miner and miller from this region, who died of lung cancer (mesothelioma), showed a high nonuniform distribution of vanadium.
This observation led to the hypothesis that the vanadium content in the lungs could be associated with inhaled particles.
Further examination of spectra of characteristic X-rays obtained by scanning particle-induced X-ray emission (microPIXE) of
an autopsy sample of this lung indicated that vanadium was indeed present in localized sites within the 20-μm spatial resolution
of the proton beam. This work points out that the microPIXE-RBS (Rutherford backscattering) test for vanadium can be used
for site localization of inhaled particles retained in the lungs. Further studies are in progress to: (i) locate uranium-bearing
particles in lung tissues of former uranium miners and millers; and (ii) evaluate the local doses of alpha radiation received
from these particles. 相似文献
143.
Many biomedical experiments require the qualitative and quantitative localization of trace elements with high sensitivity
and good spatial resolution. The feasibility of measuring the chemical form of the elements, the time course of trace element
metabolism, and conducting experiments in living biological systems are also important requirements for biological trace element
research. Nuclear analytical techniques that employ ion or photon beams have grown in importance in the past decade and have
led to several new experimental approaches. Some of the important features of these methods are reviewed here along with their
role in trace element research. Examples of their use are given to illustrate potential for new research directions. It is
emphasized that the effective application of these methods necessitates a closely integrated multidisciplinary scientific
team. 相似文献
144.
A particle-induced X-ray emission (PIXE) analysis method is presented, which allows measurement of eight elements (i.e., K,
Ca, Mn, Fe, Cu, Zn, Se, and Rb) in human brain samples of only a few mg dry weight. The precision and accuracy of the method
were investigated by analyzing animal brain matter with both PIXE and instrumental neutron activation analysis (INAA). The
method was applied to measure the 8 elements in 46 different regions of 3 human brains. The sections analyzed originated from
either the left or the right cerebral hemisphere, brain stem, and cerebellum. For one of the brains, sections were also analyzed
from 26 corresponding regions of both hemispheres. For all elements, similar concentrations were found in the corresponding
areas of the left and right sides of the brain. The concentrations (in μg/g dry weight) of the elements K, Fe, Cu, Zn, Se,
and Rb were consistently higher in cortical structures than in white matter. Deep nuclei and brain stem, which have a mixed
composition, showed intermediate values for K, Zn, Se, and Rb. A hierarchical cluster analysis indicated that the various
brain regions clustered into two large groups, one comprising gray and mixed matter regions and the other, white and mixed
matter brain areas. 相似文献
145.
T. Cichocki B. Gonsior J. Glazur M. Höfert L. Jarczyk E. Rokita A. Strzałkowski M. Sych 《Biological trace element research》1987,13(1):167-177
The elemental composition of rabbit liver was determined by the PIXE and micro-PIXE methods. The mean concentrations of P,
S, Cl, K, Fe, Cu, Zn, and Rb measured by both methods were similar. The latter method also allowed for localization of elements
within lobule territory. It has been found that some elements are more prevalent in the veins (Cl, Fe) and others in the liver
parenchyma (P, Cu, Zn). Moreover, Zn showed the characteristic intralobular distribution. Some methodological aspects of microbeam
application to biological materials were also discussed. 相似文献
146.
147.
Werner Eckert 《Biogeochemistry》1987,4(1):15-26
The process of H2S oxidation by the phototrophic bacteriaThiocapsa roseopersicina andChlorobium phaeobacteroides, respectively, was monitored using a Pt-glass-Ag0, Ag2S electrode combination without liquid junction. Due to the resulting pe(pH) and pH2S plottings three steps can be distinguished: oxidation of H2S to an S(0) state, oxidation of S (0) to SO4
2–, and oxidation of the remaining H2S directly to SO4
2–. Differences between the investigated bacteria exist with respect to their individual oxidation strategies.Thiocapsa apparently stops oxidizing H2S at pH2S 7.5 (e.g. 10–7.5M H2S) and shifts to the utilization of the intracellularly stored S (0). In contrastChlorobium utilizes its extracellularly stored sulfur parallel to the extracellular H2S fraction. The corresponding Pt-sensor responses (pe7 values) were found to be similar to the corresponding partial redox equilibria (p7 values) of H2S oxidation stoichiometries as proposed by Van Niel (1931) and Trüper (1964). It is concluded that the recording of pe enables investigators to understand (and control) in situ redox processes, independent of their thermodynamic equilibration, only bound to changes of electroactivity vs. sensor. 相似文献
148.
