全文获取类型
收费全文 | 1184篇 |
免费 | 43篇 |
国内免费 | 29篇 |
专业分类
化学 | 1050篇 |
力学 | 13篇 |
综合类 | 21篇 |
数学 | 54篇 |
物理学 | 118篇 |
出版年
2024年 | 1篇 |
2023年 | 26篇 |
2022年 | 30篇 |
2021年 | 104篇 |
2020年 | 66篇 |
2019年 | 55篇 |
2018年 | 56篇 |
2017年 | 48篇 |
2016年 | 46篇 |
2015年 | 29篇 |
2014年 | 51篇 |
2013年 | 85篇 |
2012年 | 87篇 |
2011年 | 72篇 |
2010年 | 46篇 |
2009年 | 61篇 |
2008年 | 35篇 |
2007年 | 41篇 |
2006年 | 27篇 |
2005年 | 49篇 |
2004年 | 32篇 |
2003年 | 28篇 |
2002年 | 21篇 |
2001年 | 16篇 |
2000年 | 27篇 |
1999年 | 12篇 |
1998年 | 19篇 |
1997年 | 11篇 |
1996年 | 16篇 |
1995年 | 5篇 |
1994年 | 10篇 |
1993年 | 11篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 4篇 |
1982年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有1256条查询结果,搜索用时 109 毫秒
1.
以食品发热包为主题情境,通过探析发热包成分、解密发热原理、计算原料配比、配制发热包等环节梳理酸和碱的性质、用途。从元素观、转化观的视角认识物质转化规律,体现物质研究的一般思路,凸显科学探究的基本过程。通过设计方案测定发热包中碳酸钠的含量,总结混合物成分鉴定及定量测定的方法;归纳气体定量测定的影响因素。呈现一堂学科观念、学科方法教学具体课例,凸显物质研究、定性检验、定量测定的基本方法。 相似文献
2.
A method using high-performance liquid chromatography with diode array detection (HPLC-DAD) as a powerful separation technique has been developed for the simultaneous determination of the four flavonols rutin, quercetin, kaempferol and isorhamnetin in food supplements and pharmaceutical formulations. The chromatographic separation was achieved in 36?min using a Symmetry C18 column (250?×?3?mm; 5?µm) as the stationary phase and a mixture of methanol, acetonitrile, and pH 2.5 aqueous acetic acid as the mobile phase in gradient elution mode. The analytical wavelengths were 256?nm for rutin, quercetin and isorhamnetin, and 368?nm for kaempferol. An ultrasound-assisted extraction protocol was performed using methanol as solvent. The detection and quantification limits were lower than 0.03?µg mL?1 and 0.08?µg mL?1, respectively. The inter-day and intra-day precisions were less than 4.8 and 5.1%, respectively, and the average recoveries were in the range from 96 to 107%. The method was applied for the determination of the studied flavonols in food supplements and pharmaceutical preparations. The satisfactory recovery values demonstrate the potential of the developed method for the determination of the analytes in these samples. In addition, the method is suitable for routine quality control due its ease of operation. 相似文献
3.
Antibiotic-resistant food-borne Listeriosis has been rising with up to 30% mortality threat in humans since several decades. Hence, discovering antilisterial from the extracts of ethnomedicinal plants may be of value as a novel antidote. In our preceding study, we reported that ethanolic extract of Piper betle L. var Pachaikodi leaves exhibited antibacterial activity towards Listeria monocytogenes MTCC 657. Consequently in the present study, the bioactive molecule responsible for anti-Listeria activity was purified and identified as 4-allylbenzene-1,2-diol. This identified bioactive compound may have significance while used as antimicrobials and/or food additives in food processing sector as evidenced by dual action: biofilm inhibition and pore formation on cell membrane. 相似文献
4.
