全文获取类型
收费全文 | 12302篇 |
免费 | 363篇 |
国内免费 | 24篇 |
专业分类
化学 | 8067篇 |
晶体学 | 63篇 |
力学 | 168篇 |
数学 | 1682篇 |
物理学 | 2709篇 |
出版年
2020年 | 119篇 |
2019年 | 109篇 |
2016年 | 272篇 |
2015年 | 229篇 |
2014年 | 219篇 |
2013年 | 459篇 |
2012年 | 447篇 |
2011年 | 569篇 |
2010年 | 301篇 |
2009年 | 284篇 |
2008年 | 481篇 |
2007年 | 484篇 |
2006年 | 430篇 |
2005年 | 472篇 |
2004年 | 401篇 |
2003年 | 338篇 |
2002年 | 277篇 |
2001年 | 199篇 |
2000年 | 178篇 |
1999年 | 176篇 |
1998年 | 154篇 |
1997年 | 189篇 |
1996年 | 192篇 |
1995年 | 194篇 |
1994年 | 165篇 |
1993年 | 192篇 |
1992年 | 184篇 |
1991年 | 162篇 |
1990年 | 137篇 |
1989年 | 155篇 |
1988年 | 168篇 |
1987年 | 154篇 |
1986年 | 159篇 |
1985年 | 189篇 |
1984年 | 162篇 |
1983年 | 156篇 |
1982年 | 169篇 |
1981年 | 158篇 |
1980年 | 148篇 |
1979年 | 145篇 |
1978年 | 139篇 |
1977年 | 130篇 |
1976年 | 132篇 |
1975年 | 117篇 |
1974年 | 133篇 |
1973年 | 129篇 |
1961年 | 160篇 |
1960年 | 211篇 |
1959年 | 117篇 |
1958年 | 131篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
141.
Ultra-trace analysis of U,Th, Ca and selected heavy metals in high purity refractory metals with isotope dilution mass spectrometry 总被引:1,自引:0,他引:1
A method for the determination of trace impurities (U, Th, Ca, Fe, Cr, Ni, Cu, and Cd) in the refractory metals molybdenum and tungsten with isotope dilution mass Spectrometry (IDMS) has been developed. This method enables determinations of uranium and thorium down to the lowest pg/g level with high precision and accuracy. Selective chromatographic, extractive and electrolytic methods for the trace-matrix separation were combined with positive thermal ionization mass spectrometry. Different samples of high purity (4N) and of ultra high purity (UHP) materials for advanced technologies were analysed. The detection limits reached are (in ng/g): U 0.006, Th 0.008, Ca 10, Fe 19, Cr 0.5, Ni 0.6, Cu 2.7, and Cd 0.12. A comparison of results with other sensitive analytical methods (ICP-MS, GDMS, SIMS) makes obvious the urgent necessity of a reliable calibration method like IDMS because the analytical results obtained by the other methods often spread over a wide range. 相似文献
142.
Analytical procedures for quantification of peptides in pharmaceutical research by liquid chromatography–mass spectrometry 总被引:6,自引:0,他引:6
John H Walden M Schäfer S Genz S Forssmann WG 《Analytical and bioanalytical chemistry》2004,378(4):883-897
Peptide quantification by liquid chromatography–mass spectrometry (LC–MS) combines the high resolving power of reversed-phase (RP) chromatography with the excellent selectivity and sensitivity of mass spectrometric detection. On the basis of comprehensive practical experience in the analysis of small molecules, pharmaceutical research is developing technologies for analysis of a growing number of peptidic drug candidates. This article is a detailed review of procedures based on LC–MS techniques for quantitative determination of peptides. With the focus on pharmaceutical applications several technologies for sample preparation, various aspects of peptide chromatography, important characteristics of ESI–MS, selectivity of MS-detection modes, the large variability of internal standards, and modern instrumentation are discussed. The demand for reliable, robust, sensitive, and accurate methods is discussed using numerous examples from the literature, complemented by experiments and results from our laboratory. 相似文献
143.
