全文获取类型
收费全文 | 5935篇 |
免费 | 185篇 |
国内免费 | 310篇 |
出版年
2023年 | 45篇 |
2022年 | 69篇 |
2021年 | 103篇 |
2020年 | 119篇 |
2019年 | 178篇 |
2018年 | 156篇 |
2017年 | 107篇 |
2016年 | 156篇 |
2015年 | 172篇 |
2014年 | 354篇 |
2013年 | 485篇 |
2012年 | 180篇 |
2011年 | 336篇 |
2010年 | 270篇 |
2009年 | 328篇 |
2008年 | 365篇 |
2007年 | 350篇 |
2006年 | 306篇 |
2005年 | 286篇 |
2004年 | 193篇 |
2003年 | 212篇 |
2002年 | 205篇 |
2001年 | 102篇 |
2000年 | 97篇 |
1999年 | 100篇 |
1998年 | 90篇 |
1997年 | 76篇 |
1996年 | 66篇 |
1995年 | 73篇 |
1994年 | 83篇 |
1993年 | 73篇 |
1992年 | 64篇 |
1991年 | 51篇 |
1990年 | 45篇 |
1989年 | 46篇 |
1988年 | 40篇 |
1987年 | 35篇 |
1986年 | 27篇 |
1985年 | 56篇 |
1984年 | 104篇 |
1983年 | 72篇 |
1982年 | 38篇 |
1981年 | 38篇 |
1980年 | 28篇 |
1979年 | 10篇 |
1978年 | 14篇 |
1977年 | 7篇 |
1976年 | 9篇 |
1975年 | 5篇 |
1973年 | 3篇 |
排序方式: 共有6430条查询结果,搜索用时 109 毫秒
51.
A highly sensitive enzymatic assay for diadenosine 5′,5?-P1,P3-triphosphate (Ap3A) has been established on the basis of the coupled luminescence assay for diadenosine 5′,5?-P1,P3-tetraphosphate (A. Ogilvie (1981)Anal. Biochem.115, 302–307). Snake venom phosphodiesterase splits Ap3A into AMP plus ADP which can be measured in a luminescence reaction containing pyruvate kinase, phosphoenolpyruvate and luciferin-luciferase. The procedure is linear with Ap3A levels ranging from 0.1 to 2 pmol. The assay has been used to measure Ap3A in various eukaryotic cells after ion-exchange chromatography and high-performance liquid chromatography of acidic extracts of the cells. The level of diadenosine triphosphate was higher in all instances than the level of diadenosine tetraphosphate. When growing in the abdominal cavity of mice, Ehrlich ascites tumor cells contained high amounts of Ap3A (), allowing direct optical determination in the HPLC chromatography. The quantitative measurement of Ap3A with the luminescence assay gave identical results. Ap3A extracted from Ehrlich cells was also chromatographed with authentic nucleotide in two thin-layer systems providing additional proof for the existence of Ap3A in biological material. 相似文献
52.
53.
We describe the principles of a new generation of sequential or simultaneous time-resolved fluoroimmunoassays, namely, simple, rapid, liquid-phase non-separation procedures which may be applied to the measurement of urinary steroid and drug metabolites. As an example, a method for the measurement of estrone-3-glucuronide in undiluted urine is reported. This method has a similar sensitivity, specificity and accuracy to a conventional separation fluoroimmunoassay or radioimmunoassay but in terms of speed, convenience, precision, reliability and clinical utility the new method has many advantages. The labelled antigen is a novel fluorescent europium chelate covalently linked to estrone-3-glucuronide. The antibody-binding reaction involves the incubation of the labelled antigen (2ng) with a limited concentration of polyclonal or monoclonal antibodies to estrone-3-glucuronyl-6-BSA and an aliquot of standard or sample (undiluted urine; 10 μl) in microtitre wells. After a 10 min incubation, the fluorescence which emanates from the antibody-free label is measured in a time-resolved fluorometer and is proportional to the concentration of estrone-3-glucuronide in the standard or sample. The method may be applied for the monitoring of ovarian function in women. 相似文献
54.
