全文获取类型
收费全文 | 877篇 |
免费 | 60篇 |
国内免费 | 17篇 |
专业分类
化学 | 894篇 |
晶体学 | 5篇 |
力学 | 3篇 |
综合类 | 8篇 |
数学 | 1篇 |
物理学 | 43篇 |
出版年
2023年 | 14篇 |
2022年 | 19篇 |
2021年 | 36篇 |
2020年 | 41篇 |
2019年 | 46篇 |
2018年 | 37篇 |
2017年 | 35篇 |
2016年 | 27篇 |
2015年 | 44篇 |
2014年 | 27篇 |
2013年 | 66篇 |
2012年 | 94篇 |
2011年 | 39篇 |
2010年 | 39篇 |
2009年 | 39篇 |
2008年 | 36篇 |
2007年 | 41篇 |
2006年 | 29篇 |
2005年 | 37篇 |
2004年 | 28篇 |
2003年 | 23篇 |
2002年 | 14篇 |
2001年 | 14篇 |
2000年 | 14篇 |
1999年 | 8篇 |
1998年 | 12篇 |
1997年 | 20篇 |
1996年 | 10篇 |
1995年 | 13篇 |
1994年 | 9篇 |
1993年 | 6篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 5篇 |
1989年 | 1篇 |
1988年 | 5篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 2篇 |
排序方式: 共有954条查询结果,搜索用时 265 毫秒
101.
F. A. Nour El-Dien 《光谱学快报》2013,46(3):347-357
Abstract Novel reaction between trioxalatoferrate (III) complex (A) and some di- and trihydric phenols were studied and found to form interesting mixed ligand chelates of iron (III) in the ratio 1:2:1 (Fe : oxalate : phenol) forming blue to violet colors at pH 4.0 to 6.5 and λmax = 580 to 590 nm. These reactions were used for indirect volumetric and spectrophotometric microdetermination of catechol (PC), pyrogallol (PG), dopamine hydrochloride (DHCl), adrenaline hydrogen tartrate (AHT) and sulbutamol sulfate (SS) via EDTA titration using complex (A) as an indicator. PC, DHC1 and PG were determined by EDTA titration within the concentration ranges of 0.55-2.2, 0.95-3.79 and 0.65-2.52 μg ml?1, respectively. AHT was determined in the concentration range of 96 to 204 μg ml?1 and SS was also determined in the range 5.75 to 57.7 μg ml?1. Adrenaline in ampoules coming from two Egyptian companies: sulbutamol in sulbovent liquids, and dopamine in five urine samples of Egyptian tumor patients, was also determined using the suggested procedure with high accuracy. 相似文献
102.
A new on-line Gd preconcentration and determination system associated to flow injection (FI) method was developed. 2,2′-(1,8-dihydroxy-3,6-disulfonaphthylene-2,7-bisazo) bisbenzenearsonic acid (Arsenazo III) was used as a complexing agent at pH 2.5. A reactor containing the polyamide membrane was used for the retention of the Gd complex by chemofiltration. The complex was then removed from the reactor with buffer solution pH 9. The variables affecting the combined on-line preconcentration-absorptiometric method have been evaluated and optimised. The coupling of the on-line preconcentration and spectrophotometric flow through detection led to a detection limit of 15 μg l−1 for a preconcentration time of 5 min at 2 ml/min. The method was successfully applied to biological samples. 相似文献
103.
104.
In der Arbeit wird untersucht, unter welchen analytischen Bedingungen eine quantitative Sticksoffbestimmung mit dem 15 N-Analysator NA-5A mittels Isotopenverdünnung durchzuführen ist. Es wird auf die Berechnung der N-Menge, die Abschätzung des maximalen Fehlers unter den speziellen Verhāltnissen der 15N-Analysator-Anwendung und die Auswertungsverfahren näher eingegangen. Testanalysen an unterschiedlich markierten Modell-Lösungen, bestehendaus Ammonium-, Harnstoff- und Aminosäurestickstoff, ergaben gute Übereinstimmung mit den vorgegebenen Werten. Das Verfahren wurde zur N-Bestimmung von biologish-medizinischem Material eingesetzt. Ein Arbeitsschema zur quantitativan Bestimmung von Ammonium-N, Harnstoff-N und Gesamt-N direkt in Harn wird mitgeteilt. 相似文献
105.
Shannon P. O'Grady Lindsey E. Enright Janet E. Barnette Thure E. Cerling James R. Ehleringer 《Isotopes in environmental and health studies》2013,49(4):476-483
The isotope ratio analysis of body water often involves large sample numbers and lengthy sample processing. Here we demonstrate the ability of isotope ratio infrared spectroscopy (IRIS) to rapidly and accurately analyse the isotope ratios of water in urine. We analysed water extracted from human urine using traditional isotope ratio mass spectrometry (IRMS) and compared those values with IRIS-analysed extracted water and un-extracted urine. Regression analyses for δ2H and δ18O values between (1) extracted water analysed via IRMS and IRIS and (2) urine and extracted water analysed via IRIS were significant (R 2=0.99). These results indicate that cryogenic distillation of urine was not required for an accurate estimate of the isotopic composition of urine when using IRIS. 相似文献
106.
