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Isotope ratio determination in microgram levels of osmium by deuteron activation analysis

We have demonstrated the feasibility of determining isotope ratios in very small samples of osmium by deuteron activation in the Livermore Tandem Van de Graaff accelerator. The method involves determination of cross-section ratios by using several osmium isotope mixtures carefully prepared from electromagnetically-enriched osmium isotopes. The accuracy of this method was checked by a determination of the isotope ratios in naturally-occurring osmium. Our analysis demonstrated that an osmium specimen removed from a sample of commercial grade perrhenic acid contained only 0.4% natural osmium, the rest being radiogenic¹⁸⁷Os.Work performed under the auspices of the U. S. Department of Energy by the Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.     

Restricting the conformational heterogeneity of RNA by specific incorporation of 8-bromoguanosine

In an effort to reduce the conformational heterogeneity of RNA, the modified nucleobase 8-bromoguanosine (8BrG) was introduced into oligonucleotides having the hairpin tetraloop motif YNMG (Y = U or C and M = C or A). Purine nucleobases with bromine at position eight are known to preferentially adopt the syn conformation as nucleosides. The hairpin tetraloop motif YNMG was chosen as a model system because it has a syn guanosine at position four of the loop that is essential for thermodynamic stability. Thermodynamic and structural characterization of modified oligonucleotides with the hairpin sequences UUCG, CGCG, and CGAG by UV-melting and NMR spectroscopy revealed that 8BrG substitution has a small effect upon the hairpin conformation, while the duplex conformation is strongly destabilized (DeltaDeltaG degrees 37 approximately +4.7 kcal mol-1), thus inhibiting dimerization. These results support a model in which 8BrG substitution shifts the hairpin-duplex equilibrium constant toward the hairpin conformation by destabilizing the duplex. This methodology should be useful for limiting conformational heterogeneity in large RNAs, with potential applications in structural biology and enzymology.     

Phase correction in double-pass quasi-phase-matched second-harmonic generation with a wedged crystal

Compensation for dispersive elements is necessary for efficient multiple-pass and intracavity nonlinear frequency-conversion devices. We describe the use of a wedged quasi-phase-matched crystal to compensate for the phase shifts introduced by mirrors in such devices, taking advantage of the periodic variation in the relative phases of the interacting waves in a quasi-phase-matching grating. A representative double-pass second-harmonic generation experiment with a 5-cm -long periodically poled lithium niobate crystal showed the expected conversion efficiency enhancement.     

Articulatory-acoustic kinematics: the production of American English /s/

Due to its aerodynamic, articulatory, and acoustic complexities, the fricative /s/ is known to require high precision in its control, and to be highly resistant to coarticulation. This study documents in detail how jaw, tongue front, tongue back, lips, and the first spectral moment covary during the production of /s/, to establish how coarticulation affects this segment. Data were obtained from 24 speakers in the Wisconsin x-ray microbeam database producing /s/ in prevocalic and pre-obstruent sequences. Analysis of the data showed that certain aspects of jaw and tongue motion had specific kinematic trajectories, regardless of context, and the first spectral moment trajectory corresponded to these in some aspects. In particular contexts, variability due to jaw motion is compensated for by tongue-tip motion and bracing against the palate, to maintain an invariant articulatory-aerodynamic goal, constriction degree. The change in the first spectral moment, which rises to a peak at the midpoint of the fricative, primarily reflects the motion of the jaw. Implications of the results for theories of speech motor control and acoustic-articulatory relations are discussed.     

Analysis of Airborne Particulates by Electron Spectroscopy for Chemical Analysis (ESCA)

Environmental pollution is one of the greatest challenges facing today's technological society. Our knowledge of the elemental composition and toxicology of pollutants increased substantially over the last decade. But efforts aimed at molecular characterization of these pollutants have not kept pace. The knowledge of the exact chemical form of a pollutant is quite important because some elements are much more toxic in one chemical form than in another. Mercury in the form of calomel is used as medication, but other forms of mercury, such as methyl mercury, are highly poisonous.     

