全文获取类型
收费全文 | 1455篇 |
免费 | 12篇 |
国内免费 | 1篇 |
专业分类
化学 | 902篇 |
晶体学 | 16篇 |
力学 | 15篇 |
数学 | 89篇 |
物理学 | 446篇 |
出版年
2021年 | 8篇 |
2020年 | 9篇 |
2019年 | 8篇 |
2017年 | 15篇 |
2016年 | 18篇 |
2015年 | 8篇 |
2014年 | 18篇 |
2013年 | 54篇 |
2012年 | 42篇 |
2011年 | 47篇 |
2010年 | 26篇 |
2009年 | 23篇 |
2008年 | 48篇 |
2007年 | 71篇 |
2006年 | 80篇 |
2005年 | 74篇 |
2004年 | 79篇 |
2003年 | 57篇 |
2002年 | 69篇 |
2001年 | 35篇 |
2000年 | 38篇 |
1999年 | 27篇 |
1998年 | 18篇 |
1997年 | 13篇 |
1996年 | 33篇 |
1995年 | 21篇 |
1994年 | 32篇 |
1993年 | 27篇 |
1992年 | 30篇 |
1991年 | 16篇 |
1990年 | 12篇 |
1989年 | 27篇 |
1988年 | 20篇 |
1987年 | 22篇 |
1986年 | 18篇 |
1985年 | 28篇 |
1984年 | 24篇 |
1983年 | 10篇 |
1982年 | 14篇 |
1981年 | 23篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1978年 | 25篇 |
1977年 | 18篇 |
1976年 | 13篇 |
1975年 | 10篇 |
1974年 | 21篇 |
1973年 | 24篇 |
1972年 | 12篇 |
1971年 | 8篇 |
排序方式: 共有1468条查询结果,搜索用时 15 毫秒
101.
Alex. G. Harrison R. Krishna Mohan Rao Kallury 《Journal of mass spectrometry : JMS》1980,15(5):249-256
A study of the electron impact and chemical ionization (H2, CH4, and iso-C4H10) mass spectra of stereoisomeric benzoin oximes and phenylhydrazones indicates that while the former can be distinguished only by their chemical ionization mass spectra the latter are readily distinguishable by both their electron impact and chemical ionization mass spectra. The electron impact mass spectra of the isomeric oximes are practically identical; however, the chemical ionization spectra show that the E isomer forms more stable [MH]+ and [MH? H2O]+ ions than the Z isomer for which both the [MH]+ and [MH? H2O]+ ions are relatively unstable. In electron impact the Z-phenylhydrazone shows a lower [M]+˙ ion abundance and more facile loss of H2O than does the E isomer. This more facile H2O loss also is observed for the [MH]+ ion of the Z isomer under chemical ionization conditions. 相似文献
102.
Alex. G. Harrison R. Krishna Mohan Rao Kallury 《Journal of mass spectrometry : JMS》1980,15(6):284-288
The H2 and CH4 chemical ionization mass spectra of a selection of substituted nitrobenzenes have been determined. It is shown that reduction of the nitro group to the amine is favoured by high source temperatures and the presence of water in the ion source. The H2 chemical ionization mass spectra are much more useful for distinguishing between isomeric compounds than the CH4 CI mass spectra because of the more extensive fragmentation. For ortho substituents bearing a labile hydrogen abundant [MH ? H2O]+ fragments are observed. When the substituent is electron-releasing both ortho and para substituted nitrobenzenes show abundant [MH? OH]+ fragment ions while meta substituted compounds show abundant loss of NO and NO2 from [MH]+. The latter fragmentation is interpreted in terms of protonation para to the substituent or ortho to the vitro function, while the first two fragmentation routes arise from protonation at the nitro group. When the substituent is electron-attracting the chemical ionization mass spectra of isomers are very similar except for the H2O loss reaction for ortho compounds. 相似文献
103.
Does a high UV environment ensure adequate vitamin D status? 总被引:1,自引:0,他引:1
Kimlin M Harrison S Nowak M Moore M Brodie A Lang C 《Journal of photochemistry and photobiology. B, Biology》2007,89(2-3):139-147
This study assesses the Vitamin D status of 126 healthy free-living adults aged 18-87 years, in southeast Queensland, Australia (27 degrees S) at the end of the 2006 winter. Participants provided blood samples for analysis of 25(OH)D (the measure of an individual's Vitamin D status), PTH, Calcium, Phosphate, and Albumin, completed a questionnaire on sun-protective/sun-exposure behaviours, and were assessed for phenotypic characteristics such as skin/hair/eye colour and BMI. We found that 10.2% of the participants had serum 25(OH)D levels below 25 nmol/l (considered deficient) and a further 32.3% had levels between 25 nmol/l and 50 nmol/l (considered insufficient). Our results show that low levels of 25(OH)D can occur in a substantial proportion of the population at the end of winter, even in a sunny climate. 25(OH)D levels were higher amongst those who spent more time in the sun and lower among obese participants (BMI>30) than those who were not obese (BMI<30). 25(OH)D levels were also lower in participants who had black hair, dark/olive skin, or brown eyes, when compared with participants who had brown or fair hair, fair skin, or blue/green eyes. No associations were found between 25(OH)D status and age, gender, smoking status, or the use of sunscreen. 相似文献
104.
