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  • 1
    ISSN: 1573-9171
    Keywords: ab initio quantum-chemical calculations ; direct spin-spin coupling constants1H ; specific intramolecular C-H ....X interactions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Anab initio quantum-chemical calculation of the direct13C−1H spin-spin coupling constants in the CH4−H2O system was carried out and experimental data obtained for a series ofN-vinyl compounds were analyzed. The specific C-H ....X (X = O, N) intramolecular interactions result in increasing the direct13C−1H spin-spin coupling constant of the hydrogen involved in bonding and decreasing the direct13C-1H spin-spin coupling constant for the hydrogen atom that takes no part in the interaction.
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  • 2
    ISSN: 1573-5052
    Keywords: Diversity ; Grassland ; Niche. Succession ; Time and Space
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Some diversity and niche amplitude parameters were applied to rangeland pastures of the Central Iberian Peninsula and to their succession stages after the periodical ploughing typical of the traditional management of these areas. Four different slopes within a large area of undulating terrain were selected for the monitoring of succession as they contained the characteristical geomorphological pattern of the area (denudation, transport and accumulation sectors). If we consider the total entropy theorem, H (E.P.)=H(E)+H(P/E), the total entropy of the slope H(E.P) and the entropy of species H(E) increase as succession progresses. As the value of the entropy of the sampling plots conditioned by the species H(P/E) is affected by the number of plots utilized, we employed the expression A=H(P/E)/log2 number of plots, similar to Pielou's index for niche amplitude, W=H(P/E)/H(P). This values decreases with succession, indicating that plant species tend to occupy more definite sectors along the slope. The number of low entropy species H(P/E) i or specialist species, confined to narrow sectors also increases. When computed separately within the different sectors niche amplitude results in small values for the low slope regions (accumulation sector). This effect becomes more pronounced when succession advances.
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  • 3
    ISSN: 1615-6102
    Keywords: Callus origin ; Chromosome number evolution ; Hordeum vulgare L. ; “vitro” cultures ; Stem calluses
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary A study of the chromosome numbers of calluses obtained fromin vitro cultured stem nodes of barley over a period of time was carried out. The cells in division in the calluses were found only in growth centres (g.c.) which were independent of one another and easily isolated. At the beginning of the observations, the cell dividing population showed the diploid chromosome number. Tetraploid dividing cells appeared in the 5-month-old calluses and their frequency increased gradually. After 36 months of culture, the frequency of diploid and tetraploid cells was fixed at approximately 50%. The histological observation showed that in the 2-month-old cultures, the calluses were being formed from the epidermis as well as from the stem cortical parenchyma. The medullar parenchyma of the stem was not involved in the callus formation until 5 months of culture. Thus, it was possible to correlate the increase of the tetraploid dividing cell population with the formation of the calluses from the medullar parenchyma.
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  • 4
    ISSN: 1618-2650
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary This paper describes a method for volatilizing chelate complexes in AAS using a carrier gas containing ligand vapour in order to prevent chelate decomposition on heating. The method is evaluated with iron(III) trifluoroacetylacetonate. The sample containing chelate is injected into an electrically heated system, and is carried by nitrogen gas containing trifluoroacetylacetone H(TFA) to a silica tube where atomization occurs. This system increases the reproducibility of atomic absorption signals and decreases the high blanks which occur with the conventional method as a result of chelate decomposition. The characteristic mass of the combined analytical procedure is 1.30 ng Fe, the detection limit 12.0 ng Fe, and the relative standard deviation is 4.7% ng Fe (n=10). Sensitivity is improved 16-fold with respect to the system involving conventional nebulization and atomization of aqueous solutions in air-acetylene flame.
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  • 5
    ISSN: 1618-2650
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The construction and evaluation of three types of tetrafluoroborate electrodes without internal reference solution is described. Sensors for electrodes of Type A and Type B use tetraoctylammonium tetrafluoroborate as carrier and 2-nitrophenyloctylether and dibutylphthalate, respectively, as solvent mediators. Electrodes of Type C use the 92-05-02 Orion commercial sensor based on a complex of tris(1,10-phenanthroline)-Ni(II) dissolved in p-nitrocymene. Membranes were applied on a conductive support consisting of a mixture of graphite with a non-conductive epoxy resin. The three types of electrodes showed a similar behaviour with linear responses within the concentration range 10−5−10−1 mol/l in tetrafluoroborate, with slopes of 58 mV per decade when the pH of the solutions was between 3 and 11.5. Electrodes of Type A showed the lowest short term hysteresis after treatment with interfering species and offered the best reproducibility and lifetime. Therefore, they were used in the determination of tetrafluoroborate ion in plating baths by means of the sample addition method, giving results with an average spike recovery of 100.5% and a mean coefficient of variation of 1.0%.
