全文获取类型
收费全文 | 1558篇 |
免费 | 33篇 |
国内免费 | 12篇 |
专业分类
化学 | 1020篇 |
晶体学 | 8篇 |
力学 | 24篇 |
数学 | 238篇 |
物理学 | 313篇 |
出版年
2021年 | 13篇 |
2020年 | 20篇 |
2019年 | 12篇 |
2018年 | 15篇 |
2017年 | 13篇 |
2016年 | 31篇 |
2015年 | 26篇 |
2014年 | 23篇 |
2013年 | 74篇 |
2012年 | 85篇 |
2011年 | 96篇 |
2010年 | 46篇 |
2009年 | 54篇 |
2008年 | 70篇 |
2007年 | 81篇 |
2006年 | 82篇 |
2005年 | 66篇 |
2004年 | 61篇 |
2003年 | 61篇 |
2002年 | 58篇 |
2001年 | 19篇 |
2000年 | 33篇 |
1999年 | 21篇 |
1998年 | 20篇 |
1997年 | 14篇 |
1996年 | 24篇 |
1995年 | 24篇 |
1994年 | 23篇 |
1993年 | 15篇 |
1992年 | 15篇 |
1991年 | 8篇 |
1990年 | 29篇 |
1989年 | 17篇 |
1988年 | 20篇 |
1987年 | 11篇 |
1986年 | 22篇 |
1985年 | 24篇 |
1984年 | 19篇 |
1983年 | 10篇 |
1982年 | 13篇 |
1981年 | 15篇 |
1980年 | 15篇 |
1979年 | 17篇 |
1978年 | 18篇 |
1977年 | 17篇 |
1976年 | 12篇 |
1975年 | 23篇 |
1974年 | 20篇 |
1973年 | 21篇 |
1971年 | 8篇 |
排序方式: 共有1603条查询结果,搜索用时 31 毫秒
11.
12.
Adler J Becker JJ Blaylock GT Bolton T Brient J Browder T Brown JS Bunnell KO Burchell M Burnett TH Cassell RE Coffman D Cook V Coward DH DeJongh F Dorfan DE Drinkard J Dubois GP Eigen G Einsweiler KF Eisenstein BI Freese T Gatto C Gladding G Grab C Hamilton RP Hauser J Heusch CA Hitlin DG Izen JM Kim PC Köpke L Li A Lockman WS Mallik U Matthews CG Mincer AI Mir R Mockett PM Mozley RF Nemati B Odian A Parrish L Partridge R Pitman D Plaetzer SA Richman JD Sadrozinski HF Scarlatella M Schalk TL 《Physical review letters》1989,62(16):1821-1824
13.
Determination of 11 aminoglycosides in meat and liver by liquid chromatography with tandem mass spectrometry 总被引:2,自引:0,他引:2
A method using liquid chromatography with tandem mass spectrometry was developed for the determination of 11 commonly used aminoglycoside antibiotics in meat. The proposed method is sufficiently sensitive (detection limits of 15 to 40 ppb for the various antibiotics) and highly selective. It is suitable for the quantitation and confirmation of aminoglycosides in a variety of matrixes (pork muscle, fish, and veal liver). Any multiresidue method for aminoglycosides must take into account their high affinity toward sample proteins and the significantly different pK values of the various analytes. The developed method uses a low-pH extraction with trichloracetic acid to ensure complete extraction of the analytes from the matrix. An anion-exchange step is used to remove the acid from the centrifuged extract. Aminoglycosides in this solution of low ionic strength can be quantitatively retained and afterwards eluted from a weak cation-exchanger solid-phase extraction (SPE) cartridge. The highly selective SPE steps produce clean extracts, which minimize possible suppression of the mass spectrometer signal. 相似文献
14.
15.
Blaylock GT Bolton T Brown JS Bunnell KO Burnett TH Cassell RE Coffman D Cook V Coward DH Dorfan DE Dubois GP Eigen G Eisenstein BI Freese T Gladding G Grab C Heusch CA Hitlin DG Izen JM Köpke L Li A Lockman WS Mallik U Matthews CG Mir R Mockett PM Mozley RF Nemati B Odian A Parker J Parrish L Partridge R Pitman D Sadrozinski HF Scarlatella M Schalk TL Schindler RH Seiden A Simopoulos C Stockdale IE Stockhausen W Thaler JJ Toki W Tripsas B Villa F Wasserbaech S Wattenberg A Weinstein AJ 《Physical review letters》1987,58(21):2171-2174
16.
