全文获取类型
收费全文 | 3668篇 |
免费 | 61篇 |
国内免费 | 259篇 |
专业分类
化学 | 1906篇 |
晶体学 | 35篇 |
力学 | 34篇 |
综合类 | 1篇 |
数学 | 85篇 |
物理学 | 1927篇 |
出版年
2024年 | 5篇 |
2023年 | 35篇 |
2022年 | 45篇 |
2021年 | 31篇 |
2020年 | 63篇 |
2019年 | 40篇 |
2018年 | 36篇 |
2017年 | 46篇 |
2016年 | 61篇 |
2015年 | 54篇 |
2014年 | 71篇 |
2013年 | 152篇 |
2012年 | 177篇 |
2011年 | 206篇 |
2010年 | 191篇 |
2009年 | 232篇 |
2008年 | 197篇 |
2007年 | 289篇 |
2006年 | 233篇 |
2005年 | 167篇 |
2004年 | 182篇 |
2003年 | 170篇 |
2002年 | 128篇 |
2001年 | 134篇 |
2000年 | 123篇 |
1999年 | 133篇 |
1998年 | 100篇 |
1997年 | 98篇 |
1996年 | 86篇 |
1995年 | 87篇 |
1994年 | 60篇 |
1993年 | 69篇 |
1992年 | 49篇 |
1991年 | 65篇 |
1990年 | 28篇 |
1989年 | 23篇 |
1988年 | 21篇 |
1987年 | 10篇 |
1986年 | 7篇 |
1985年 | 9篇 |
1984年 | 9篇 |
1983年 | 9篇 |
1982年 | 13篇 |
1981年 | 4篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 7篇 |
排序方式: 共有3988条查询结果,搜索用时 15 毫秒
91.
A simple solid phase extraction procedure for speciation of selenium(IV) and selenium(VI) in environmental samples has been proposed prior to graphite furnace atomic absorption spectrometry. The method is based on the solid phase extraction of the selenium(IV)-ammonium pyrrolidine dithiocarbamate (APDC) chelate on the Diaion HP-2MG. After reduction of Se(VI) by heating the samples in the microwave oven with 4 mol l−1 HCl, the system was applied to the total selenium. Se(VI) was calculated as the difference between the total selenium content and Se(IV) content. The experimental parameters, pH, amounts of reagents, eluent type and sample volume were optimized. The recoveries of analytes were found greater than 95%. No appreciable matrix effects were observed. The adsorption capacity of sorbent was 5.20 mg g−1 Se (IV). The detection limit of Se (IV) (3sigma, n = 11) is 0.010 μg l−1. The preconcentration factor for the presented system was 100. The proposed method was applied to the speciation of selenium(IV), selenium(VI) and determination of total selenium in natural waters and microwave digested soil, garlic, onion, rice, wheat and hazelnut samples harvested various locations in Turkey with satisfactory results. In order to verify the accuracy of the method, certified reference materials (NIST SRM 2711 Montana Soil, NIST SRM 1568a Rice Flour and NIST SRM 8418 Wheat Gluten) were analyzed and the results obtained were in good agreement with the certified values. The relative errors and relative standard deviations were below 6 and 10%, respectively. 相似文献
92.
原子吸收法在有机分析中的应用(Ⅲ):茶叶中茶多酚的测定 总被引:14,自引:0,他引:14
研究了间接测定茶叶中茶多酚的原子吸收,用甲基异丁酮和丁醇的混合溶剂萃取茶汤中的茶多酚,与氨基酸分离后,加入碱性Cu3(PO4)2悬浮液反萃取并同茶多酚作用,离心沉降未作用的Cu3(PO4)2后,用原子吸收法测定Cu,可求得茶多酚的含量,本法可消除茶叶共存氨基酸的干扰,测定结果同标准方法一致,回收率范围为94%-104%,平均标准偏差为3.1%。 相似文献
93.
94.
Jordi de Mier‐Vinu ngel M. Montaa Virtudes Moreno María J. Prieto 《无机化学与普通化学杂志》2005,631(11):2054-2061
Two 1,4‐diamine ligands were synthesized having 1,2‐bis(aminomethyl)‐cyclohexane and 1,2‐bis(aminomethyl)‐benzene structures. The two ligands have different electron density in the six‐membered ring: a cyclohexane versus a phenyl ring. The organic synthesis of the ligands was carried out by synthetic pathways of seven and four steps, respectively, starting from 1,2,3,6‐tetrahydrophthalic anhydride and diethyl phthalate. The coordination of platinum to these ligands afforded platinum(II) complexes which are analogue to the clinical drug cisplatin but form a seven‐membered chelate ring. The interaction of the platinum compounds with DNA was studied in order to know the relationship between the electron density of ligands and their capability to chelate DNA, by using three techniques: Circular Dichroism, Agarose Gel Electrophoresis and Atomic Force Microscopy. The degree of interaction of both compounds with DNA was slightly different, but both complexes showed a cisplatin‐like behaviour and are promising candidates to follow an extensive study of their cytotoxic activity. 相似文献
95.
Yong Li 《Theoretical chemistry accounts》2007,117(1):163-165
We present the theoretical results for the positions and widths of the initial anticrossing between Stark manifold n and n + 1 in sodium, obtained by using the method of diagonalization in which zero-field wave functions are chosen from a kind of atomic potential model. These results can provide the quantitative information for understanding the microwave ionization process, whose rate-limiting step is probably the n to n + 1 transition performed at the first crossing between the outermost states of these two manifolds. 相似文献
96.
