Determination of traces of chloride in solution by microwave-induced plasma emission spectrometry using chlorine generation

The method described for the determination of traces of chloride in solution by microwave-induced argon plasma emission spectrometry is based on the evolution of chlorine from a potassium permanganate solution in sulphuric acid and measurement of the chlorine molecular emission at 257 nm. The detection limit is 10 ng of chloride and the log—log calibration plots are linear over ranges of 0.025–100 μg of chloride, depending on the orientation of the plasma. Potential interfering elements are shown not to present any major problems.     

Molecular dynamics simulation studies of the transport and adsorption of a charged macromolecule onto a charged adsorbent solid surface immersed in an electrolytic solution

Molecular dynamics simulations were performed in order to study the transport and adsorption of a charged macromolecule (desmopressin) onto a charged solid surface in an electrolytic solution. The strong Coulombic interaction from the charged solid surface represents the major force for accelerating, orienting, entrapping in the electrical double layer, and adsorbing the macromolecule onto the charged solid surface. The macromolecule is flattened as it approaches the charged surface, giving rise to a stronger surface exclusion effect that shields surface sites. When adsorbed, the macromolecule is restrained by a surface interaction more than one hundred times stronger than the thermal energy, of which 99.8% results from the strong dominant Coulombic interaction, and trapped by a hydration layer adjacent to the surface. This leads to zero lateral displacement of the adsorbed macromolecule and indicates that surface diffusion is a physically implausible mechanism in similar systems. Explicit solvent is required for realistic representation of the macromolecular structure and the surface interaction energy. The adsorbed macromolecule also decreased the electrostatic potential gradient perpendicular to the charged solid surface and introduced additional electrostatic potential gradients laterally. The results obtained from the molecular dynamics simulations confirm the importance of electrophoretic migration and support the physical mechanisms used in a macroscopic continuum model that predicts an overshoot in the concentration of a charged macromolecule in the adsorbed phase under certain conditions of pH and ionic strength.     

Some cyclic C9H6 isomers and their potential trefoil aromaticity

Cyclonona-1,2,4,5,7,8-hexaene (2) and cyclonona-1,2,4,8-tetraen-6-yne (4) are relatively strain-free according to MINDO/3 and MNDO calculations. The valence isomeric structures (1) and (3) with trefoil aromaticity are less stable by ～100 and 37 kcal mo1^-1 respectively.     

Construction by molecular dynamics modeling and simulations of the porous structures formed by dextran polymer chains attached on the surface of the pores of a base matrix: characterization of porous structures

Significant increases in the separation of bioactive molecules by using ion-exchange chromatography are realized by utilizing porous adsorbent particles in which the affinity group/ligand is linked to the base matrix of the porous particle via a polymeric extender. To study and understand the behavior of such systems, the M3B model is modified and used in molecular dynamics (MD) simulation studies to construct porous dextran layers on the surface of a base matrix, where the dextran polymer chains and the surface are covered by water. Two different porous polymer layers having 25 and 40 monomers per main polymer chain of dextran, respectively, are constructed, and their three-dimensional (3D) porous structures are characterized with respect to porosity, pore size distribution, and number of conducting pathways along the direction of net transport. It is found that the more desirable practical implications with respect to structural properties exhibited by the porous polymer layer having 40 monomers per main polymer chain, are mainly due to the higher flexibility of the polymer chains of this system, especially in the upper region of the porous structure. The characterization and analysis of the porous structures have suggested a useful definition for the physical meaning and implications of the pore connectivity of a real porous medium that is significantly different than the artificial physical meaning associated with the pore connectivity parameter employed in pore network models and whose physical limitations are discussed; furthermore, the methodology developed for the characterization of the three-dimensional structures of real porous media could be used to analyze the experimental data obtained from high-resolution noninvasive three-dimensional methods like high-resolution optical microscopy. The MD modeling and simulations methodology presented here could be used, considering that the type and size of affinity group/ligand as well as the size of the biomolecule to be adsorbed onto the affinity group/ligand are known, to construct different porous dextran layers by varying the length of the polymeric chain of dextran, the number of attachment points to the base matrix, the degree of side branching, and the number of main polymeric chains immobilized per unit surface area of base matrix. After the characterization of the porous structures of the different porous dextran layers is performed, then only a few promising structures would be selected for studying the immobilization of adsorption sites on the pore surfaces and the subsequent adsorption of the bioactive molecules onto the immobilized affinity groups/ligands.     

Computer-aided MS analysis of the first decomposition step of 2,5-diaryl-2H-tetrazoles

The product mixtures formed during the first step of thermal decomposition of some 2,5-diaryl-2H-tetrazoles were investigated by MS analysis after quenching of the mixtures to room temperature and dissolution in methylene chloride. The multivariate evaluation program SEKOS was used. This is based on the computer-aided analysis of intensityvs. time curves of multicomponent mixtures in a mass-spectrometer. The working principles and the advantages of the evaluation program are discussed.

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Zusammenfassung Das bei der thermischen Zersetzung von 2,5-Diaryl-2H-tetrazolen im ersten Schritt gebildete Produktengemisch wurde nach Abschrecken auf Zimmertemperatur und Auflösen in Dichlormethan massenspektrometrisch untersucht. Dazu wurde das multivariate Auswertungsprogramm SEKOS benutzt, das auf einer Computer-gestützten Analyse von Intensität-Zeit-Kurven mehrkomponentiger Mischungen im Massenspektrometer beruht. Das Arbeitsprinzip und die Vorteile des Auswerteprogramms werden erklärt.

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- , 2,5--2-, . SEKOS , — - . .

     

Eine volumetrische Methode zur Bestimmung des Bleis

Ohne Zusammenfassung     

Radiolysis of confined water: molecular hydrogen formation.

The formation of molecular hydrogen in the radiolysis of water confined in nanoscale pores of well-characterised porous silica glasses and mesoporous molecular sieves (MCM-41) is examined. The comparison of dihydrogen formation by irradiation of both materials, dry and hydrated, shows that a large part of the H2 comes from the surface of the material. The radiolytic yields, G(H2)=(3+/-0.5)x10(-7) mol J(-1), calculated using the total energy deposited in the material and the water, are only slightly affected by the degree of hydration of the material and by the pore size. These yields are also not modified by the presence of hydroxyl radical scavengers. This observation proves that the back reaction between H2 and HO(.) is inoperative in such confined environments. Furthermore, the large amount of H2 produced in the presence of different concentrated scavengers of the hydrated electron and its precursor suggests that these two species are far from being the only species responsible for the H2 formation. Our results show that the radiolytic phenomena that occur in water confined in nanoporous silica are dramatically different to those in bulk water, suggesting the need to investigate further the chemical reactivity in this type of environment.