全文获取类型
收费全文 | 217篇 |
免费 | 2篇 |
国内免费 | 2篇 |
专业分类
化学 | 139篇 |
力学 | 4篇 |
数学 | 57篇 |
物理学 | 21篇 |
出版年
2023年 | 3篇 |
2022年 | 10篇 |
2021年 | 9篇 |
2020年 | 4篇 |
2019年 | 2篇 |
2018年 | 3篇 |
2017年 | 7篇 |
2016年 | 8篇 |
2015年 | 6篇 |
2014年 | 14篇 |
2013年 | 13篇 |
2012年 | 19篇 |
2011年 | 17篇 |
2010年 | 16篇 |
2009年 | 10篇 |
2008年 | 11篇 |
2007年 | 11篇 |
2006年 | 6篇 |
2005年 | 8篇 |
2004年 | 5篇 |
2003年 | 10篇 |
2002年 | 6篇 |
2001年 | 1篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1996年 | 5篇 |
1995年 | 4篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有221条查询结果,搜索用时 15 毫秒
101.
Ricardo Jorge Neves Bettencourt da Silva Maria Filomena Gomes Ferreira Crujo Camões 《Analytica chimica acta》2010,674(1):9-19
Testing safety of foodstuffs of plant origin involves the analysis of hundreds of pesticide residues. This control is only cost-effective through the use of methods validated for the analysis of many thousands of analyte/matrix combinations. Several documents propose representative matrices of groups of matrices from which the validity of the analytical method can be extrapolated to the represented matrices after summarised experimental check of within group method performance homogeneity. Those groups are based on an evolved expert consensus based on the empirical knowledge on the current analytical procedures; they are not exhaustive, they are not objectively defined and they propose a large list of representative matrices which makes their application difficult. This work proposes grouping 240 matrices, based on the nutritional composition pattern equivalence of the analytical portion right after hydration and before solvent extraction, aiming at defining groups that observe method performance homogeneity. This grouping was based on the combined outcome of three multivariate tools, namely: Principal Component Analysis, Hierarchical Cluster Analysis and K-Mean Cluster Analysis. These tools allowed the selection of eight groups for which representative matrices with average characteristics and objective criteria to test inclusion of new matrices were established. The proposed matrices groups are homogeneous to nutritional data not considered in their definition but correlated with the studied multivariate nutritional pattern. The developed grouping that must be checked with experimental test before use was tested against small deviations in food composition and for the integration of new matrices. 相似文献
102.
Filomena Martins Luís Moreira Nélson Nunes Ruben E. Leitão 《Journal of Thermal Analysis and Calorimetry》2010,100(2):483-491
Solution enthalpies of 1-bromoadamantane, 1-adamantanol, and 2-adamantanone in a large set of protic and aprotic solvents
are reported at 298.15 K. Solvent effects on the solution processes of these solutes are analyzed in terms of a modified TAKA
equation, involving δcav
h
s as the cavity term. The nature and magnitude of the major interactions which influence these processes are assessed and discussed
in terms of the solutes’ characteristics. New insights on the solution processes under scrutiny are presented. 相似文献
103.
