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排序方式: 共有310条查询结果,搜索用时 15 毫秒
301.
Masahiko Yoshimura Michael Dieckmann Pierre-Yves Dakas Raymonde Fonné-Pfister Claudio Screpanti Katrin Hermann Stefano Rendine Pierre Quinodoz Beyza Horoz Saron Catak Alain De Mesmaeker 《Helvetica chimica acta》2020,103(4):e2000017
Strigolactones are plant hormones, which play pivotal roles in plant growth and development with potential application in sustainable agriculture. Recently, zealactone 1a/b has been identified as the major strigolactone from the root exudates of corn. Although zealactone is a promising molecule affecting signaling in the rhizosphere as well as in planta, evaluating its biological activities has been hampered by its low natural abundance and its relative chemical instability. Herein, we present the total synthesis of zealactone 1a/b based on our studies employing a [2+2]-cycloaddition strategy and a chemoselective Baeyer-Villiger oxidation to forge the γ-butyrolactone fragment. Furthermore, we disclose the biological activities of zealactone 1a/b on corn and in soil in comparison with related synthetic analogues. 相似文献
302.
The classical problem of testing the equality of the covariance matrices from k ? 2 p-dimensional normal populations is reexamined. The likelihood ratio (LR) statistic, also called Bartlett’s statistic, can be decomposed in two ways, corresponding to two distinct component-wise decompositions of the null hypothesis in terms of the covariance matrices or precision matrices, respectively. The factors of the LR statistic that appear in these two decompositions can be interpreted as conditional and unconditional LR statistics for the component-wise null hypotheses, and their mutual independence under the null hypothesis allows the determination of the overall significance level. 相似文献
303.
Carlos Romero 《Annals of Operations Research》2000,94(1-4):57-68
This paper develops an approach to deal with risk in agricultural decisions. Although the model is in line with the Prospect ranking theory and the Partitioned multiobjective risk method, which recognise the multidimensional character of any risk measure taken in agricultural decisionmaking problems, its behavioural hypothesis and analytical development are totally different. The way in which the approach works is illustrated through a simple but wellknown example in agricultural planning. 相似文献
304.
Ana Cristina Soria Isabel Martínez-CastroJesús Sanz 《Journal of chromatography. A》2009,1216(15):3300-3304
Data precision in the analysis by purge-and-trap coupled on-line to gas chromatography–mass spectrometry (P&T-GC–MS) of honey volatiles has been studied by statistical analysis. The contribution of non-random factors to dispersion of quantitative results was proven by comparing several statistical parameters (correlation coefficients, principal component analysis (PCA) eigenvalues and loadings) from both experimental and simulated data. PCA was also useful for grouping volatiles with similar dispersion behaviour; these groups being generally related to compounds with common properties or structural features. The use of area ratios improves data precision for compounds within the same group. Results from this study could be used for a better selection of internal standards in quantitative analysis of volatiles by P&T-GC–MS. 相似文献
305.
306.
Christin Scheller Finja Krebs Rebecca Wiesner Hermann Wätzig Imke Oltmann-Norden 《Electrophoresis》2021,42(14-15):1521-1531
SDS gel electrophoresis is a commonly used approach for monitoring purity and apparent molecular mass (Mr) of proteins, especially in the field of quality control of biopharmaceutical proteins. The technological installation of CE-SDS as the replacement of the slab gel technique (SDS-PAGE) is still in progress, leading to a continuous improvement of CE-SDS instruments. Various CE-SDS instruments, namely Maurice (CE-SDS/CE-SDS PLUS) and Wes by ProteinSimple as well as the microchip gel electrophoresis system LabChip® GXII Touch™ HT by PerkinElmer were tested for precision and repeatability compared to SDS-PAGE (Bio-Rad). For assessing these quality control parameters, standard model proteins with minor post-translational modifications were used. Overall, it can be concluded that the CE-SDS-based methods are similar to SDS-PAGE with respect to these parameters. Quality characteristics of test systems gain more significance by testing proteins that do not behave like model proteins. Therefore, glycosylated proteins were analyzed to comparatively investigate the influence of glycosylation on Mr determination in the different instruments. In some cases, high deviations were found both among the methods and with regard to reference values. This article provides possible explanations for these findings. 相似文献
307.