Enhancement of the Photoresponse in Organic Field‐Effect Transistors by Incorporating Thin DNA Layers

A mechanistic study of the DNA interfacial layer that enhances the photoresponse in n‐type field‐effect transistors (FET) and lateral photoconductors using a solution‐processed fullerene derivative embedded with disperse‐red dye, namely PCBDR, is reported. Incorporation of the thin DNA layer simultaneously leads to increasing the electron injection from non‐Ohmic contacts into the PCBDR active layer in dark and to increasing the photocurrent under irradiation. Such features lead to the observation of the enhancement of the photoresponsivity in PCBDR FETs up to 10³. Kelvin probe microscopy displays that in the presence of the DNA layer, the surface potential of PCBDR has a greater change in response to irradiation, which is rationalized by a larger number of photoinduced surface carriers. Transient absorption spectroscopy confirms that the increase in photoinduced carriers in PCBDR under irradiation is primarily ascribed to the increase in exciton dissociation rates through the PCBDR/DNA interface and this process can be assisted by the interfacial dipole interaction.     

Reduction of aldehydes and hydrogen cyanide yields in mainstream cigarette smoke using an amine functionalised ion exchange resin

Background

Cigarette smoking is a well recognized cause of diseases such as lung cancer, chronic obstructive pulmonary disease and cardiovascular disease. Of the more than 5000 identified species in cigarette smoke, at least 150 have toxicological activity. For example, formaldehyde and acetaldehyde have been assigned as Group 1 and Group 2B carcinogens by IARC, and hydrogen cyanide has been identified as a respiratory and cardiovascular toxicant. Active carbon has been shown to be an effective material for the physical adsorption of many of the smoke volatile species. However, physical adsorption of acetaldehyde, formaldehyde and also hydrogen cyanide from smoke is less effective using carbon. Alternative methods for the removal of these species from cigarette smoke are therefore of interest. A macroporous, polystyrene based ion-exchange resin (Diaion^®CR20) with surface amine group functionality has been investigated for its ability to react with aldehydes and HCN in an aerosol stream, and thus selectively reduce the yields of these compounds (in particular formaldehyde) in mainstream cigarette smoke.

Results

Resin surface chemistry was characterized using vapour sorption, XPS, TOF-SIMS and ¹⁵N NMR. Diaion^®CR20 was found to have structural characteristics indicating weak physisorption properties, but sufficient surface functionalities to selectively remove aldehydes and HCN from cigarette smoke. Using 60 mg of Diaion^®CR20 in a cigarette cavity filter gave reductions in smoke formaldehyde greater than 50% (estimated to be equivalent to >80% of the formaldehyde present in the smoke vapour phase) independent of a range of flow rates. Substantial removal of HCN (>80%) and acetaldehyde (>60%) was also observed. The performance of Diaion^®CR20 was found to be consistent over a test period of 6 months. The overall adsorption for the majority of smoke compounds measured appeared to follow a pseudo-first order approximation to second order kinetics.

Conclusions

This study has shown that Diaion^®CR20 is a highly selective and efficient adsorbent for formaldehyde, acetaldehyde and HCN in cigarette smoke. The reductions for these compounds were greater than those achieved using an active carbon. The results also demonstrate that chemisorption can be an effective mechanism for the removal of certain vapour phase toxicants from cigarette smoke.

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A simple oxidative procedure for the removal of ruthenium residues from metathesis reaction products

Ruthenium residues can be easily and rapidly removed from Grubbs metathesis products by washing with 15% aqueous hydrogen peroxide, which converts any ruthenium complexes into highly insoluble ruthenium dioxide, which then catalyzes the conversion of excess peroxide into water and oxygen. Ruthenium levels lower than 2 ppm can be routinely obtained; an additional advantage is that any phosphines are also rapidly oxidized to the corresponding, more polar phosphine oxides thereby facilitating their removal as well in many cases.