全文获取类型
收费全文 | 230235篇 |
免费 | 4842篇 |
国内免费 | 2378篇 |
专业分类
化学 | 123479篇 |
晶体学 | 3347篇 |
力学 | 10935篇 |
综合类 | 101篇 |
数学 | 23272篇 |
物理学 | 76321篇 |
出版年
2022年 | 1710篇 |
2021年 | 1897篇 |
2020年 | 1991篇 |
2019年 | 1960篇 |
2018年 | 2085篇 |
2017年 | 1972篇 |
2016年 | 3618篇 |
2015年 | 3052篇 |
2014年 | 3999篇 |
2013年 | 10711篇 |
2012年 | 9121篇 |
2011年 | 11237篇 |
2010年 | 7267篇 |
2009年 | 7381篇 |
2008年 | 10156篇 |
2007年 | 9974篇 |
2006年 | 9445篇 |
2005年 | 8633篇 |
2004年 | 7731篇 |
2003年 | 6690篇 |
2002年 | 6501篇 |
2001年 | 8582篇 |
2000年 | 6431篇 |
1999年 | 4946篇 |
1998年 | 3735篇 |
1997年 | 3652篇 |
1996年 | 3485篇 |
1995年 | 3102篇 |
1994年 | 2904篇 |
1993年 | 2685篇 |
1992年 | 3257篇 |
1991年 | 3215篇 |
1990年 | 2885篇 |
1989年 | 2745篇 |
1988年 | 2731篇 |
1987年 | 2720篇 |
1986年 | 2500篇 |
1985年 | 3608篇 |
1984年 | 3479篇 |
1983年 | 2719篇 |
1982年 | 2881篇 |
1981年 | 2742篇 |
1980年 | 2638篇 |
1979年 | 2713篇 |
1978年 | 2822篇 |
1977年 | 2798篇 |
1976年 | 2632篇 |
1975年 | 2534篇 |
1974年 | 2461篇 |
1973年 | 2499篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Locatelli C 《Talanta》1996,43(1):45-54
The determination of manganese in the presence of iron and chromium by differential pulse voltammetry and fundamental harmonic alternating current voltammetry was compared, including the case of very high element concentration ratios. The voltammetric measurements were carried out using a stationary mercury electrode in ammonia-ammonium chloride buffer (pH 9.6). The analytical procedure was verified by the analysis of the standard reference materials Portland Cement BCS 372, Spectrographic Zinc Spelter NBS-SRM 631, Stainless Steel (AISI 321) NBS-SRM 121d and Highly Alloyed Steel Eurostandard 281-1. Precision and accuracy, expressed as relative standard deviation and relative error respectively, were of the order of 3-5%, while the detection limit for each element was around 1 x 10(-9) M. The standard addition technique improved the resolution of the voltammetric methods, within a maximum experimental error of 5%, even in the case of very high concentration ratios, that is outside the non-interference concentration ratios 69:1 >c(Fe):c(Mn) > 1:74; 35:1 > c(Fe):c(Cr) > 1:30 and 63:1 > c(Fe):c(Mn) > 1:65; 32:1 > c(Fe):c(Cr) > 1:31 for the differential pulse and alternating current techniques respectively, extrapolating the linear section of the i(p) vs. concentration analytical calibration function for the element present at the lowest concentration. In contrast, the element with the greatest concentration was determined by the relevant calibration curve. 相似文献
952.