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  • 6
    ISSN: 1588-2837
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Abstract Различные катализаторы были приготовлены пропитыванием коммерческой окиси алюминия растворами, содержащими комплексный ион — дихлор-бис ( (трихлорстаннато)-платинат(II) [PtCl2(SnCl3)2]−2. После восстановления, каталитическая активность в реакции восстановления бензола изменялась в 1–20 раз. Последняя прямо пропорциональна стабильности комплекса в пропитывающем растворе.
    Notes: Abstract Various catalysts were prepared by impregnating a commercial alumina with solutions containing the complex dichlorobis(trichlorostannato)platinate (II). [Pt Cl2 (SnCl3)2]2−. After reduction, 1–20 fold variations in the catalytic activity in benzene hydrogenation were observed in direct proportion to the stability of the complex in the impregnating solution.
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  • 7
    ISSN: 1612-1112
    Keywords: Gas chromatography ; Solid-phase extraction disks ; Pesticides ; Soil analysis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary A systematic study comparing the methodology and analytical results obtained in an investigation of seven pesticide residues (Molinate, Atrazine, Carbofuran, Pirimicarb, Prometryn, Malathion and Tetrachlorvinphos) in soil samples is reported. Solid-phase extraction (SPE) using glass columns and 47 mm disks of octyl and octadecyl-bonded silica was used in the pesticide analysis. The best extraction efficiency and clearest extracts are obtained with C8 disks. The analyses were carried out by capillary gas chromatography with nitrogen and phosphorus detection. Recovery experiments were performed at ppb levels in spiked soil samples. The average recoveries of the compounds were 53–77%. Detection limits are between 5 and 30 ng g−1 based on 5 g moist soil sample. The method was validated by comparing it with conventional liquid-liquid extraction.
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  • 8
    ISSN: 1573-4935
    Keywords: cAMP ; acetylcholine ; morphine ; guinea-pig ileum
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract Neither acute nor prolonged exposure to morphine altered cAMP content or spontaneous movements of longitudinal muscle-myenteric plexus strips of the guinea-pig ileum. By contrast, exogenous acetylcholine or electrical stimulation of the strips elicited both a decrease of cAMP concentration and a twitch response. Atropine blocked the effects of stimulation on these parameters. Addition of morphine to electrically stimulated strips inhibited the twitch response but did not affect cAMP levels. Incubation with morphine led to the development of tolerance to the inhibitory effect on twitch activity and prevented the fall in cAMP normally elicited by electrical stimulation. These results suggest that muscarinic activation is associated with a reduction of cAMP content, an effect which would be impaired in opiate-tolerant tissues.
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  • 9
    ISSN: 1573-4935
    Keywords: somatostatin binding ; gastric mucosa ; alloxan ; diabetes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract Diabetes was induced by administration of alloxan (150 mg/Kg) to 24 h-fasted rabbits. Somatostatinlike immunoreactivity (SLI) and cytosolic binding sites for somatostatin in gastric fundic mucosa were studied using radiolabelled Tyr11-somatostatin. Three months after the onset of the disease, the specific binding of somatostatin to these sites was seen to be significantly lower, due to a reduction in the number (but not the affinity) of specific somatostatin binding sites of high-affinity and a disappearance of the specific, somatostatin binding sites of low-affinity. These changes were associated with an increase in the SLI concentration in both gastric fundic mucosa and plasma.
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  • 10
    ISSN: 1573-4803
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract A procedure was considered for determining theoretical expressions of the area under the first radial distribution function (RDF) peak, and of the relative co-ordination numbers, n ii which depend on the co-ordination hypotheses and on the numbers of bonds between elements of a single type of pair. In this method, two important facts were taken into account. First, the products of atomic factors are functions of s (the scattering vector module) and so they cannot always be considered constant; they were therefore approximated by polynomic functions which best fitted the results obtained from the atomic factors given in international tables. Secondly, consideration of the influence of the structural hypotheses (co-ordinations, existence of certain types of bond) on the area and co-ordination numbers mentioned enabled the most probable local order to be postulated. In order to test the reliability of this method, the method was applied to a set of alloys, quoted in the literature, and the theoretical results obtained agreed very satisfactorily with the experimental values.
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