Dielectrophoretic force on a sphere near a planar boundary

The small gap distance separating a spherical colloidal particle in electrophoretic motion from a planar nonconducting surface is a required parameter for calculating its electrophoretic mobility. In the presence of an externally applied electric field, this gap distance is determined by balancing the van der Waals, electrical double layer interaction, and gravitational forces with a dielectrophoretic (DEP) force. Here, the DEP force was determined analytically by integration of the Maxwell stress over the surface of the particle. The account of this force showed that its previous omission from the analysis always resulted in underpredicted gap distances. Furthermore, the DEP force dominated under conditions of low particle density or high electric field strength and led to much higher gap distances on the order of a few microns. In one particular case, a combination of low particle density and small particle size produced two possible equilibrium gap distances for the particle. However, the particle was unstable in the second equilibrium position when subjected to small perturbations. In general, larger particles had smaller gap sizes. The effects of four other parameters on gap distance were studied, and gap distances were found to increase with lower particle density, higher electric field strength, higher particle and wall zeta potentials, and lower Hamaker constants. Retardation effects on van der Waals attraction were considered.     

Excited-state electron-transfer quenching by a series of water photoreduction mediators

Rate constants k_q for the quenching of the excited state of Ru(bipy)₃²⁺ by a series of viologen salts having different redox potential E_{$\text{1}\text{2}$} have been determined in deaerated aqueous solutions at pH = 5 by laser flash photolysis. The k_q values are found to decrease with increasing —E_{$\text{1}\text{2}$} and to correlate with the reaction free-energy change ΔG. Such a correlation is shown to be consistent with the Rehm—Weller model for electron-transfer reactions.     

Laser-excited ionic fluorescence spectrometry of rare-earth elements in the inductively-coupled plasma

A pulsed tunable dye laser pumped with an excimer laser is used to excite ionic fluorescence of the rare earth elements in the inductively-coupled plasma. Because several fluorescence lines were observed after laser excitation, it was possible to draw partial energy-level diagrams for most of the rare earths. Non-resonance fluorescence lines were used for all measurements in order to minimize spectral interferences. Detection limits at given excitation wavelengths are reported for each element. Laser-excited ionic fluorescence eliminates the problem of spectral interferences which has been associated with the determination of the rare earths by atomic emission spectrometry in the inductively-coupled plasma.     

Hydrogen activation on Mo-based sulfide catalysts,a periodic DFT study

Hydrogen adsorption on Mo[bond]S, Co[bond]Mo[bond]S, and Ni[bond]Mo[bond]S (10 1 macro 0) surfaces has been modeled by means of periodic DFT calculations taking into account the gaseous surrounding of these catalysts in working conditions. On the stable Mo[bond]S surface, only six-fold coordinated Mo cations are present, whereas substitution by Co or Ni leads to the creation of stable coordinatively unsaturated sites. On the stable MoS(2) surface, hydrogen dissociation is always endothermic and presents a high activation barrier. On Co[bond]Mo[bond]S surfaces, the ability to dissociate H(2) depends on the nature of the metal atom and the sulfur coordination environment. As an adsorption center, Co strongly favors molecular hydrogen activation as compared to the Mo atoms. Co also increases the ability of its sulfur atom ligands to bind hydrogen. Investigation of surface acidity using ammonia as a probe molecule confirms the crucial role of sulfur basicity on hydrogen activation on these surfaces. As a result, Co[bond]Mo[bond]S surfaces present Co[bond]S sites for which the dissociation of hydrogen is exothermic and weakly activated. On Ni[bond]Mo[bond]S surfaces, Ni[bond]S pairs are not stable and do not provide for an efficient way for hydrogen activation. These theoretical results are in good agreement with recent experimental studies of H(2)[bond]D(2) exchange reactions.     

Solid-phase synthesis of hydroxysteroid derivatives using the diethylsilyloxy linker

Four different types of hydroxysteroids (primary alcohol, secondary alcohols, and phenol), bearing either an oxirane or an azide as a precursor of molecular diversity, were linked in good yields to solid support using the butyldiethylsilane polystyrene (PS-DES) resin. These molecules were then used as scaffolds to generate hydroxysteroid derivatives containing two levels of diversity. The proposed libraries were tested by running steroidal alcohols through a model sequence of reactions (solid-phase coupling, aminolysis of oxirane or reduction of azide, amidation, and final cleavage). As a result, two linked secondary alcohols (17beta-hydroxy-spiro-3(R)-oxirane-5alpha-androstane and 3beta-hydroxy-spiro- 17(S)-oxirane-5alpha-androstane) and a primary alcohol (spiro-17(S)-oxirane-3-(hydroxymethyl)-1,3,5(10)-estratriene) afforded good overall yields (>45%) and high HPLC purities (>90%) of hydroxysteroids derivatized as alkylamides without purification. One limitation was noted for the fourth library: the phenolic steroid linked by the diethylsilyloxy linker gave a poor overall yield of 8% of the desired model compound. Finally, the diethylsilyloxy linker was used successfully for a rapid solid-phase synthesis of a model library of twenty C19-steroid derivatives (3beta-amido-3alpha-hydroxy-5alpha-androstane-17-ones), with an average yield of 53% and average HPLC purity of 97% without purification steps.     

Convexity preserving interpolation by algebraic curves and surfaces

The problem of interpolation by a convex curve to the vertices of a convex polygon is considered. A natural 1-parameter family ofC algebraic curves solving this problem is presented. This is extended to a solution, of a general Hermite-type problem, in, which the curve also interpolates to one or two prescribedtangents at any desired vertices of the polygon. The construction of these curves is a generalization of well known methods for generatingconic sections. Several properties of this family of algebraic curves are discussed. In addition, the method is generalized to convexC interpolation of strictly convex data sets inR ³ by algebraicsurfaces.     

FDA Quality assurance for radioactivity in foods and radiopharmaceuticals

Summary The Food and Drug Administration (FDA) has regulatory responsibility for radionuclides in foods and radiopharmaceuticals and must maintain an ongoing Quality Assurance Program. These Quality Control Programs involve the U.S. Environmental Protection Agency (EPA) [1, 2] and the National Institute of Standards and Technology (NIST) [3], who supply the necessary reference materials and standards to ensure that the measurements are accurate and reproduceable. Data from EPA (1987 to 1991) and from a NIST Blind Source study (1985 to 1991) show the results are accurate with an average variation from NIST values of 1.8% [4–10]. The use of standard materials with the same matrices as the samples being analyzed provides credibility to the measurements.