Validation of analytical procedures is important for their efficient and reliable application. The International Conference on Harmonisation (ICH), Food and Drug Administration (FDA) and pharmacopoeia guidelines achieved a great deal in harmonising the definitions of the required validation characteristics. It is well known that poor reproducibility limits the practical implementation of capillary electrophoresis (CE). A precision study on four different MEKC methods was performed with 11 samples, containing seven antibiotics, by two analysts, in few days, on two capillary electrophoresis instruments. Five pharmaceutical preparations and three animal feeds were used. Precision was statistically analysed using migration time, peak area and height of each compound, as well as electroosmotic front (EOF). In 25 of 31 cases, the reproducibility of peak area, peak height and migration time was good (<5%). In most cases the reproducibility of peak area was much better than the reproducibility of peak height. The worst reproducibility that we observed was 12.7% for peak height and 7.6% for peak area. 相似文献
308.
Anas M. Abdel Rahman Judy Pawling Michael Ryczko Amy A. Caudy James W. Dennis 《Analytica chimica acta》2014
Metabolomics is the identification and quantitation of small bio-molecules (metabolites) in biological samples under various environmental and genetic conditions. Mass spectrometry provides the unique opportunity for targeted identification and quantification of known metabolites by selective reaction monitoring (SRM). However, reproducibility of this approach depends on careful consideration of sample preparation, chemical classes, and stability of metabolites to be evaluated. Herein, we introduce and validate a targeted metabolite profiling workflow for cultured cells and tissues by liquid chromatography–triple quadrupole tandem mass spectrometry. The method requires a one-step extraction of water-soluble metabolites and targeted analysis of central metabolites that include glycolysis, amino acids, nucleotides, citric acid cycle, and the hexosamine biosynthetic pathway. The sensitivity, reproducibility and molecular stability of each targeted metabolite were assessed under experimental conditions. Quantitation of metabolites by peak area ratio was linear with a dilution over a 4 fold dynamic range with minimal deviation R2 = 0.98. Inter- and intra-day precision with cells and tissues had an average coefficient of variation <15% for cultured cell lines, and somewhat higher for mouse liver tissues. The method applied in triplicate measurements readily distinguished immortalized cells from malignant cells, as well as mouse littermates based on their hepatic metabolic profiles. 相似文献
309.
In order to set up the precision limit that can be reached with laser-induced breakdown spectrometry microprobe, a steel sample was scanned by using a 6-µm diameter spot. Besides being close to the limit of the technique, such a spot diameter resulted in a small plasma size that minimized self-absorption effects. To minimize shot noise, Cr and Fe were used as test elements because of their high contents. Scan consisted of 25 successive matrices formed by 5 × 6 shots, i.e. a total of 750 shots. Results were studied as a whole, as well as between matrices and within matrices, to search for inhomogeneity, outliers and drift. Except a few outliers, the main contribution in the experimental RSD was the drift either within a matrix or between matrices. Drift attributed to laser warm up could be compensated for either by using a polynomial fitting or by using the other element. %RSD significantly below 2 were then obtained demonstrating that there is no penalty in terms of precision to perform laser microprobe using a series of single shots. 相似文献
310.
This paper studies optimal investment and the dynamic cost of income uncertainty, applying a stochastic programming approach. The motivation is given by a case study in Finnish agriculture. The investment decision of a representative farm is modelled as a Markov decision process, extended to account for risk. A numerical framework for studying the dynamic uncertainty cost is presented, modifying the classical expected value of perfect information to a dynamic setting. The uncertainty cost depends on the volatility of income: e.g. with stationary income, the dynamic uncertainty cost corresponds to a dynamic option value of postponing investment. The model can be applied to agricultural policy planning. In the case study, the investment decision is sensitive to risk. 相似文献