全文获取类型
收费全文 | 4758篇 |
免费 | 944篇 |
国内免费 | 559篇 |
专业分类
化学 | 3516篇 |
晶体学 | 108篇 |
力学 | 301篇 |
综合类 | 30篇 |
数学 | 443篇 |
物理学 | 1863篇 |
出版年
2024年 | 15篇 |
2023年 | 171篇 |
2022年 | 240篇 |
2021年 | 287篇 |
2020年 | 319篇 |
2019年 | 272篇 |
2018年 | 259篇 |
2017年 | 201篇 |
2016年 | 290篇 |
2015年 | 307篇 |
2014年 | 367篇 |
2013年 | 369篇 |
2012年 | 477篇 |
2011年 | 501篇 |
2010年 | 263篇 |
2009年 | 312篇 |
2008年 | 289篇 |
2007年 | 222篇 |
2006年 | 205篇 |
2005年 | 156篇 |
2004年 | 112篇 |
2003年 | 83篇 |
2002年 | 81篇 |
2001年 | 59篇 |
2000年 | 57篇 |
1999年 | 58篇 |
1998年 | 42篇 |
1997年 | 34篇 |
1996年 | 54篇 |
1995年 | 24篇 |
1994年 | 32篇 |
1993年 | 17篇 |
1992年 | 18篇 |
1991年 | 17篇 |
1990年 | 14篇 |
1989年 | 10篇 |
1988年 | 8篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1974年 | 1篇 |
1957年 | 3篇 |
1936年 | 1篇 |
排序方式: 共有6261条查询结果,搜索用时 15 毫秒
971.
972.
A growing number of peptides are being used today in bioanalytical laboratories. Because of this, there is an increasing interest in the development of highly sensitive, specific and robust liquid chromatography/tandem mass spectrometry (LC/MS/MS) assays for the quantitative analysis of peptides in biological samples. Among the mass spectrometers previously used for peptide quantification, triple quadrupole mass spectrometers are generally not considered the instrument of choice. With this instrumentation, collision cascades or multiple fragmentations tend to generate multiple peaks that have weak intensities. This leads to a loss in detection sensitivity. However, in cases where immonium product ions were formed in abundance, it was found that peptide quantification succeeded. A common feature of these peptides is their intra-loop structure. To elucidate the usefulness of this feature in fragmentation, several peptide analytes with intra-chain disulfide bonds were investigated in this study, including a newly synthesized analog having a single amino acid substitution. The results presented here indicate that abrupt bond cleavage from the intra-loop structure of peptides could be one of the premises for intense immonium ion generation. In contrast, any preferential cleavage of peptide bonds (e.g., proline effect) that gives rise to a linearized sequence would break the intactness of the loop and prevent it from completely dissociating. In addition, the utilization of immonium product ions in LC/MS/MS was demonstrated for the determination of peptides with intra-chain disulfide bonds in biological fluids. 相似文献
973.
Quan‐Bin Han Lina Wong Fanny Lai Nian‐Yun Yang Jing‐Zheng Song Chun‐Feng Qiao Hong‐Xi Xu 《Journal of separation science》2009,32(2):309-313
In order to provide the chemical markers for the quality control of herbal medicines, four diterpenoids, pseudolaric acids A and B (PAA and PAB), and their glucosides were isolated from the methanol extract of the Chinese herb Pseudolarix kaempferi using high‐speed counter‐current chromatography (HSCCC). The diphase solvent system was n‐hexane/EtOAc/MeOH/H2O which was used at two ratios (5:5:5:5 and 1:9:4:6 by volume) in the separation of pseudolaric acids and their glycosides, respectively. As a result, PAA (14 mg), PAB (129 mg), PAA‐O‐β‐D ‐glucopyranoside (8 mg, PAAG), and PAB‐O‐β‐D ‐glucopyranoside (42 mg, PABG) were obtained from 0.5 g of the crude extract. Their purities were determined to be above 97% by HPLC analysis. Their chemical structures were confirmed by 1H and 13C NMR analysis or HPLC comparison with the reference compounds. 相似文献
974.
Qian Qiao Guo‐Qiang Wu Tian‐Di Tang Seik Weng Ng 《Acta Crystallographica. Section C, Structural Chemistry》2009,65(4):m146-m148
In the title PbII coordination polymer, [Pb(C16H10O4)(C14H8N4)(C3H7NO)]n, each PbII atom is eight‐coordinated by two chelating N atoms from one pyrazino[2,3‐f][1,10]phenanthroline (L) ligand, one dimethylformamide (DMF) O atom and five carboxylate O atoms from three different 4,4′‐ethylenedibenzoate (eedb) ligands. The eedb dianions bridge neighbouring PbII centres through four typical Pb—O bonds and one longer Pb—O interaction to form a two‐dimensional structure. The C atoms from the L and eedb ligands form C—H...O hydrogen bonds with the O atoms of eedb and DMF ligands, which further stabilize the structure. The title compound is the first PbII coordination polymer incorporating the L ligand. 相似文献
975.
Huijie Qiao 《Journal of Mathematical Analysis and Applications》2009,355(2):725-738
In this paper we study a class of infinite horizon backward stochastic differential equations (BSDEs) of the form
976.
977.
A correction vector method within the multireference determinant single and double configuration interaction approximation coupled with the semiempirical intermediate neglect of differential overlap Hamiltonian has been developed for the computation of single and multiphoton absorption spectra of conjugated molecules. We study the effect of pi conjugation on these properties in the extended rylenebis(dicarboximide)s. The one-, two-, and three-photon absorption cross sections of the lowest-lying excited states show a power law dependence on the conjugation length, with exponents of about 1.3, 2.6, and 5.6, respectively. The maximum value of the three-photon absorption cross section in these molecules is calculated to be 1.06x10(-78) cm6 s2photon2 for photon energy at 0.57 eV. 相似文献
978.
979.
Flavonoids are the main active constituents of Dalbergia odorifera. The excretion of the major flavonoids in rat urine after oral administration of D. odorifera extract was investigated by HPLC-UV and HPLC-MS methods. Utilizing the HPLC-MS technique, 18 flavonoids, including five isoflavones, four isoflavanones, four neoflavones, two flavanones, two chalcones and one isoflavanonol were identified in free form in a urine sample based on the direct comparison of the corresponding tR, UV maximum absorbance (lambda(max)) values and MS data with the authentic standards. The amounts of the prominent flavonoids, (3R)-4'-methoxy-2',3,7-trihydroxyisoflavanone and vestitone, were determined by HPLC-UV with the internal standard method, and the validation procedure confirmed that it afforded reliable analysis of these two analytes in urine after oral administration of D. odorifera extract. 相似文献
980.
A novel circular Dammann grating is proposed to generate uniform-intensity impulse rings corresponding to different diffraction orders in the far field. The intensities of the rings are determined by the coefficients of the circular sine series decomposition of the grating function. The definition of diffraction efficiency and uniformity for this novel device are described. Numerical solutions of binary phase circular Dammann gratings are presented. A binary phase three-order circular Dammann grating of pi phase depth is fabricated by an e-beam direct writing technique and is experimentally demonstrated. 相似文献