G Bianco F Lelario FG Battista SA Bufo TR Cataldi 《Journal of mass spectrometry : JMS》2012,47(9):1160-1169
An liquid chromatography-mass spectrometry method using electrospray ionization in negative ion mode coupled with a hybrid quadrupole linear ion trap and Fourier transform ion cyclotron resonance (FTICR) mass spectrometer was applied to characterize of intact glucosinolates (GLSs) in crude sample extracts of wild bud flowers of Capparis spinosa (Capparis species, family Capparaceae). Structural information of GLSs was obtained upon precursor ions' isolation within the FTICR trapping cell and subsequent fragmentation induced by infrared multiphoton dissociation (IRMPD). Such a fragmentation was found very useful in terms of chemical identification of all precursor ions [M-H](-) including sulfur-rich GLSs reported here for the first time. Along with most common GLSs already found in capers such as glucocapparin, isopropyl/n-propyl-GLS, mercapto-glucocapparin, and two indolic GLS, i.e., 4-hydroxyglucobrassicin and glucobrassicin, the occurrence of the uncommon glycinyl-glucocapparin as well as two sulfur-rich GLSs is reported. IRMPD showed an increased selectivity towards disulfide bond cleavages with thiol migration, suggesting the side chain structure of non-targeted compounds, i.e., disulfanyl-glucocapparin and trisulfanyl-glucocapparin. Glucocapparin [2.05?±?0.25?mg/g, dry weight (dw)] was the most abundant GLS, followed by glucobrassicin (232?±?18?μg/g, dw) and 4-hydroxyglucobrassicin (89?±?12?μg/g, dw). All other compounds were present at very low content ranging from 0.5 to 1.5?μg/g dw. Copyright ? 2012 John Wiley & Sons, Ltd. 相似文献
104.
Ricardo J. N. Bettencourt da Silva Alejandro Arias Cristina M. R. R. Oliveira M. Filomena G. F. C. Cam?es 《Accreditation and quality assurance》2012,17(2):147-157
The water-soluble ionic composition of atmospheric aerosols is a key parameter for estimating its anthropogenic or natural origin. This evaluation depends on the reliability of the performed measurements. The differential approach for the evaluation of the measurement uncertainty was used for estimating, separately, the uncertainty associated with the extraction step, by difference from the observed intermediate precision and the combination of all the other uncertainty components affecting the estimated intermediate precision. The intermediate precision was estimated from the difference of results of the analysis, in different days, of several pairs of filters resulting from cross-changing of their halves. The precision associated with the symmetry of filters division, affecting the homogeneity of studied combined pairs of filters, was subtracted from the observed dispersion of results. The resulting detailed model of measurement performance allowed defining strategies for cost of analysis or magnitude of measurement uncertainty reduction. Measurements are fit for the analysis of urban aerosols since the expanded relative measurement uncertainty is smaller than a target value of 30 %. 相似文献
105.
The conflict between pH as empirical number in routine control and the pH value regarded as conveying some information concerning the effective concentration or activity of hydrogen ions, aH, has caused much confusion.There are, however, reasons to conclude that the overwhelming amount of thermodynamic data is not sufficiently accurate—either due to ignorance of metrological concepts or due to insufficiently specified measurement processes of fundamental chemical quantities pH.The commonly used seven reference buffer solutions to which primary pH values have been conventional assigned, represent a selection out of a more extensive list, recommended by NBS (now NIST) in 1962. From then onwards conventions concerning the Debye-Hückel model of electrolyte solutions and ionic strength have been revised and the pH(S) values reassessed in conformity but only for these seven reference buffer solutions. The others have, so far remained unchanged, locking harmonisation of the conventionally assigned pH(S) values.In this work, ionic strength is calculated through complete equations derived from the acidity constants. Concentrations of the various species involved in the conventional assignment of pH and their corresponding activity coefficients are therefore, more rigorously known. The process proves particularly useful for poliprotic acids with overlapping acidity constants, where the ratio is less than 103.As a consequence, conventionally assigned pH values of reference buffer solutions are recalculated and corrections are introduced as appropriate. 相似文献
106.
Mariangela Marrelli Filomena Conforti Carmen Formisano Nelly Apostolides Arnold Francesco Menichini 《Natural product research》2016,30(6):735-739
The essential oils from Origanum dictamnus, Origanum libanoticum and Origanum microphyllum were analysed by GC-MS, finding carvacrol, p-cymene, linalool, γ-terpinene and terpinen-4-ol as major components. The antioxidant activity by the DPPH and FRAP tests and the antiproliferative activity against two human cancer cell lines, LoVo and HepG2, were investigated, showing that the essential oil of O. dictamnus was statistically the most inhibitory on both the cell lines, while all the oils exerted a weak antioxidant activity. Furthermore, the samples were tested against 10 Gram-negative and Gram-positive bacteria; all the oils were active on Gram-positive bacteria but O. dictamnus essential oil was the most effective (MIC = 25–50 μg/mL), showing also a good activity against the Gram-negative Escherichia coli (MIC = 50 μg/mL). Data suggest that these essential oils and particularly O. dictamnus oil could be used as valuable new flavours with functional properties for food or nutraceutical products. 相似文献
107.
