Determination of the interchangeable heavy-metal fraction in soils by isotope dilution mass spectrometry

An isotope dilution technique using enriched stable isotopes is applied to determine the interchangeable heavy-metal fraction in soils. Metals in two soil samples are extracted at constant pH, with water, NH₄NO₃, and EDTA. A spike of enriched stable isotopes is added to the suspension of sample and eluant at the beginning of the extraction. The heavy-metal fraction which exchanges with the added spike during the extraction is called the interchangeable fraction. The extractable heavy-metal fractions are obtained from the heavy-metal concentrations in the eluates. Isotope ratios and concentrations are determined by HR-ICP-MS. The isotope dilution technique described enables both the extractable and the interchangeable heavy-metal fractions to be determined in the same experiment. The combination of both results gives additional information on elemental availability under different conditions that cannot be obtained by analyzing the extractable heavy-metal fractions alone. It is demonstrated that in some cases different eluants just shift the distribution of the interchangeable fraction of an element between the solid and liquid phases (e.g., Pb and Cd in a topsoil sample) while the amount of the interchangeable fraction itself remains constant. For other elements, as Ni, Zn, and Cr, the use of different eluants (different pH, complexing agents) sometimes enlarges the interchangeable fraction. Received: 8 December 1998 / Revised: 30 June 1999 / Accepted: 2 July 1999     

Centrifugally spun poly(ethylene oxide) fibers rival the properties of electrospun fibers

Centrifugal force spinning (CFS), also known as centrifugal spinning, forcespinning, or rotary jet spinning, provides considerably higher production rates than electrospinning (ES), but the more widespread use of CFS as an alternative depends on the ability to produce fibers with robust thermal and mechanical properties. Here, we report the CFS of poly(ethylene oxide) (PEO) fibers made using a spinning dope formulated with acetonitrile (AcN) as the volatile solvent, and we describe the thermal and mechanical properties of the centrifugally-spun fibers. Even though the formation, diameter, and morphology of electrospun and centrifugally-spun PEO fibers are relatively well-studied, the article presents three crucial contributions: the pioneering use of PEO solutions in AcN as spinning dope, characterization of crystallinity and mechanical properties of the centrifugally-spun PEO fibers, and a comparison with the corresponding properties of electrospun fibers. We find that fiber formation occurrs for the chosen CFS conditions if polymer concentration exceeds the entanglement concentration, determined from the measured specific viscosity. Most significantly, the centrifugally spun PEO fibers display crystallinity, modulus, elongation-at-break, and fiber diameter that rival the properties of electrospun PEO fibers reported in the literature.     

Synthesis of gem‐Difluoro Olefins through C−H Functionalization and β‐fluoride Elimination Reactions

A palladium catalyzed C?H functionalization and consecutive β‐fluoride elimination reaction between indole heterocycles and fluorinated diazoalkanes is reported. This approach provides for the first time a facile method for the rapid synthesis of gem‐difluoro olefins using fluorinated diazoalkanes under mild reaction conditions. Cyclopropanation products were obtained when N‐arylated rather than N‐alkylated indoles were applied in this reaction. Mechanistic studies reveal the importance of the β‐fluoride elimination step in this transformation. This method presents a new concept for the simple and direct transfer of a 1‐aryl‐(2,2‐difluorovinyl) group to access gem‐difluoro olefins.     

Ultradense Polymer Brushes by Adsorption

Standing room only : Dense polymer brushes can be prepared by adsorbing a diblock copolymer comprising a neutral block and a polyelectrolyte block to an oppositely charged polyelectrolyte brush (see picture). The density of the resulting neutral brush is determined by charge compensation, leading to brush densities well over 1 nm^?2. The diblock copolymer can be desorbed by changing the solution conditions.

     

Light‐Driven Enzymatic Decarboxylation of Fatty Acids

The photoenzymatic decarboxylation of fatty acids to alkanes is proposed as an alternative approach for the synthesis of biodiesel. By using a recently discovered photodecarboxylase from Chlorella variabilis NC64A (CvFAP) we demonstrate the irreversible preparation of alkanes from fatty acids and triglycerides. Several fatty acids and their triglycerides are converted by CvFAP in near‐quantitative yield and exclusive selectivity upon illumination with blue light. Very promising turnover numbers of up to 8000 were achieved in this proof‐of‐concept study.     

Are conformational changes, induced by osmotic pressure variations, the underlying mechanism of controlling the adhesive activity of mussel adhesive proteins?

The mussel adhesive protein Mefp-1, under physiological conditions, presumably has a self-avoiding random walk conformation with helix-like or turned deca-peptide segments. Such a conformation may coil up under osmotic pressure induced by surrounding macromolecules. As a consequence, the orientation of the 3,4-dihydroxy-phenylalanine groups (dopa), essential for the adhesive strength as well as the cohesive strength in Mefp-1, will be altered. Changing the concentration of the protein itself or of different-type surrounding macromolecules may therefore be a tool to control the protein's adhesive activity. The effect of osmotic pressure on the conformation and dopa reactivity of Mefp-1 is studied by the addition of (poly)ethylene oxide (PEO) as a model macromolecule (Mw = 100 kD). From UV-spectroscopy measurements, it can be concluded that dopa reactivity in Mefp-1 changes with increasing PEO concentration. Fitting of the measured absorbance intensity data of the oxidation product dopaquinone versus time with a kinetic model points to the decreased accessibility of dopa groups in the Mefp-1 structure, a faster oxidation, and diminished cross linking under the influence of increasing PEO concentration up to 2.4 g/L, corresponding to an osmotic pressure of approximately 73 Pa. At higher PEO concentrations, the accessibility of the dopa groups for oxidation as well as cross-link formation decreases until about 20% of the dopa groups are oxidized at a PEO concentration of 3.8 g/L, corresponding to an osmotic pressure of approximately 113 Pa. FTIR measurements on the basis of amide I shifts qualitatively point to a transition to a more continuously turned structure of Mefp-1 in the presence of PEO. Therefore, it seems that conformational changes caused by variations of osmotic pressure determine the extent of steric hindrance of the dopa groups and hence the adhesive reactivity of Mefp-1.     

Multi-residue liquid chromatography/tandem mass spectrometric analysis of beta-agonists in urine using molecular imprinted polymers

Ion suppression, a matrix effect that affects quantitative mass spectrometry, is one of the main problems encountered in liquid chromatography/tandem mass spectrometry. Two different clean-up steps for the multi-residue analysis of beta-agonists in urine were evaluated with respect to minimisation of ion suppression, namely, a mixed-phase solid phase extraction (SPE) column, i.e., clean screen Dau (CSD), and a molecular imprinted polymer (MIP) SPE column. Ion suppression experiments revealed that CSD sample clean-up can lead to false negative results for some beta-agonists, and that clean-up using MIP columns is more selective for beta-agonists than the use of CSD columns.