Fluorogenic probes dual-labeled with reporter and quencher dyes use a change in fluorescence to monitor biochemical events (e.g., substrate binding or enzyme digestion). Such events change the reporter-quencher distance, which affects fluorescence. Recently, it is has been shown that static quenching through intramolecular dimers is an important mechanism that can sometimes be more efficient than F?rster resonance energy transfer (FRET). 相似文献
17.
The first total syntheses of (-)-vincamajinine (5) (from Na-methyl-d-tryptophan methyl ester) and (-)-11-methoxy-17-epivincamajine (6) (from Na-methyl-6-methoxy-d-tryptophan ethyl ester) are described. The syntheses have been completed in a highly stereocontrolled manner (>98% ee). Key steps included the asymmetric Pictet-Spengler reaction, enolate-mediated palladium cross-coupling reaction, and acid-catalyzed formation of the C(7)-C(17) bond. In addition, the triethylsilane/TFA-mediated incorporation of the 2alpha-H (11 to 12) and the borohydride generation of the C(17) hydroxyl function (R) were also stereospecific. The unique highly conjested carbon skeletons of the two alkaloids were completed in a concise manner and in regiospecific fashion. 相似文献
18.
Exposure of ent-kaura-2,16-dien-19-ol (1) or its succinate (2) to resuspended mycelia of G. fujikuroi has produced a complex mixture of acids which after methylation gave the esters of two C19 (24) and (30) and five C20 gibberellins (4, 11, 20, 32 and 33). The triester (32) and the lactone ester (24) have been prepared before from the esters of gibberellin A13 (8) and gibberellin A4 (26) respectively. The structures of the other metabolites were assigned on spectroscopic data and by chemical transformations. Thus the lactone diester (4) has been converted to the known keto triester (6). The epoxide (11) has been related to gibberellin A14 (14) and the aldehyde (33) has been related to gibberellin A13 trimethyl ester (8) by way of the triol (34). Selective de-epoxidation of the 16,17-epoxy function in diepoxides has provided a route from the dienes (20 and 24) to the epoxides (11 and 30) respectively, but not from the ester of gibberellin A5 (23) to that of gibberellin A6 (29). On the other hand the latter can be obtained by epoxidation of gibberellin A5 methyl ester trifluoroacetate. Backfeeding experiments carried out with the epoxy diacid (12), the diene diacid (21) and the derived diol (39) indicate pathways connecting the various metabolites. The natural gibberellins A5 and A6 were shown to be formed in some of the backfeeding experiments. 相似文献
19.
Johnny D. Reynolds Kelsey D. Cook 《Journal of the American Society for Mass Spectrometry》1990,1(3):233-237
The abundance of ion pairs (CA+) relative to that of doubly charged ions (C2+) in electrohydrodynamic (EH) mass spectra of a series of anions with a common dication in glycerol was found to increase in the order acetate < nitrite < chloride < bromide ≈ nitrate < iodide < perchlorate. Correlation with enthalpies of hydration for the anions suggests that this trend reflects the solution chemistry of ion association. These spectra also reveal that solvation rather than interactions with the extracting field is more important in determining the overall EH mass spectrometric sensitivity to doubly charged ions. Therefore, the use of anions that promote more extensive ion pairing enhances the overall sensitivity to multiply charged ions that otherwise interact strongly with the solvent, but reduces sensitivity to singly charged ions. These observations hold in fast atom bombardment mass spectrometry, surviving the invasive effects of the primary beam. 相似文献
20.
A core model approach to the calculation of deuteron quadrupole coupling constants is investigated using NH3 as an example. First the deuteron quadrupole coupling constant is calculated from a CNDO wave function. This result is subsequently improved by recomputing the N—D bond orbital by means of a variational calculation using the CNDO function to construct a core potential for the bond Hamiltonian. In order to simplify integrations a single-center basis is chosen to represent the variational wave function. A projection operator formalism is used as a computational scheme to maintain orthogonality of the bond orbital to core orbitals. Excellent agreement with experiment is obtained. The procedure is applicable to more complicated molecules. 相似文献