AbstractHerein the bioaccessibility of Mn, Cu, Zn, Cd, and Pb, selected from essential and toxic elements, was determined in commercially sold vegetable purees intended for infant and toddler consumption. Chemical fractionation studies using water, acetone, diethyl ether, chloroform:methanol, and n-hexane were employed to predict the importance of the protein and lipid parts of a matrix to assess the bioaccessibility data. In addition, in-vitro gastrointestinal digestion was performed to determine the bioaccessibility of the elements using a five level, three factor central composite design (CCD) to maximize the elemental solubility. The total elemental concentrations in all of the fractions were determined by inductively coupled plasma – mass spectrometry (ICP-MS). Based on the consumption of one jar of vegetable/rice-based baby food, Zn was 1.3% of the recommended dietary allowance and Mn was 4.2% of the adequate intake level, while Cu was almost 100% of the adequate intake level. Additionally, Pb was always below the detection limit and Cd was sometimes under the detection limit for the percent bioaccessibility. However, in some samples, Cd was as high as 80% of the tolerable weekly intake level depending on the body weight. 相似文献
5.
Chunlai Wu Chuanyun Peng 《International journal of environmental analytical chemistry》2019,99(7):670-682
To develop an accurate and precise method for separation and pre-concentration of Hg(II), a novel thionin functionalised core shell structure magnetic material has been prepared and characterised. The extraction ability of the material was evaluated by magnetic solid-phase extraction coupled with inductively coupled plasma mass spectrometry determination of Hg(II) in food and water samples. Combining the advantages of magnetic separation with selective extraction of thionin towards Hg(II), the material exhibits enhanced enrich selectivity and efficiency for Hg(II). The experimental parameters influencing Hg(II) extraction efficiency, including pH of the aqueous solution, the dosage of the adsorbent, extraction time and sample volume, were systematically investigated. Under the optimised conditions, concentration of Hg(II) at 1.0 μg L?1 can be successfully enriched by the material without the interference of the common co-existing ions. The enrichment factor and adsorption capacity were 250 and 75.2 mg g?1, and precise of the method was confirmed by analysing the spiked food, water samples and standard water reference samples with the recoveries of 92.5–101.8%. 相似文献
6.
当今全球范围内有机食品行业发展迅速,体现出消费者对食品质量安全的重视。相比于普通鸡蛋,有机鸡蛋因严格的生产条件以及更高的营养价值生产成本更高、售价更贵。市面上所销售的有机鸡蛋虽取得了严格有机食品认证标识,但依旧不能阻止不法份子将普通鸡蛋冒充有机鸡蛋销售,从而谋取利润。这一行为不仅会损害有机鸡蛋生产商的利益,也降低了人们对有机食品的信任度,为此需要一种有效的对有机鸡蛋和普通鸡蛋无损鉴别方法。使用高光谱技术透射成像的方式,可以获取物质内部的信息,以有机鸡蛋和普通鸡蛋为试验对象,采集鸡蛋样本364~1 025 nm波长范围的高光谱图像数据,从采集到的数据中提取出鸡蛋蛋清和蛋黄感兴趣区域(ROI)的平均透射光谱数据。根据透射光谱曲线图筛选出有机鸡蛋与普通鸡蛋光谱响应差异明显的波段区间,分别通过偏最小二乘判别分析(PLS-DA)和支持向量机(SVM)建立鸡蛋类别的鉴别模型,结果表明模型分别根据蛋黄区域与蛋清区域数据进行建模的鉴别准确率相近,进一步对蛋黄区域数据进行分析。由于高光谱数据量大且存在大量冗余信息,给数据采集、存储、传输和建模处理都带来不便,因此分别通过连续投影算法(SPA)和竞争性自适应重加权算法(CARS)对蛋黄ROI数据进行降维处理,剔除了大量冗余信息后再建模。最终,使用对蛋黄ROI区域运用SPA降维后得到的23个特征波长建立的SPA-SVM鉴别模型在测试集的准确率最高达到94.2%。结果表明,通过高光谱技术对有机鸡蛋和普通鸡蛋进行无损鉴别有一定效果。 相似文献
7.
