全文获取类型
收费全文 | 1865篇 |
免费 | 40篇 |
国内免费 | 9篇 |
专业分类
化学 | 1356篇 |
晶体学 | 31篇 |
力学 | 47篇 |
数学 | 242篇 |
物理学 | 238篇 |
出版年
2022年 | 9篇 |
2021年 | 16篇 |
2020年 | 27篇 |
2019年 | 22篇 |
2018年 | 14篇 |
2017年 | 19篇 |
2016年 | 43篇 |
2015年 | 27篇 |
2014年 | 51篇 |
2013年 | 104篇 |
2012年 | 108篇 |
2011年 | 140篇 |
2010年 | 63篇 |
2009年 | 63篇 |
2008年 | 125篇 |
2007年 | 125篇 |
2006年 | 140篇 |
2005年 | 107篇 |
2004年 | 86篇 |
2003年 | 81篇 |
2002年 | 82篇 |
2001年 | 35篇 |
2000年 | 15篇 |
1999年 | 24篇 |
1998年 | 21篇 |
1997年 | 28篇 |
1996年 | 21篇 |
1995年 | 34篇 |
1994年 | 36篇 |
1993年 | 12篇 |
1992年 | 18篇 |
1991年 | 16篇 |
1990年 | 15篇 |
1989年 | 18篇 |
1988年 | 14篇 |
1987年 | 9篇 |
1986年 | 10篇 |
1985年 | 19篇 |
1984年 | 12篇 |
1983年 | 11篇 |
1982年 | 12篇 |
1981年 | 7篇 |
1980年 | 14篇 |
1979年 | 8篇 |
1978年 | 13篇 |
1977年 | 7篇 |
1976年 | 4篇 |
1975年 | 6篇 |
1974年 | 9篇 |
1973年 | 5篇 |
排序方式: 共有1914条查询结果,搜索用时 31 毫秒
11.
12.
Limonene is a common component found in consumer goods ranging from beverages to cleaning compounds. Limonene oxidation products, however, have a less desirable flavor and fragrance. Early detection of limonene oxide formation would aid quality control. A method is developed to determine the concentration of limonene oxide in essential oils and beverages using solid-phase microextraction (SPME). A headspace sampling technique is used to reduce or eliminate the presence of less volatile components. Several different SPME fibers are tested, varying in polymer thickness, polymer cross-linking and bonding, and polarity of the polymer. For each fiber tested, the sampling time is optimized for reproducible results. The 7-microm-thick bonded poly(dimethylsiloxane) fiber provides the best results. External standards are used for quantitation. 相似文献
13.
Ballou S Goodpaster J MacCrehan W Reeder D 《Analytical and bioanalytical chemistry》2003,376(8):1149-1150
14.
Angelo C. Pinto Susan K. Do Prado Raimundo Braz Filho William E. Hull Andras Neszmelyi Gabor Lukacs 《Tetrahedron letters》1982,23(50):5267-5270
With the help of Natural Abundance 13C - 13C Coupling Constants, observed via double quantum coherence, the structural determination of VELLOZIOLONE (1), a new diterpene, has been carried out. 相似文献
15.
Torabi F Ramanathan K Larsson PO Gorton L Svanberg K Okamoto Y Danielsson B Khayyami M 《Talanta》1999,50(4):787-797
An electrochemical method for the measurement of NAD(+) and NADH in normal and cancer tissues using flow injection analysis (FIA) is reported. Reticulated vitreous carbon (RVC) electrodes with entrapped l-lactate dehydrogenase (LDH) and a new redox polymer containing covalently bound toluidine blue O (TBO) were employed for this purpose. Both NAD(+) and NADH were estimated coulometrically based on their reaction with LDH. The latter was immobilized on controlled pore glass (CPG) by cross-linking with glutaraldehyde and packed within the RVC. The concentrations of NAD(+) and NADH in the tissues, estimated using different electron mediators such as ferricyanide (FCN), meldola blue (MB) and TBO have also been compared. The effects of flow rate, pH, applied potential (versus Ag/AgCl reference) and adsorption of the mediators have also been investigated. Based on the measurements of NAD(+) and NADH in normal and cancer tissues it has been concluded that the NADH concentration is lower, while the NAD(+) concentration is higher in cancer tissues. Amongst the electron mediators TBO was found to be a more stable mediator for such measurements. 相似文献
16.