A combined system of chemiluminescence detection and high performance liquid chromatography (CL–HPLC) was developed to determine primary peroxidation products in biological tissues, such as phosphatidylcholine hydroperoxide (PCOOH). The CL–HPLC assay consists of separation of lipid classes with HPLC and detection of hydroperoxide-specific chemiluminescence. Hydroperoxides react with heme compounds to produce oxidants as suggested by our early studies on tissue low-level chemiluminescence in which singlet molecular oxygen is generated as one of the excited species in several biological systems involving free radical events. In the CL–HPLC method, a cytochrome c–luminol mixture was used as a hydroperoxide-specific luminescent reagent, and the quantification of hydroperoxide was performed by detecting chemiluminescence due to the luminol oxidation caused by the oxidant produced during the lipid hydroperoxides with heme. The detection limit of PCOOH was 10 pmole hydroperoxide–O2. PCOOH in normal human blood was found to be 10–500 pmol/ml plasma and significantly higher levels of PCOOH were observed in some hospitalized patients. 相似文献
55.
用间接酶联免疫吸附实验(ELISA)对新近诊断的179例血液病患者血清巨细胞病毒(HCMV)IgM和IgA抗体进行了检测。阳性率分别为11,17%11,73%,明显高于对照人群(4,76%和3,97%),提示血液病患者由于免疫功能下降,易于发生HCMV活动性感染。 相似文献
56.
The distribution density of opioid receptors in the brain of El mice (seizure-susceptible strain) was examined to determine the relation between seizures and the opioid system. Saturation curves and Scatchard plots of [3H]2-d-alamine-5-d-leucine enkephalin binding revealed that the opioid delta receptor density in adult El mice during interictal periods was significantly increased in the cerebral cortex, hippocampus, and septal area. It was further shown that the concentration of such receptors in 25-day-old El mice that had no seizures was also significantly increased in the hippocampus and septal area, with no changes in apparent affinities, as compared with in the corresponding regions in ddY mice (seizure-nonsusceptible strain; the mother strain of El). Such up-regulation of opioid receptors in the El mouse brain could result from deficits in endogenous opioid peptides, which could be associated with the pathogenesis of seizure diathesis in the El mouse. 相似文献
57.
本文提出一种测定FeMo-co催化活力的反应体系,用此反应体系,在测定FeMo-co催化活力的过程中,FeMo-co与变种UW45抽提液的重组活性始终保持不变。讨论了水含量、还原剂对FeMo-co催化活力和重组活力的影响。 相似文献
58.
DANIEL P. LAVIN CHRISTOS HATZIS FRIEDRICH SRIENC A. G. FREDRICKSON 《The Journal of eukaryotic microbiology》1990,37(3):157-163
Flow cytometry has been used to make direct measurements of rates of uptake of latex microspheres from dilute, monodisperse suspensions by Tetrahymena pyriformis. Measurements were made for five different sizes of microspheres, ranging from 1.09 to 6.17 μm diameter. Fractions of cells in the population that did not ingest the microspheres offered were also determined. In addition, the size distributions, as indicated by the forward angle light scattering intensity which is measured by the instrument, were determined for the whole population and for the subpopulations of cells that did and did not ingest the particles, for each particle size used. It was found that the fraction of cells that did not ingest the particles was small and independent of particle size when this was less than about 2.7 μm, but increased with particle size when particle size was increased above this value. The so-called maximum clearance rate, which can be calculated from the data, was found to increase monotonically with particle size if it were based only on those cells which actually ingested the particles offered. However, a plot of maximum clearance rate vs. particle size exhibited a maximum if the clearance rate were based on all cells present in the population. 相似文献
59.
Dr. Marie-Joelle Virolle Victor J. Morris Mervyn J. Bibb 《Journal of industrial microbiology & biotechnology》1990,5(5):295-301
Summary A simple, reliable and sensitive assay for alpha-amylase activity is reported, together with its theoretical derivation, that overcomes many of the problems encountered with other assays, especially when attempting to assay alpha-amylase activity in crude cell extracts or culture supernatants. The method relies on the reduction in turbidity that occurs upon digestion of a starch suspension with alpha-amylase. The initial rate of decrease in turbidity is shown to be proportional to a wide range of enzyme concentrations, permitting a rapid spectrophotometric and kinetic determination of alpha-amylase activity. 相似文献
60.