纸基包装材料中抗氧化剂及增塑剂向干性食品模拟物Tenax TA迁移的研究 总被引:1,自引:0,他引:1
采用改性聚苯醚多孔聚合物微球Tenax TA为干性食品模拟物,用于纤维素纸基食品接触材料中3-叔丁基-4-羟基苯甲醚(BHA)、2,6-二叔丁基-4-甲基苯酚(BHT)和邻苯二甲酸二环己酯(DCP)向Tenax TA迁移的研究,以超高效液相色谱进行测定。迁移试验以白卡纸和牛皮纸为研究对象,通过设计不同迁移时间和迁移温度下3种目标物向Tenax TA迁移的实验,获取有机化合物从纸基纤维素向Tenax TA迁移的规律。结果表明,有机化合物从纸基材料向多孔聚合物微球的迁移经历了气体吸附和脱附两个步骤。在Tenax TA吸附目标化合物的过程中,纤维素纸基材料的微观孔径尺寸越大,与迁移物的相互作用越小,迁移物迁移的速率越大,Tenax TA的吸附效率越高;当吸附达到平衡后,迁移时间的延长会引发Tenax TA中迁移物的脱附,进而导致目标化合物的迁移率下降。对45种纸基食品接触材料中目标迁移物的筛查表明,BHA和DCP分别存在于两种不同类型的纸基食品接触材料中,迁移量分别为0.027μg/dm~2和0.81μg/dm~2,均小于欧盟指令规定的特定迁移限量。 相似文献
8.
Dr. Sarvesh K. Srivastava Prof. Dr. Oliver G. Schmidt 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(27):9072-9076
A proof‐of‐concept design for autonomous, self‐propelling motors towards value‐added product synthesis and separation is presented. The hybrid motor design consists of two distinct functional blocks. The first, a sodium borohydride (NaBH4) granule, serves both as a reaction prerequisite for the reduction of vanillin and also as a localized solid‐state fuel in the reaction mixture. The second capping functional block consisting of a graphene–polymer composite serves as a hydrophobic matrix to attract the reaction product vanillyl alcohol (VA), resulting in facile separation of this edible value‐added product. These autonomously propelled motors were fabricated at a length scale down to 400 μm, and once introduced in the reaction environment showed rapid bubble‐propulsion followed by high‐purity separation of the reaction product (VA) by the virtue of the graphene–polymer cap acting as a mesoporous sponge. The concept has excellent potential towards the synthesis/isolation of industrially important compounds, affinity‐based product separation, pollutant remediation (such as heavy metal chelation/adsorption), as well as localized fuel‐gradients as an alternative to external fuel dependency. 相似文献
9.
气相色谱-质谱联用法测定日用品及食品中9种凉味剂的含量 总被引:1,自引:0,他引:1
建立了日用品及食品中薄荷脑等9种新型凉味剂含量的测定方法.样品经乙醇超声提取后,用DB-1701 MS毛细管色谱柱(30 m×0.25 mm×0.25 μm)分离,选择离子监测模式(SIM)进行质谱检测.在优化分析条件下,9种凉味剂分离良好,其在0.25~250 μg/mL浓度范围内线性关系良好,相关系数均大于0.996,除薄荷氧基.1,2丙二醇(0.26 μg/g)外,其余凉味剂的检出限均小于0.1μg/g;在3个不同加标浓度下的平均回收率为82.8%~103.5%,相对标准偏差(RSD,n=6)均不高于3.7%.该方法灵敏、准确、可靠,能较好地分离薄荷脑等9种凉味剂,具有分析时间短、抗干扰性好、定量准确等优点,可为含薄荷(清凉)口味食品的质量控制及未知凉味剂含量样品的分析提供科学数据. 相似文献
10.