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排序方式: 共有606条查询结果,搜索用时 12 毫秒
1.
Some bounds on the entries and on the norm of the inverse of triangular matrices with nonnegative and monotone entries are found. All the results are obtained by exploiting the properties of the fundamental matrix of the recurrence relation which generates the sequence of the entries of the inverse matrix. One of the results generalizes a theorem contained in a recent article of one of the authors about Toeplitz matrices. 相似文献
2.
Strategically designed sample composition (SSC) is a new technique that decreases the number of analytical determinations needed in routine screening to as few as the number of original sample specimens while providing information that is specific to them. Although this new technique has been applied to environmental studies, this paper describes its first application to food safety studies. Contamination of milk samples with polychlorinated biphenyls (PCBs) was chosen as a case study to show the usefulness and potential of the SSC technique with a fast analytical procedure that involves saponification of the samples and solid-phase microextraction of the PCBs. A total of 20 sample specimens can be analyzed in 11 determinations with excellent predictions of the positive samples and the concentration levels of the contaminants. The robustness of the strategy was investigated and demonstrated. 相似文献
3.
Domenico Armenise Nicolino De Laurentis Antonia Reho Antonio Rosato Flaviano Morlacchi 《Journal of heterocyclic chemistry》2004,41(5):771-775
A series of 6‐fluoro‐4‐(5 or 7)‐chloro‐2‐(difluorobenzoyl)aminobenzothiazoles 3a‐r were prepared to investigate their potential biological activity. In this work, the results of their in vitro antifungal activity against some strains of Candida albicans are reported. It was found that some derivatives displayed antifungal activity higher than that for 3k [1a] compound already described in literature. 相似文献
4.
Giuseppe Trapani Andrea Latrofa Antonia Reho Massimo Franco Gaetano Liso 《Journal of heterocyclic chemistry》1992,29(5):1155-1159
The title compounds 3a-j together with the N-alkylacylketene S,N-acetals 12a-j were obtained by reaction of N,N'-dialkyldithiodianilines with β-ketoesters compounds. A possible reaction pathway is suggested. 相似文献
5.
All three derivatives of the temperature fluctuation in a fully developed turbulent channel flow have been measured using a pair of parallel cold wires with fixed separations in either the x (streamwise), y (normal to the wall) or z (spanwise) direction. At several locations, measurements were also made, each for a range of y separations between the wires. These latter measurements were important for establishing a suitable method of correcting the data for the effect of wire separation, thus allowing the fixed separation data to be corrected. In the near-wall region, the corrected temperature dissipation is in good agreement with results from direct numerical simulations. The time scale ratio deduced from measurements and simulations agrees well with the theoretical wall value. 相似文献
6.
Ana I. Olives Emiliano E. Romero-Ale M. Antonia Martín Benito del Castillo Pilar López-Alvarado J. Carlos Menéndez 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):553-559
Carbazole-9-carbonyl chloride (C9CC) and 9-carbazolylacetic acid (9CAA) were selected as model fluorescent reagents. The effect
of different chemically modified cyclodextrins (CDs) added to the aqueous solutions of these reagents was studied in water
and in buffered aqueous solutions at pH 4.5 and 8.8. The CDs employed were 2-hydroxypropyl-β-cyclodextrin (HP-βCD), 2,3-di-O-methyl-β-cyclodextrin (DM-βCD) and 2,3,6-tri-O-methyl-β-cyclodextrin (TM-βCD). The inclusion of these reagents inside the cavities of the CDs was verified and this process
can affect the derivatization reaction because CDs can modify the reactivity of the guest molecules. The basic conditions
necessary for the derivatization reaction between C9CC and amines lead to the formation of carbazole anion through hydrolysis
followed by decarboxylation. In the presence of CDs, the hydrolysis-decarboxylation of carbazole-9-carbonyl chloride is faster
than in buffered aqueous homogeneous solutions. The behaviour observed for these reagents in aqueous solutions of CDs was
compared to the one observed in basic ethanolic solutions. These changes are particularly noticeable in the case of 2,3-di-O-methyl-β-CD and 2-hydroxypropyl-β-CD. The characteristics of the fluorescent reagents are compared to carbazole and 9-methylcarbazole
as model compounds.