Jean-Claude Muller Henri Rarouz John Daly Peter Schönholzer 《Helvetica chimica acta》1982,65(5):1454-1466
The structure of new 2-oxo-2,8-dihydro-[l,2,4]oxadiazolo-[2,3-a]pyrimidine-carbamates 1 was established by correlating the IR. and NMR. data with these of analogous compounds. A X-ray crystallographic analysis of 1c gave an unequivocal proof of structural assignment. 相似文献
144.
Yang L Mayr M Wurst K Buchmeiser MR 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(22):5761-5770
The synthesis of novel ruthenium-based metathesis catalysts containing the saturated 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene ligand, that is, [RuCl2(NHC)[=CH-2-(2-PrO)-5-NO(2)-C6H3]] (1) and [Ru(CF3COO)2(NHC)[=CH-2-(2-PrO)-5-NO2-C6H3]] (2) (NHC=1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene) is described. Both catalysts are highly active in ring-closing metathesis (RCM) and ring-opening cross-metathesis (ROCM). Compound 1 shows moderate activity in enyne metathesis. Compound 2 is not applicable to enyne metathesis since it shows high activity in the cyclopolymerization of diethyl dipropargylmalonate (DEDPM). Poly(DEDPM) prepared by the action of 2 consists of 95% five-membered rings, that is, poly(cyclopent-1-enevinylene)s, and 5 % of six-membered rings, that is, poly(cyclohex-1-ene-3-methylidene)s. The polymerization proceeds in a nonliving manner and results in polyenes with broad polydispersities (1.9< or =PDI< or =2.3). Supported analogues of 2 were prepared by immobilization on hydroxymethyl-Merrifield resin and a monolithic support derived from ring-opening-metathesis polymerization (ROMP). Catalyst loadings of 1 and 2.5%, respectively, were obtained. Both supported versions of 2 showed excellent reactivity. With 0.24-2% of the supported catalysts, yields in RCM and ROCM were in the range of 76-100%. Leaching of ruthenium was low and resulted in Ru contaminations of the products of less than 0.000014% (0.14 ppm). 相似文献
145.
Burford N Cameron TS Clyburne JA Eichele K Robertson KN Sereda S Wasylishen RE Whitla WA 《Inorganic chemistry》1996,35(19):5460-5467
The phosphadiazonium cation [MesNP](+) reacts quantitatively with the fluorenylide anion, MesNH(2), and MesOH (Mes = 2,4,6-tri-tert-butylphenyl), resulting in formal insertion of the N-P moiety into the H-Y (Y = C, N, O) bonds. Specifically, reaction of MesNPCl with fluorenyllithium gives the aminofluorenylidenephosphine [crystal data: C(31)H(38)NP, monoclinic, P2(1)/c, a = 9.568(8) ?, b = 24.25(2) ?, c = 11.77(1) ?, beta = 101.38(8) degrees, Z = 4]. Similarly, reaction of [MesNP][GaCl(4)] with MesNH(2) gives the diaminophosphenium salt [MesN(H)PN(H)Mes][GaCl(4)] [crystal data: C(36)H(60)Cl(4)GaN(2)P, monoclinic, C2/c, a = 24.921(2) ?, b = 10.198(4) ?, c = 16.445(2) ?, beta = 93.32(1) degrees, Z = 4], and reaction with MesOH gives the first example of an aminooxyphosphenium salt [MesN(H)POMes][GaCl(4)]. It is proposed that the reactions involve nucleophilic attack at phosphorus followed by a 1,3-hydrogen migration from Y to N. Experimental evidence for the formation of sigma-complex intermediates is provided by the isolation of [MesNP-PPh(3)][SO(3)CF(3)] [crystal data: C(37)H(44)F(3)NO(3)P(2)S, triclinic, P&onemacr;, a = 10.663(1) ?, b = 19.439(1) ?, c = 10.502(1) ?, alpha = 103.100(7) degrees, beta = 113.311(7) degrees, gamma = 93.401(7) degrees, Z = 2]. As part of the unequivocal characterization of the aminooxyphosphenium salt, detailed solid-state (31)P NMR studies and GIAO calculations on the phosphenium cations have been performed. Contrary to popular belief, the phosphorus shielding in dicoordinate cations is not caused by the positive charge but results from efficient mixing between the phosphorus lone pair and pi orbitals. 相似文献
146.
