Voltammetrische Bestimmung von Coffein in Kaffee,Tee und coffeinhaltigen Getränken

Zusammenfassung Zur Bestimmung von Coffein in Erfrischungsgetränken (Cola), Kaffee und Tee wird die Differential-Pulsvoltammetrie verwendet. Bei dieser Methode wird Coffein bei pH 1,2 an einer glasartigen Kohlenstoffelektrode oxydiert und quantitativ bestimmt. Die voltammetrisch ermittelten Analysenergebnisse werden mit den Resultaten der Flüssigkeistchromatographie verglichen. Um einen weiteren Einblick in den Reaktionsverlauf zu erhalten, wurde Coffein anodisch bei pH 1,2, 2,3 und pH 6 oxydiert. Die Reaktionsprodukte wurden kathodisch durch Differential-Pulspolarographie identifiziert.

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Voltammetric determination of caffeine in coffee, tea, and soft drinks

Summary Differential pulse voltammetry is used for the determination of caffeine in soft drinks, coffee, and tea. In this method caffeine is oxidised at a glassy carbon electrode at pH 1.2 and determined quantitatively. The voltammetric results are compared with those found by HPLC. To elucidate the mechanism of the electrode processes the reaction products were produced by anodic oxidation of caffeine at pH 1.2, 2.3 und pH 6 and then identified by cathodic differential pulse polarography.

     

Quercetin Prevents UV-lnduced Local Immunosuppression, but Does Not Affect UV-lnduced Tumor Growth in SKH-1 Hairless Mice

Abstract— Ultraviolet is thought to induce skin tumors by its dual activity as a mutagenic agent and a suppressor of cell-mediated immunity. In the present study the effects of quercetin, a flavonoid-containing compound, on carcinogenesis and immunosuppression were studied in SKH hairless mice exposed to suberythemal doses of UV for up to 17 weeks. It was found that quercetin did not affect the onset or growth of non-melanoma skin tumors resulting from UV exposure. In contrast, it prevented the suppression in contact hypersensitivity (CHS) to picryl chloride induced by UV. The mechanism of this prevention might be explained by the observation that the decreased number of epidermal Langerhans'cells is partly prevented by the quercetin. Quercetin did not alter the effects of UV in increasing numbers of spleen and lymph node cells, only partly in decreasing the CD8-positive cells in spleen cell populations and decreasing the lym-phoproliferative response of spleen cells to the mitogens concanavalin A and phytohemagglutinin. Thus oral quercetin did not prevent UV-induced carcinogenesis although it restored the skin-associated CHS response probably by protecting the antigen-presenting cells in the skin.     

Bestimmung von Quecksilber(II) durch „Inverse Differential-Puls-Polarographie“

Zusammenfassung Die inverse Differential-Puls-Polarographie wurde zur Bestimmung kleinster Quecksilbermengen herangezogen. Bei Verwendung einer wachsimprägnierten Graphitelektrode gelang es, 10 ng Hg/ml zu bestimmen. Mit Hilfe einer Glassy Carbon-Elektrode konnte noch 1 ng/ml erfaßt werden.

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Determination of mercury(II) by Differential-Pulse-Anodic-Stripping voltammetry

Summary The Differential Pulse Anodic Stripping Voltammetry was used for the determination of very small amounts of mercury. The application of a wax-impregnated graphite electrode allowed to determine 10 ng Hg/ml. By means of a glassy carbon electrode it was possible to decrease the limit of detection to 1 ng/ml.

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Wir danken dem Fonds zur Förderung der wissenschaftlichen Forschung für die Anschaffung des Gerätes.     

Fusion around the barrier for 7Li+12C

Fusion cross-sections for the ⁷Li + ¹²C reaction have been measured at energies above the Coulomb barrier by the direct detection of evaporation residues. The heavy evaporation residues with energies below 3 MeV could not be separated out from the α-particles in the spectrum and hence their contribution was estimated using statistical model calculations. The present work indicates that suppression of fusion cross-sections due to the breakup of ⁷Li may not be significant for ⁷Li + ¹²C reaction at energies around the barrier.     

An infinite-time relaxation theorem for differential inclusions

The fundamental relaxation result for Lipschitz differential inclusions is the Filippov-Wazewski Relaxation Theorem, which provides approximations of trajectories of a relaxed inclusion on finite intervals. A complementary result is presented, which provides approximations on infinite intervals, but does not guarantee that the approximation and the reference trajectory satisfy the same initial condition.

     

Turing computability with neural nets

This paper shows the existence of a finite neural network, made up of sigmoidal neurons, which simulates a universal Turing machine. It is composed of less than 10⁵ synchronously evolving processors, interconnected linearly. High-order connections are not required.     

Monotone Chemical Reaction Networks

We analyze certain chemical reaction networks and show that every solution converges to some steady state. The reaction kinetics are assumed to be monotone but otherwise arbitrary. When diffusion effects are taken into account, the conclusions remain unchanged. The main tools used in our analysis come from the theory of monotone dynamical systems. We review some of the features of this theory and provide a self-contained proof of a particular attractivity result which is used in proving our main result.