全文获取类型
收费全文 | 42528篇 |
免费 | 1484篇 |
国内免费 | 134篇 |
专业分类
化学 | 22531篇 |
晶体学 | 207篇 |
力学 | 1115篇 |
综合类 | 5篇 |
数学 | 7004篇 |
物理学 | 13284篇 |
出版年
2023年 | 276篇 |
2021年 | 350篇 |
2020年 | 534篇 |
2019年 | 412篇 |
2018年 | 540篇 |
2017年 | 541篇 |
2016年 | 1148篇 |
2015年 | 883篇 |
2014年 | 975篇 |
2013年 | 2047篇 |
2012年 | 1935篇 |
2011年 | 2096篇 |
2010年 | 1378篇 |
2009年 | 1161篇 |
2008年 | 1951篇 |
2007年 | 1831篇 |
2006年 | 1738篇 |
2005年 | 1570篇 |
2004年 | 1341篇 |
2003年 | 1106篇 |
2002年 | 950篇 |
2001年 | 902篇 |
2000年 | 719篇 |
1999年 | 525篇 |
1998年 | 458篇 |
1997年 | 444篇 |
1996年 | 559篇 |
1995年 | 504篇 |
1994年 | 515篇 |
1993年 | 546篇 |
1992年 | 602篇 |
1991年 | 429篇 |
1990年 | 421篇 |
1989年 | 379篇 |
1988年 | 355篇 |
1987年 | 362篇 |
1986年 | 338篇 |
1985年 | 534篇 |
1984年 | 515篇 |
1983年 | 429篇 |
1982年 | 464篇 |
1981年 | 468篇 |
1980年 | 449篇 |
1979年 | 440篇 |
1978年 | 431篇 |
1977年 | 421篇 |
1976年 | 392篇 |
1975年 | 390篇 |
1974年 | 376篇 |
1973年 | 321篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
991.
α-(1-adamantyl)-β-diketones are now accessible by the alkylation of Co(II) complexes of β-diketones with 1-bromoadamantane. The intermediacy of adamantyl cation is proposed. Co(II), Co(III) and Zn(II) complexes of pentane-2,4-dione react with alkyl halides precursors of stabilized carbenium ions, to give 3-alkylpentane-2,4-diones. 相似文献
992.
The characteristic behaviour of a series of important barbiturates in field desorption mass spectrometry (f.d.m.s.) is described. Detection limits are reported for three standard barbiturates by f.d.m.s. and high-performance liquid chromatography (h.p.l.c.). The off-line combination of h.p.l.c. and f.d.m.s. is used in forensic investigations to identify these drugs unequivocally in body fluids such as urine and stomach fluids and in extracts from human liver and kidney tissues. Eighteen barbiturates were tested. The use of f.d.m.s. as an off-line h.p.l.c. detector provides detection limits (S/N =3:1) ranging from 1 ng g-1 to 10 ng g-1, depending on the compound and on the preliminary clean-up used. 相似文献
993.
In the EURACHEM/CITAC draft ”Quantifying uncertainty in analytical measurement” estimations of measurement uncertainty in
analytical results for linear calibration are given. In this work these estimations are compared, i.e. the uncertainty deduced
from repeated observations of the sample vs. the uncertainty deduced from the standard residual deviation of the regression.
As a result of this study it is shown that an uncertainty estimation based on repeated observations can give more realistic
values if the condition of variance homogeneity is not correctly fulfilled in the calibration range. The complete calculation
of measurement uncertainty including assessment of trueness is represented by an example concerning the determination of zinc
in sediment samples using ICP-atomic emission spectrometry.
Received: 9 February 2002 Accepted: 17 April 2002 相似文献
994.
In this study, a rapid pre-concentration procedure, which employs powdered activated carbon as a clean-up and pre-concentration material, is described for the gas chromatographic analysis of N-nitrosodiethylamine (NDEA) and N-nitrosodimethylamine (NDMA) in aqueous solutions. It was found that powdered activated carbon is suitable for the adsorption of volatile N-nitrosamine compounds from aqueous solutions. Adsorption efficiency with spiked beer samples (alcohol content 5% v/v) was found to be 80.5% (NDMA) and 89.4% (NDEA) and recovery of extraction from activated carbon was calculated as 82.1% (NDMA) and 89.7% (NDEA), respectively. The effect of 100µgmL–1 of tannic acid on the adsorption was also studied, and no significant effect on the adsorption and extraction of volatile N-nitrosamine compounds was found. 相似文献
995.