147.
The behaviour of ng- and μg-amounts of amines of biochemical interest is compared. ng-amounts are co-precipitated from aqueous solutions with the tetraphenyloborates of potassium, ammonium and creatinine. A separation from potassium chloride, ammonium chloride and creatinine is achieved by extraction of the amines with methanol. The elution volumes of about 50 amines during ion-exchange chromatography using a weakly acidic resin are compared. A procedure has been elaborated by which the amines can be collected from urine. 相似文献
148.
Klaus Sommer 《无机化学与普通化学杂志》1971,383(2):136-143
Hydrolysis of 1-chloro-arsolane, C4H8AsCl, and 1-chloro-arsenane, C5H10AsCl, leads to oxides C8H10As2O and C10H20As2O, respectively. On direct alkylation of arsenic trioxide with 1.4-butane- and 1.5-pentane-bis-magnesium bromide these oxides are formed only in trifling amounts. Better results are gained by alkylation of N,N-dimethylaminodichloroarsane into amino-arsolane and amino-arsenane, which are also transformed into these oxides by hydrolysis. By hydrolysis of 1.4-dichloro-diarsenane only a small amount of 1.4-epoxo-arsenane is formed, whereas mainly a dimer of this oxide results. Solvolysis of 2.6-dichloro-diarsa-[3.3.0]-bicyclooctane with water, hydrogen sulphide or methylamine leads to 7-oxa-, 7-thia- and 7-methylaza-2.5-diarsa-noradamantane, respectively. 相似文献
149.
A graphite rod electrothermal atomizer has been used for the AAS determination of traces of gold in hydrochloric and in hydrobromic acid solutions, and also after extraction into HBr-saturated methyl isobutyl ketone. Photographic film samples were decomposed first by enzyme action then by nitric acid/peroxide oxidation, and the gold was extracted into MIBK. For 10-μl aliquots of solution the 3s limits of detection were 3 × 10?10g for aqueous solutions, 7 × 10?10g for MIBK, and 7 × 10?9 g/cm2 for film. 相似文献
150.
L. Grosse und W. Klaus 《Fresenius' Journal of Analytical Chemistry》1972,259(3):195-203
Zusammenfassung Es werden allgemeine und qualitative Merkmale von Natrium-Carboxymethylcellulose, Methylcellulose und Hydroxyäthylcellulose sowie gravimetrische und titrimetrische Bestimmungsmöglichkeiten der wasserlöslichen Celluloseäther insbesondere im Zusammenhang mit dem Substitutionsgrad beschrieben.Verfahren zur Ermittlung der Viscosität von Lösungen, des Polymerisationsgrades und des Gelteilchengehaltes werden besprochen. Abschließend werden charakteristische Beispiele für das analytische Vorgehen bei Substanzgemischen gegeben, die nur einen geringen Anteil Celluloseäther enthalten.
Erweiterte und umgearbeitete Fassung eines Vortrages, gehalten am 28. 6. 1971 in Baden-Baden.
Frau Dr. G. Bartelmus, Analyt. Laboratorium der Fa. Kalle AG, sprechen wir an dieser Stelle unseren herzlichen Dank für wertvolle Beratung aus. 相似文献
Analysis of water-soluble cellulose ethers
General and qualitative characteristics of sodium carboxymethylcellulose, methylcellulose and hydroxyethylcellulose are presented and gravimetric and titrimetric methods for the determination of water-soluble cellulose ethers are described, especially with regard to the degree of substitution. Procedures for the measurement of the viscosity of solutions, of the degree of polymerisation and of the content of gel particles are discussed. Characteristic examples are presented for the treatment of mixtures containing only a small percentage of cellulose ethers.
Erweiterte und umgearbeitete Fassung eines Vortrages, gehalten am 28. 6. 1971 in Baden-Baden.
Frau Dr. G. Bartelmus, Analyt. Laboratorium der Fa. Kalle AG, sprechen wir an dieser Stelle unseren herzlichen Dank für wertvolle Beratung aus. 相似文献