Steroidal glycoalkaloids (SGAs) extracted from tomato leaves and berries (Lycopersicon esculentum Mill.) were separated and identified using optimized reversed-phase liquid chromatography with electrospray ionization (ESI) and ion trap mass spectrometry (ITMS). The ESI source polarity and chromatographic conditions were evaluated. The ESI spectra contain valuable information, which includes the mass of SGAs, the mass of the aglycones, and several characteristic fragment ions. Cleavage at the interglycosidic bonds proximal to the aglycones is the most prominent process in the ESI process. A protonated molecule, [M+H]+, accompanied by a mixed adduct ion, [M+H+Na]2+, was observed for alpha-tomatine (i.e., m/z 1034.7 and 528.9) and dehydrotomatine (i.e., m/z 1032.6 and 527.9) in positive ion mode spectra. The structures of these tomato glycoalkaloids were confirmed using tandem mass spectrometry. The identification of a new alpha-tomatine isomer glycoalkaloid, named filotomatine (MW 1033), which shares a common tetrasaccharide structure (i.e., lycotretraose) with alpha-tomatine and dehydrotomatine, and soladulcidine as an aglycone, is described for the first time. It occurs in significant amounts in the extracts of wild tomato foliage. Multistage mass spectrometry both of the protonated molecules and of the doubly charged ions was used for detailed structural elucidation of SGAs. Key fragmentations and regularities in fragmentation pathways are described and the fragmentation mechanisms involved are proposed. 相似文献
108.
Filomena Pacella 《Annali di Matematica Pura ed Applicata》1982,131(1):311-332
Sunto In questo lavoro si prova l'esistenza di una soluzione periodica di classe C dell'equazione quasiellittica totalmente non lineare: v + F(D
x
4
v, Dsky/4v)=f.
Lavoro eseguito nell'ambito del G.N.A.F.A. del C.N.R. 相似文献
Lavoro eseguito nell'ambito del G.N.A.F.A. del C.N.R. 相似文献
109.
Filomena Gon?alves George E. Marnellos Evangelos A. Efthimiadis José L. Figueiredo 《Reaction Kinetics and Catalysis Letters》2003,80(1):153-159
The simultaneous reduction of N2O+NO with carbon supported Ni/K catalysts was studied. High conversions of both compounds were obtained at 400°C, and a synergystic
effect between Ni and K was observed.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
110.
Given a smooth compact k-dimensional manifold Λ embedded in ? m , with m≥2 and 1≤k≤m?1, and given ?>0, we define B ? (Λ) to be the geodesic tubular neighborhood of radius ? about Λ. In this paper, we construct positive solutions of the semilinear elliptic equation $$\begin{cases} \Delta u + u^p = 0 &\mbox{in } B_{\epsilon}(\varLambda) \\ u = 0 & \mbox{on } \partial B_{\epsilon}(\varLambda) , \end{cases} $$ when the parameter ? is chosen small enough. In this equation, the exponent p satisfies either p>1 when n:=m?k≤2 or $p\in(1, \frac{n+2}{n-2})$ when n>2. In particular, p can be critical or supercritical in dimension m≥3. As ? tends to 0, the solutions we construct have Morse index tending to infinity. Moreover, using a Pohozaev type argument, we prove that our result is sharp in the sense that there are no positive solutions for $p>\frac{n+2}{n-2}$ , n≥3, if ? is sufficiently small. 相似文献