17.
Plumb RS Stumpf CL Gorenstein MV Castro-Perez JM Dear GJ Anthony M Sweatman BC Connor SC Haselden JN 《Rapid communications in mass spectrometry : RCM》2002,16(20):1991-1996
The application of liquid chromatography/mass spectrometry (LC/MS) followed by principal components analysis (PCA) has been successfully applied to the screening of rat urine following the administration of three candidate pharmaceuticals. With this methodology it was possible to differentiate the control samples from the dosed samples and to identify the components of the mass spectrum responsible for the separation. These data clearly show that LC/MS is a viable alternative, or complementary, technique to proton NMR for metabonomics applications in drug discovery and development. 相似文献
18.
Jaehun Park Doug S. English Yvonne Wannemuehler Susan Carpenter Jacob W. Petrich 《Photochemistry and photobiology》1998,68(4):593-597
The light-induced antiviral activity of hypericin and hypocrellin in the presence and absence of oxygen was examined under experimental conditions where the effect of oxygen depletion could be quantified. There was a significant reduction of light-induced antiviral activity of hypericin and hypocrellin under hypoxic conditions. Interestingly, antiviral activity of hypocrellin was not observed at low oxygen levels at which hypericin retained measurable virucidal activity. This suggests that additional pathways, such as the generation of protons from excited states of hypericin, may enhance the biological activity of activated oxygen species. 相似文献
19.
Conway B Hevia E Kennedy AR Mulvey RE Weatherstone S 《Dalton transactions (Cambridge, England : 2003)》2005,(8):1532-1544
Synthesised either by an unusual tert-butyl metathesis between tert-butyllithium and a n,s-butylmagnesium amide or by reaction of an alkyl Grignard reagent and a sodium amide, five tert-butylmagnesium amides, Bu(t)MgDBA (5)(DBA=dibenzylamide), Bu(t)MgDA (6)(DA=diisopropylamide), Bu(t)MgHMDS (7)(HMDS=1,1,1,3,3,3-hexamethyldisilazide), Bu(t)MgTMP (8)(TMP=2,2,6,6-tetramethylpiperidide) and Bu(t)MgNCy2 (9)(cy=cyclohexyl) have been isolated as crystalline solids. All five amides have been characterised by X-ray crystallography and solution NMR spectroscopic studies. The former studies reveal a common dimeric molecular structure with amido bridges in a planar (MgN)2 ring and terminal Bu(t) ligands on the Mg atoms. Also described is the dodecameric primary amide [Bu(n)MgN(H)Dipp]12 (10a) and its monomeric solvate Bu(n)MgN(H)Dipp.TMEDA (10b)(Dipp=2,6-diisopropylphenyl; TMEDA=N,N,N',N'-tetramethylethylenediamine). The crystal structures of the oxo-insertion products Bu(t)MgOBu(t).THF (11), Bu(t)Mg(mu-OBu(t))(mu-TMP)MgTMP (12) and Mg(OBu(n))HMDS.solv [solv=THF (13a) or Et2O (13b)], made fortuitously during the course of this work, are also presented. 相似文献
20.
By using low temperatures and largely deuterated solvents, the rate of OH proton exchange for aqueous solutions of alcohols is reduced sufficiently to give separate NMR signals from water and alcohol OH protons. The limiting shifts for dilute alcohols in water are down-field of both the water resonance and those of the pure alcohols. This contrasts with the limiting shift for water in the alcohols, which is to high field of the bulk water resonance. The resonance shifts initially to low fields as [ROH] increases, the rate of shift being greatest for t-butyl alcohol. For dilute aqueous solutions, all the alcohols reduce the total concentration of free OH groups, as judged by the overtone infrared spectra. Some of these results are interpreted in terms of a scavenging of free OH groups by the excess lone-pairs of the alcohol molecules. An extra, temperature dependent, down-field shift in the water proton resonance induced by t-butyl alcohol is assigned to a clathrate cage effect. 相似文献