105.
The reactions of metastable [C5H10O]+ ˙ radical cations produced by ionization of 4-penten-1-ol are reported and discussed. These [C5H10O]+ ˙ species undergo mainly ethyl radical loss, with smaller contributions of methyl radical and water expulsion. 2H-Labelling studies reveal different specificities of hydrogen selection in these three fragmentations. The behaviour of these [C5H10O]+ ˙ ions is compared to those reported previously for isomeric radical cations containing linear alkenyl chains and a terminal hydroxyl group. 相似文献
106.
The [M - H]- ions of a variety of di- to pentapeptides containing H or alkyl side chains have been prepared by electrospray ionization and low-energy collision-induced dissociation (CID) of the deprotonated species carried out in the interface region between the atmospheric pressure source and the quadrupole mass analyzer. Using the nomenclature applied to the fragmentation of protonated peptides, deprotonated dipeptides fragment to give a2 ions (CO2 loss) and y1 ions, where the y1 ion has two fewer hydrogens than the y"1 ions formed from protonated peptides. Deprotonated tri- and tetrapeptides fragment to give primarily y1, c1, and "b2 ions, where the "b2 ion has two fewer hydrogens than the b2 ion observed for protonated peptides. More minor yields of y2, c2, and a2 ions also are observed. The a ion formed by loss of CO2 from the [M - H]- ion shows loss of the N-terminal residue for tripeptides and sequential loss of two amino acid residues from the N-terminus for tetrapeptides. The formation of c(n) ions and the sequential loss of N-terminus residues from the [M - H - CO2]- ion serves to sequence the peptide from the N-terminus, whereas the formation of y(n) ions serves to sequence the peptide from the C-terminus. It is concluded that low-energy CID of deprotonated peptides provides as much (or more) sequence information as does CID of protonated peptides, at least for those peptides containing H or alkyl side chains. Mechanistic aspects of the fragmentation reactions observed are discussed. 相似文献
107.
Validation of Sun Exposure Reported Annually Against Interim Self‐report and Daily Sun Diaries 下载免费PDF全文
Laura King Fan Xiang Ashwin Swaminathan Keith Dear Simone L. Harrison Ingrid van der Mei Michael G. Kimlin Catherine D'Este Robyn M. Lucas 《Photochemistry and photobiology》2017,93(5):1294-1302
Data on personal sun exposure over a period exceeding the immediate past days or weeks are typically self‐reported in brief questionnaire items. The validity of such self‐reporting of longer term personal sun exposure, for example over a year, including detail on variation across seasons, has not previously been investigated. In a volunteer sample (n = 331) of Australian adults aged 18 years and over, we assessed the 12‐month reliability of sun exposure reported separately for each season, and its accuracy compared to a daily sun diary in the same season. Seasonal time outdoors displayed fair‐to‐good reliability between baseline and end of study (12 months), with responses showing higher agreement at lower levels of time outdoors. There was good agreement for ranking of individuals' time outdoors with the daily sun diary data, although the actual diary time outdoors was typically considerably lower than the self‐reported questionnaire data. Place of residence, education, being a smoker, day of the week (i.e. working day vs nonworking day) and working mainly outdoors were significant predictors of agreement. While participants overestimated their actual time outdoors, the self‐report questionnaire provided a valid ranking of long‐term sun exposure against others in the study that was reliable over time. 相似文献
108.
William T. A. Harrison R. Alan Howie Janet M. S. Skakle James L. Wardell 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(6):m351-m354
The title compounds, [Sn(C6H5)2(C5H4S5)] and [Sn(C5H4S5)2], respectively, are of interest because they can be regarded as intermediate in nature between chelates and heterocyclic compounds containing the C3S5 fragment. In contrast with the essentially normal bond lengths and angles within the molecules, the molecular conformations are somewhat unexpected, as are the intermolecular contacts found in the case of the latter compound. 相似文献
109.
We have designed and constructed an atmospheric pressure laser desorption/chemical ionization (AP-LD/CI) source that utilizes a laser pulse to desorb intact neutral molecules, followed by chemical ionization via reagent ions produced by a corona discharge. This source employs a heated capillary atmospheric pressure inlet coupled to a quadrupole ion trap mass spectrometer and allows sampling under normal ambient air conditions. Preliminary results demonstrate that this technique provides approximately 150-fold increase in analyte ions compared to the ion population generated by atmospheric pressure infrared matrix-assisted laser desorption/ionization (AP-IR-MALDI). 相似文献
110.