Summary The native enantioselectivity in binding of human serum albumin (HSA) towards 2-aryl propionic acid non-steroidal anti-inflammatory drugs (2-APA-NSAIDs, the profens) was found to be preserved when the protein was immobilized within a commercially available diol high-performance liquid chromatographic column. High capacity factors were obtained, reflecting the previously observed extensive binding of the 2-APA-NSAIDs to free HSA. The capacity factors were modified by the addition of octanoic acid to the mobile phase. Chiral resolution of the enantiomers of all nine 2-APA-NSAIDs studied was achieved. Preliminary studies show that in addition to being a useful chiral analytical tool for this therapeutically important series of compounds, the HSA chiral stationary phase may provide useful information on the affinity and binding mechanism of small molecules to HSA. 相似文献
97.
Summary The first-order polarized basis sets for the use in high-level-correlated investigations of molecular electric properties have been generated for Pb, Bi, Po, and At. The performance of the standard [10.17.14.5/13.11.8.2] and extended [20.17.14.9/13.11.8.4] basis sets has been examined in nonrelativistic and quasirelativistic calculations for atoms and simple closed-shell hydrides. The relativistic contributions to electric dipole properties of those systems have been evaluated by using the recently developed quasirelativistic scheme. The predicted dipole polarizability of At is in good agreement with the results of other relativistic calculations. The calculated quasirelativistic dipole moments of BiH3 (–0.499 a.u.), PoH2 (–0.207 a.u.), and AtH (+0.036 a.u.) involve a significant relativistic contribution which amounts to —0.230 a.u., –0.177 a.u., and –0.097 a.u., respectively. The basis set details append this paper. They are also available as a part of the basis set library of the MOLCAS system. 相似文献
98.
Biosurface fabrication using the Fab′ fragment of immunoglobulin (IgG) was carried out by self-assembly (SA) technique. The pepsin-digested monoclonal antibody (Mab) against bovine insulin containing the F(ab′)2 fragment and residual proteins was separated using affinity chromatography and dialysis. To prevent the nonspecific binding of F(ab′)2 onto gold (Au) substrate, the native disulfide bridge was reduced using dithiothreitol (DTT) to convert F(ab′)2 into Fab′, which made the immobilization to be carried out via the native thiol (–SH) group. The fabricated biosurface using SA technique showed the formation of stable thin film through AFM topography. Through the concentration change of DTT and Fab′, the absorption characteristics against the Au surface were investigated using surface plasmon resonance (SPR) with the flow cell. The amount of immobilized antibody fragment and the antigen binding capacity were regulated with respect to the reduction state and concentration of F(ab′)2. Based on the biosurface of the fabricated Fab′, the insulin-detection was carried out by the measurement of SPR. The proposed antibody surface could successfully detect the bovine insulin at the concentration from 100 ng/mL to 10 μg/mL. 相似文献
99.
Behpour M Attaran AM Ghoreishi SM Soltani N 《Analytical and bioanalytical chemistry》2005,382(2):444-447
A sensitive, selective and simple preconcentration method for ultra-trace gold determination has been developed that uses naphthalene–methyltrioctyl ammonium chloride (Aliquat 336s) as an adsorbent. Gold, in the form of AuCl4–, was retained by the adsorbent in the column at a flow rate of 1 ml min–1. After filtration, the solid mass consisting of the gold complex and naphthalene was dissolved out of the column with 5 ml of N,N-dimethylformamide (DMF), and the metal was then determined by atomic absorption spectrometry. In the initial solution, the calibration graph of absorbance versus gold concentration was found to be linear in the range 0.5–150 ng ml–1 Au(III) with r=0.997 (n =9), and the 3 s detection limit was 0.428 ng ml–1. The relative standard deviation for eight replicate measurements of 20 g of gold was 2.14%. Preconcentration factors of 390 and 650 were achieved using 5 ml and 3 ml of DMF, respectively. The proposed method was successfully applied to the determination of gold in wastewater, processed pool water, slurry pool water, and raw well-water from the Moteh gold mine, and synthetic samples. 相似文献
100.
Bismuth as BiCl4− and BH4− ware successively retained in a column (150 mm × 4 mm, length × i.d.) packed with Amberlite IRA-410 (strong anion-exchange resin). This was followed by passage of an injected slug of hydrochloric acid resulting in bismuthine generation (BiH3). BiH3 was stripped from the eluent solution by the addition of a nitrogen flow and the bulk phases were separated in a gas–liquid separator. Finally, bismutine was atomized in a quartz tube for the subsequent detection of bismuth by atomic absorption spectrometry. Different halide complexes of bismuth (namely, BiBr4−, BiI4− and BiCl4−) were tested for its pre-concentration, being the chloride complexes which produced the best results. Therefore, a concentration of 0.3 mol l−1 of HCl was added to the samples and calibration solutions. A linear response was obtained between the detection limit (3σ) of 0.225 and 80 μg l−1. The R.S.D.% (n = 10) for a solution containing 50 μg l−1 of Bi was 0.85%. The tolerance of the system to interferences was evaluated by investigating the effect of the following ions: Cu2+, Co2+, Ni2+, Fe3+, Cd2+, Pb2+, Hg2+, Zn2+, and Mg2+. The most severe depression was caused by Hg2+, which at 60 mg l−1 caused a 5% depression on the signal. For the other cations, concentrations between 1000 and 10,000 mg l−1 could be tolerated. The system was applied to the determination of Bi in urine of patients under therapy with bismuth subcitrate. The recovery of spikes of 5 and 50 μg l−1 of Bi added to the samples prior to digestion with HNO3 and H2O2 was in satisfactory ranges from 95.0 to 101.0%. The concentrations of bismuth found in six selected samples using this procedure were in good agreement with those obtained by an alternative technique (ETAAS). Finally, the concentration of Bi determined in urine before and after 3 days of treatment were 1.94 ± 1.26 and 9.02 ± 5.82 μg l−1, respectively. 相似文献