A simple analytical procedure has been developed for the determination of indican and tryptophan in biological fluids by reversed-phase liquid chromatography using a new electrochemical detector consisting of a tubular anode obtained by moulding graphitized carbon black and polyethylene. The hydrodynamic voltammetry of these compounds has been carried out and it has been found that, by operating in isocratic conditions with phosphate buffer (pH 4.0)-methanol (93:7), the reported compounds can be determined directly. The procedure can be applied for the determination of the free compounds on ultrafiltered serum as well as of their total content on serum deproteinized with methanol. Levels of both compounds in normal and uraemic patients have been measured and the relative ratios between free and total content yield a useful marker for patients with renal disease. The limits of quantitation of indican and tryptophan in serum were 5 and 10 ng/ml, respectively. The within-day assay coefficient of variation for total indican and tryptophan ranged from 3.0 to 3.6% and from 3.8 to 4.1%, respectively. The day-to-day assay coefficient of variation for total indican and tryptophan ranged from 3.4 to 3.7% and from 4.6 to 5.0%, respectively. 相似文献
953.
It has been shown that the “internal” isotope distribution within a given molecular species at the natural abundance level is accessible by a new method, SNIF-NMR, which is based on deuterium NMR. Relative internal factors, Ri/j,have been defined which enable the isotope content of a given site, i, to be compared to that of another molecular site, j, taken as the reference. Several referencing methods intended to provide direct access to relative , Ti , and , (D/H)i , site-specific parameters, are now discussed from both the theoretical and the experimental points of view. In the , which involves a time-consuming chemical transformation of the sample, the risk exists of more or leas systematic errors resulting from discriminating fractionation effects. However this technique offers, conversely, an interesting way of investigating kinetic isotope effects without the need for specific labelling. In spite of its lower spectral precision the has the advantage of being fast and less sensitive to systematic errois and may be used for direct rough routine determinations of the site-specific isotope contents. More precise results can be obtained, at the price of contaminating the sample, when an is added and signal heights are used, remembering however that the intensity parameters then have no strict physical meaning in terms of absolute isotope contents. The site-specific parameters, Ti and (D/H)i thus accessible, provide new information on the mechanisms of the fractionation effects occurring in natural conditions and examples are considered. 相似文献
954.
Summary Simple, direct, and sensitive low temperature phosphorimetric methods for the determination of Chlorpropham, Propham, Propoxur, and Terbucarb in EPA solvent are described. The minimum detectable amounts are 0.01 p. p. m. for Chlorpropham and Propham and 0.02 and 0.3 p. p. m. for Propoxur and Terbucarb respectively. The ranges of linearity of the analytical calibration graphs are 0.05-7.5, 0.05–10, 0.5–50, and 1–50 p. p. m. for Chlorpropham, Propham, Propoxur, and Terbucarb respectively. The procedure for Chlorpropham has been applied to the analysis of a solid commercial formulation.
Bestimmung der Pesticide Chlorpropham, Propham, Propoxur und Terbucarb bei Tief-Temperatur-Phosphorimetrie
Zusammenfassung Einfache, direkte und empfindliche Methoden für die Bestimmung der genannten Pesticide in dem Lösungsmittel Diethylether-i-Pentan-abs. Ethanol wurden beschrieben. Die geringsten nachweisbaren Mengen sind 0,01 ppm für Chlorpropham und Propham sowie 0,02 und 0,3 ppm für Propoxur und Terbucarb. Die Eichkurven verlaufen zwischen 0,05 und 7,5, 0,05 und 10, 0,5 und 50 sowie zwischen 1 und 50 ppm für Chlorpropham, Propham, Propoxur bzw. Terbucarb linear. Das Verfahren für Chlorpropham wurde zur Analyse eines festen Handelsproduktes angewendet.相似文献
955.
I. S. El-Yamani M. Y. Farah E. N. Abd El-Messieh 《Journal of Radioanalytical and Nuclear Chemistry》1978,45(1):147-153
The distribution of Be between aqueous HCl solutions and organic phases of di-(2-ethyl hexyl)-phosphoric acid, has been described. The dependence of extraction on aqueous acidity, metal and extractant concentration as also on the diluent type, was thoroughly examined. The possible mechanism of extraction has been discussed in the light of results obtained. 相似文献
956.
Monoalkylketene acetals and monoarylketene acetals react with diphenylketene to give acylketene acetals (VII). These compounds can further react with diphenylketene to give 2,4-pyronone 4-acetals (IX), which, by mild acid hydrolysis, yields 2,4-pyronones (X).