Photosynthetically active vesicles prepared from Chlamydomonas reinhardtii retained a light-dependent glutamate synthase activity which was highly specific for 2-oxoglutarate (Km=2.1 mM) and L-glutamine (Km=0.9 mM) as amido group acceptor and donor respectively. This activity was inhibited by azaserine, p-hydroxymercuribenzoate and 3-(p-chlorophenyl)-1,1-dimethyl urea.Light-dependent synthesis of glutamate was also obtained by coupling Chlamydomonas photosynthetic particles to purified ferredoxin-glutamate synthase, using ascorbate and 2,6-dichlorophenol-indophenol as electron donor. This system was also specific for 2-oxoglutarate (Km=1 mM) and L-glutamine (Km=0.8 mM) as substrates, and was stimulated by dithioerythritol. Azaserine and p-hydroxymercuribenzoate, but not 3-(p-chlorophenyl)-1,1-dimethyl urea, inhibited the reconstituted activity; high concentrations of 2-oxoglutarate were inhibitory.Abbreviations A
Absorbance
- CCP
p-Trichlorometoxi-carbonylcyanide-phenylhydrazone
- Chl
Chlorophyll
- CMU
3-(p-Chlorophenyl)-1,1-dimethyl urea
- DPIP
2,6-Dichlorophenol-indophenol
- DTE
Dithioerythritol
- MSX
L-Methionine, D-L, sulfoximine
- MV
Methyl viologen 相似文献
149.
Yasumasa Kunifuji Terumasa Nakamura Masayuki Takasugi 《Biological trace element research》1987,14(3):237-248
Cd induced changes of Zn and Cd distribution in the liver and kidneys were studied in relation to Cd metallothionein (MT)
synthesis. Wistar male rats were given CdCl2 by sc injection of .8, 1.5, and 3.0 mg Cd/kg three times a week for three weeks. Cd levels of liver and kidneys increased
with the increment of Cd dosage and 80–90% of Cd was found in the cytosol. The MT fractions contained 80–89% cytosolic Cd
in the liver and 55–75% Cd in the kidneys. Zn concentrations in the liver increased following Cd administration, But Zn in
the kidneys showed only slight increase. There was a distinct decrease of Cu concentration in the liver of the 3.0 mg group.
In contrast, Cu concentrations in the kidneys increased about three times in the .8 and 1.5 mg Cd groups, but Cu in the 3.0
mg group showed only 1.5 times increase. The changes of these metal concentrations were observed mainly in the cytosol. Non-MT-Cd
in the kidneys was maximum in the 1.5 mg group, but the 3.0 mg group showed significant decrease. In parallel with this decrease
of Cd, Cu and Zn in the kidneys showed similar decrease. When the kidneys are injured, Zn and Cu appear to leak from this
organ. 相似文献
150.
Evelyn Fox Keller 《Biology & philosophy》1987,2(4):383-396
In much of the discourse of evolutionary theory, reproduction is treated as an autonomous function of the individual organism — even in discussions of sexually reproducing organisms. In this paper, I examine some of the functions and consequences of such manifestly peculiar language. In particular, I suggest that it provides crucial support for the central project of evolutionary theory — namely that of locating causal efficacy in intrinsic properties of the individual organism. Furthermore, I argue that the language of individual reproduction is maintained by certain methodological conventions that both obscure many of the problems it generates and serve to actively impede attempts to redress those difficulties that can be identified. Finally, I suggest that inclusion of the complexities introduced by sexual reproduction — in both language and methodology — may radically undermine the individualist focus of evolutionary theory.I am indepted to the Rockefeller Foundation for a Humanities Fellowship that supported this research during the spring of 1986. I am also grateful to Richard Lewontin, Diane Paul, and Lisa Lloyd for many extremely helpful conversations. 相似文献