Michael Zech Marianne Benesch Johannes Hepp Steven Polifka Bruno Glaser 《Isotopes in environmental and health studies》2013,49(4):394-403
ABSTRACTInspired by a previous ‘Sauna, sweat and science’ study [Zech et al. Isot Environ Health Stud. 2015;51(3):439–447] and out of curiosity and enthusiasm for stable isotope and sauna research we aimed at answering the question ‘do we sweat (isotopically) what we drink’? We, therefore, pulse-labelled five test persons in a sauna experiment with beverages that were 2H-enriched at about +25,600?‰. Sweat samples were collected during six sauna rounds and the hydrogen isotope composition δ2Hsweat was determined using an isotope ratio mass spectrometer. Before pulse labelling, δ2Hsweat – reflecting by approximation body water – ranged from –32 to –22?‰. This is ~35?‰ enriched compared to usual mid-European drinking water and can be explained with hydrogen-bearing food as well as with the respiratory loss of 2H-depleted vapour. The absence of a clearly detectable 2H pulse in sweat after pulse labelling and δ2Hsweat results of ≤+250?‰ due to a fast 2H equilibration with body water are moreover a clearly negative answer to our research question also in a short-term consideration. Given that the recovery of the tracer based on an isotope mass balance calculation is clearly below 100?%, we finally answer the question ‘where did the rest of the tracer go?’ 相似文献
107.
E. Winkler H. Faust K. Wetzel H.-D. Czarnetzky W. Hartig 《Isotopes in environmental and health studies》2013,49(11):367-372
Klinische Untersuchungen zum menschlichen Stickstoffmetabolismus, die unter Verwendung des stabilen Stickstoffisotops 15N durchgeführt wurden, werden mit Hilfe eines 3-Compartmentmodells ausgewertet und interpretiert. Möglichkeiten zur Erweiterung dieses Modells werden diskutiert, und durch Computerstudien wird die Aussagekraft der aus Modellen gewonnenen Resultate überprüft. 相似文献
108.
109.
Waritda Pookmanee Siriwan Thongthip Jeeranut Tankanitlert Mathirut Mungthin Chonlaphat Sukasem Supeecha Wittayalertpanya 《Molecules (Basel, Switzerland)》2021,26(14)
The method for the determination of primaquine (PQ) and 5,6-orthoquinone primaquine (5,6-PQ), the representative marker for PQ active metabolites, via CYP2D6 in human plasma and urine has been validated. All samples were extracted using acetonitrile for protein precipitation and analyzed using the ultra-high-performance liquid chromatography–tandem mass spectrometry (UHPLC-MS/MS) system. Chromatography separation was carried out using a Hypersil GOLDTM aQ C18 column (100 × 2.1 mm, particle size 1.9 μm) with a C18 guard column (4 × 3 mm) flowed with an isocratic mode of methanol, water, and acetonitrile in an optimal ratio at 0.4 mL/min. The retention times of 5,6-PQ and PQ in plasma and urine were 0.8 and 1.6 min, respectively. The method was validated according to the guideline. The linearity of the analytes was in the range of 25–1500 ng/mL. The matrix effect of PQ and 5,6-PQ ranged from 100% to 116% and from 87% to 104% for plasma, and from 87% to 89% and from 86% to 87% for urine, respectively. The recovery of PQ and 5,6-PQ ranged from 78% to 95% and form 80% to 98% for plasma, and from 102% to from 112% to 97% to 109% for urine, respectively. The accuracy and precision of PQ and 5,6-PQ in plasma and urine were within the acceptance criteria. The samples should be kept in the freezer (−80 °C) and analyzed within 7 days due to the metabolite stability. This validated UHPLC-MS/MS method was beneficial for a pharmacokinetic study in subjects receiving PQ. 相似文献
110.
Massimo Chessa Mario Panebianco Sara Corbu Milena Lussu Angelica Dessì Roberta Pintus Flaminia Cesare Marincola Vassilios Fanos 《Molecules (Basel, Switzerland)》2021,26(14)
Bicuspid aortic valve (BAV) is the most common congenital heart defect responsible for valvular and aortic complications in affected patients. Causes and mechanisms of this pathology are still elusive and thus the lack of early detection biomarkers leads to challenges in its diagnosis and prevention of associated cardiovascular anomalies. The aim of this study was to explore the potential use of urine Nuclear Magnetic Resonance (NMR) metabolomics to evaluate a molecular fingerprint of BAV. Both multivariate and univariate statistical analyses were performed to compare the urinary metabolome of 20 patients with BAV with that of 24 matched controls. Orthogonal partial least squared discriminant analysis (OPLS-DA) showed statistically significant discrimination between cases and controls, suggesting seven metabolites (3-hydroxybutyrate, alanine, betaine, creatine, glycine, hippurate, and taurine) as potential biomarkers. Among these, glycine, hippurate and taurine individually displayed medium sensitivity and specificity by receiver operating characteristic (ROC) analysis. Pathway analysis indicated two metabolic pathways likely perturbed in BAV subjects. Possible contributions of gut microbiota activity and energy imbalance are also discussed. These results constitute encouraging preliminary findings in favor of the use of urine-based metabolomics for early diagnosis of BAV. 相似文献