Reaction of methylketene diethylacetal with two equivalents of diphenylketene yields 3,3-di-phenyl-5-methyl-6-diphenylmethylentetrahydropyran-2,4-dione 4-diethylacetal (XIV) in addition to small quantities of the pyronone acetal IXb. 相似文献
957.
The retention of the enantiomers of mandelic acid and N-benzoylalanine was studied on columns prepared by immobilizing bovine serum albumin (BSA) on an anion exchanger. The amount of BSA fixed on the column is easy to adjust and measure. The adsorption isotherms were determined. For each enantiomer, the isotherm is well accounted for by a bi-Langmuir equation. One term of the isotherm (which is the same for both enantiomers) corresponds to non-selective interactions and the other term to the chiral selective interactions. The column saturation capacity of this second term is 8% larger for the less strongly retained enantiomer. This saturation capacity corresponds approximately to one enantiomer molecule adsorbed for five BSA molecules immobilized. This result is in agreement with the assumption of the hydrophobic cavity of BSA being the chiral selective site. 相似文献
958.
The adsorption and thermal decomposition of ketene on Si(l 11)-7 × 7 were investigated using various surface analysis techniques. When the surface was exposed to ketene at 120 K, two CO stretching modes at 220 and 273 meV appeared in HREELS, corresponding to two adsorbed ketene states. After the sample was annealed at ?250 K, the 273 and the 80 meV peaks vanished, indicating the disappearance of one of the adsorption states by partial desorption of the adsorbate. In a corresponding TPD measurement, a desorption peak for ketene species was noted at 220 K. Annealing the sample at 450 K caused the decomposition of the adsorbate, producing CHx and O adspecies. Further annealing of the surface at higher temperatures resulted in the breaking of the CH bond, the desorption of H and O species and the formation of Si carbide. The desorption of H at 800 K was confirmed by the appearance of the D2 (m/e = 4) TPD peak at that temperature when CD2CO was used instead of CH2CO. 相似文献
959.
Two novel metal-organic frameworks (MOFs) [Zn(TITMB)(OAc)](OH).8.5H(2)O (1) and [Ag(TITMB)N(3)].H(2)O (2) [TITMB = 1,3,5-tris(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene, OAc = acetate anion] were synthesized and their structures were determined by X-ray crystallography. Complex 1 crystallizes in tetragonal space group P(-)4 with a = 23.2664(7) and c = 11.9890(3) A and Z = 8. 1 has a one-dimensional tubelike structure with large inner pore size of approximately 17 A. Complex 2 crystallizes in monoclinic space group C2 with a = 20.7193(10), b = 11.5677(8), and c = 12.2944(6) A, beta = 125.5770(10) degrees, and Z = 4. 2 consists of two-dimensional honeycomb networks that interpenetrate each other to generate a polycatenated structure. In these two complexes, both zinc(II) and silver(I) atoms are four-coordinated with the same tetrahedral coordination geometry. The topologies of 1 and 2 are predominated by the conformations of TITMB, which are cis, trans, trans in 1 and cis, cis, cis in 2, respectively. 相似文献
960.
Kotera M Roupioz Y Defrancq E Bourdat AG Garcia J Coulombeau C Lhomme J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(22):4163-4169
On the basis of molecular modeling studies, the 7-nitroindole nucleoside 1 was selected as a suitable photochemical precursor for photochemical generation of the C1' deoxyribosyl radical under irradiation, which led to 2'-deoxyribonolactone. The nitro-indole nucleoside derivatives 1a and 1b were prepared and their conformation was determined by X-ray crystallography and NMR spectroscopy. The photoreaction of these nucleosides gave the corresponding deoxyribonolactone derivatives efficiently, with release of 7-nitrosoindole. This reaction was successfully applied to synthesis of oligonucleotides containing the deoxyribonolactone lesion. 相似文献