This paper was presented at XIIIth International Cyclodextrin Symposium. Torino, Italy, May 14–17, 2006. 相似文献
7.
Alpha-amino acids are soluble in acetonitrile when treated with phosphazene bases. As a result, the protection/deprotection events that are usually required for peptide coupling reactions can be minimized. This is illustrated in the synthesis of the important angiotensin-converting enzyme (ACE) inhibitor enalapril. [reaction: see text] 相似文献
8.
A kinetic method is presented for the determination of 0.5–5 μg ml?1 gallium based on its activating effect on the copper(II)-catalyzed oxidation of 4,4′-dihydroxybenzophenone thiosemicarbazone by hydrogen peroxide. The reaction is monitored spectrophotometrically at 415 nm. Two sets of reaction conditions are established; one for the direct determination of gallium, and another, in which indium affects the gallium response, for determination of indium. Mixtures of these cations can be determined at μg ml?1 levels and in gallium/indium ratios from 7.5:1 to 1:1.6, with an accuracy and precision of ca. 4.5%. 相似文献
9.
Andrés G. León Ana I. Olives M. Antonia Martín Benito del Castillo 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):577-583
The use of cyclodextrins (CDs) in HPLC as mobile phase additives provides a flexible alternative for the separation of chemically
related compounds because these separations can be performed on conventional columns and are economically advantageous over
the use of chiral stationary phases. The present paper describes the influence of the presence of β-cyclodextrin (β-CD) and
2-hydroxypropyl-β-cyclodextrin (HPβ-CD) on the separation of the β-carboline alkaloids norharmane, harmane and harmine. The
nature of the stationary phase (reverse phases C1 and C18) affects the chromatographic separations and also the stability of the inclusion complexes that are developed. The changes
in the proportion of the organic solvents at constant concentration of CDs (3 mM for β-CD and 15 mM for HPβ-CD) modify the
retention factors (k′) for all alkaloids studied. The nature of the organic solvent in the mobile phase also changes the chromatographic parameters.
The logarithm of the capacity factor (k′) is linearly increased with the proportion of water in the hydro-organic mobile phase (ethanolic or methanolic) but the
slopes obtained vary depending on the CD added to the mobile phase. The role of competitive equilibria, i.e., chromatographic
distribution and inclusion complexes formation is discussed.
This paper was presented at XIIIth International Cyclodextrin Symposium. Torino, Italy, May, 14–17, 2006 相似文献
10.
Isabel Pizarro Ma Milagros Gomez Carmen Cámara Ma Antonia Palacios 《International journal of environmental analytical chemistry》2013,93(10):879-890
Gradient ion chromatographic separation coupled with ICP-MS was used to resolve and determine the most common arsenic species in environmental and biological samples of carrots, trout, soil, sediment and river water from Region II of Chile. The carrot and trout samples showed a concentration of 49 and 168 µg g?1, respectively, of total As. Both concentrations are high considering the basal level. In the carrots, percentage of 45 and 31% of total As were found for As(III) and As(V) species, respectively. In the trout, the higher concentration related to AsB at 39% of the total As. As(III) and DMA were also present in relatively high concentrations. The River Loa and the soil in which the carrots are growing also present very high As(V) concentrations of 100 and 17 µg g?1, respectively. The ratio between the concentration for the same As species found in the living organisms (carrots and trout) and the environment in which they grow (soil and water) can provide important information about the possible absorption or biotransformation of As species in living beings. As(III) and DMA are the species in which the greatest accumulation occurs with respect to the medium in which they are present, and biotransformation also appears to take place. 相似文献