PtCl2 constitutes a convenient catalyst for intramolecular hydroalkoxylation, carboalkoxylation, hydroamination, and carboamination reactions of alkynes, effecting the formation of substituted benzo[b]furan, indole-, and isochromene-1-one derivatives, respectively. This procedure allows for the transfer of (substituted) allyl, methoxymethyl (MOM), benzyloxymethyl (BOM), and (trimethylsilyl)ethoxymethyl (SEM) groups from oxygen to carbon and is compatible with functional groups that are susceptible to oxidative insertion of low valent metal species previously used for similar purposes. Although some of the reactions can even be carried out in air, the rates are significantly increased when conducted under an atmosphere of CO. A mechanistic rationale is proposed, implying activation of the alkyne by the carbophilic Pt(2+) template as the primary step of the catalytic cycle. 相似文献
996.
Dieter Leibfritz Erhard Haupt Norbert Dubischar Heinrich Lachmann Raymond Oekonomopulos Günther Jung 《Tetrahedron》1982,38(14):2165-2181
The uncommon amino-acid 2-methylalanine (α-aminoisobutyric acid, Aib) was investigated by 13C-NMR. The chemical shifts of amino- or carboxy-protected derivatives of Aib and of protected oligopeptides are discussed with respect to neighbouring groups and amino acids. The pH-dependence of the 13C-NMR spectra of Aib, Aib-Ala, Ala-Aib, Aib-Ala-Aib and Aib-Ala-Aib-Ala-Aib was studied. Using these examples, a new and advantageous method is demonstrated for the first time for the evalutions of NMR titration curves, which uses so-called chemical shift diagrams (CS diagrams). 相似文献
997.
Stereocontrolled syntheses of dictyopterene B (hormosirene) (1) and its enantiomer (ent-1) are reported. Key steps are highly stereoselective ScN′ reactions of esters of alcohols 2 and 5 derived from (+)-camphor. 相似文献
998.
The SINDO1 method is modified to include the calculation of molecular anions. Two versions of modifications are presented which are based on charge dependent orbital exponents. Calculated heats of formation and electron affinities are much improved compared to the standard version with fixed orbital exponents.Dedicated to Professor J. Koutecký on the occasion of his 65th birthday 相似文献
999.
A selective and sensitive chromatographic method is described for the determination of nine organochlorine and organophosphorus pesticides in vegetable samples by gas chromatography-mass spectrometry. The proposed method combines the use of positive and negative chemical ionisation and tandem mass spectrometric fragmentation, resulting in a significant increase in selectivity and allowing the simultaneous confirmation and quantification of trace levels of pesticides in complex vegetable matrices. Parameters relative to ionisation and fragmentation processes were optimised to obtain maximum sensitivity. Repeatability and reproducibility studies yielded relative standard deviations lower than 25% in all cases. Identification criteria, such as retention time and relative abundance of characteristic product ions, were also evaluated in order to guarantee the correct identification of the target compounds. The method was applied to real vegetable samples to demonstrate its use in routine analysis. 相似文献
1000.
Burba P Jakubowski B Kuckuk R Küllmer K Heumann KG 《Fresenius' Journal of Analytical Chemistry》2000,368(7):689-696
The analytical fractionation of aquatic humic substances (HS) by means of immobilized metal-chelate affinity chromatography (IMAC) on metal-loaded chelating ion exchangers is described. The cellulose HYPHAN, loaded with different trivalent ions, and the chelate exchanger Chelex 100, loaded to 90% of its capacity with Fe(III), were used. The cellulose HYPHAN, loaded with 2% Fe(III), resulted in HS distribution coefficients Kd of up to 10(3.7) mL/g at pH 4.0 continuously decreasing down to 10(1.5) at pH 12, which were appropriate for HS fractionation by a pH-depending chromatographic procedure. Similar distribution coefficients Kd were obtained for HS sorption onto Fe(III)-loaded Chelex 100. On the basis of Fe-loaded HYPHAN both, a low-pressure and high-pressure IMAC technique, were developed for the fractionation of dissolved HS applying a buffer-based pH gradient for their gradual elution between pH 4.0 and 12.0. By coupling the Chelex 100 column under high-pressure conditions with an inductively coupled plasma mass spectrometer an on-line characterization of HS metal species could be achieved. Using these fractionation procedures a number of reference HS were characterized. Accordingly, the HA (humic acids) and FA (fulvic acids) studied could be discriminated into up to 6 fractions by applying cellulose HYPHAN, significantly differing in their Cu(II) complexation capacity but hardly in their substructures assessed by conventional FTIR. In the case of using Chelex 100 exchanger resin two major UV active HS fractions were obtained, which significantly differ in their complexation properties for Cu(II) and Pb(II), respectively. 相似文献