The determination of phenolic anti-oxidants in edible oils and fats by high-performance liquid chromatography

A simple procedure for the extraction of phenolic anti-oxidants with methanol is described. The extracted compounds are determined by gradient elution with retention times of 4.0–13.2 min; an internal standard technique is used for quantification.     

Two shape preserving lagrangeC 2-interpolants

Summary We present a LagrangeC ²-interpolant to scattered convex data which preserves convexity. We also present a LagrangeC ²-interpolant to uniformly spaced monotone data sites which preserves monotonicity. In both cases no further conditions are required on the data values. These interpolants are explicitely described and local. Error isO(h ³) when the function to be interpolated isC ³.     

1-Acetamido-17-carbomethoxydihydrothebainone

Treatment of 1-(5′-benzyloxy-4′-methoxy-2′-nitrobenzyl)-3,4-dihydroisoquinoline 2 with lithium/ammonia followed by the addition of ethanol gave a mixture of the dibenz[b,g]indolizines 3 and 4 . Omission of ethanol resulted in the formation of 3 only. The dihydroisoquinoline from 3,4-dimethoxyphenylethyl amine behaved similarly. Reduction of 2a with sodium/ammonia resulted in cleavage to give the nitro-guaiacol 8. Oxidation of 2a in the presence of potassium t-butoxide gave a complex mixture from which the benzoyldihydroisoquinoline 5 and 2-benzyloxy-5-nitroanisole were isolated in low yield. Treatment of the hexahydrobenzylisoquinoline 10 with diazotized sulfanilic acid, followed by reduction and then acetylation gave a diacetyl compound 12 which was hydrolyzed to 13. Ring closure with boron trifluoride gave the dihydrothebainone 14.     

Entanglement friction and dynamics in blends of starlike and linear polymer molecules

We investigate relaxation dynamics in a series of six‐arm star/linear 1,4‐polybutadiene blends with mechanical rheometry measurements. Blend systems are formulated to systematically probe constraint release and arm relaxation dynamics. Zero shear viscosity and terminal relaxation times of star/linear polymer blends with fixed star arm molecular weights (M_a) and compositions (?_S) are found to follow nonmonotonic dependencies on the linear polymer molecular weight (M_L). At low values of ?_S, at least two scaling regimes are apparent from the data (ξ₀ ～ M and ξ₀ ～ M), where ξ₀ refers to the zero shear viscosity or terminal relaxation time of the blend. The two regimes are separated by a critical linear polymer molecular weight M^* that is more than 20 times larger than the critical molecular weight for entanglements. When the linear polymer contribution to blend properties is removed, a clear transition from dilution dynamics, ξ₀ ～ M, to Rouse‐like constraint‐release dynamics, ξ₀ ～ M, is apparent at low values of ?_S. At higher ?_S values, a new activated constraint‐release dynamic regime is evident in which ξ₀ ～ M and ξ₀ ～ ?, where α changes continuously from approximately 2 to 0.5 as ?_S increases and β varies from 2.0 to 1.0 as M_L increases. The experimental results are compared with theoretical predictions based on a drag coupling model for entangled polymer liquids. All features observed experimentally are captured by this model, including the value of M^* for the transition from dilution to Rouse constraint‐release dynamics. Predictions of the drag coupling model are also compared with published data for the zero shear viscosity and terminal relaxation time in bidisperse linear polymer blends and pure entangled starlike molecules. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2501–2518, 2001     

Effect of topological asymmetry on the electrophoretic mobility of branched DNA structures with and without single-base mismatches

The electrophoretic mobility of three-arm star DNA structures with varying degrees of branch length asymmetry has been investigated in polyacrylamide (PAA) hydrogels. We report the effect of single-base mismatches, adjacent to the branch point, on the mobility of branched DNA with three different arm lengths. Branched DNA structures were formed using wild-type and mutated fragments of the p53 tumor suppressor gene, which is believed to play an important role in cancer development. Branching was directed at the site of several previously characterized mutations in exon 7 of p53. At a given gel concentration, the mobility of branched DNA with fully complementary base pairing is found to increase as the degree of branch length asymmetry is increased. Ferguson analysis of the gel electrophoresis data leads to a retardation coefficient that is strongly dependent on topology. This finding can be explained in terms of a minimum molecular cross-section for each molecule. Specifically, we show that structures with the smallest molecular cross-section can access more pores in the gel, which leads to higher mobility. Our results can also be understood by considering the rotational diffusivity of branched DNA. Asymmetric DNA stars with higher calculated rotational diffusivities also have higher mobilities. When a mutated base is present in junctions with low degrees of branch length asymmetry, adjacent to the branch point, the mobility increases in comparison to the fully complementary molecules. The reason for this increased mobility is unclear, here, we propose that the mismatched base introduces additional flexibility to the arm containing the mutation leading to higher conformational freedom and enhanced mobility in gels. When a mismatched base is present in junctions with high degrees of branch length asymmetry, the opposite result is obtained. Here, the mutated species has a lower mobility. This result is argued to arise from incomplete hybridization and/or frayed ends. Finally, we have shown that by using two of the branch point oligonucleotides as probe molecules, mutations known to occur at specific sites can be detected through the mobility shift. If the sequences of the probe chains are changed in a controlled manner, the location and base of the mutant can also be determined.     

Monte Carlo simulation of structure and nanoscale interactions in polymer nanocomposites

Off-lattice Monte Carlo simulations in the canonical ensemble are used to study polymer-particle interactions in nanocomposite materials. Specifically, nanoscale interactions between long polymer chains (N=550) and strongly adsorbing colloidal particles of comparable size to the polymer coils are quantified and their influence on nanocomposite structure and dynamics investigated. In this work, polymer-particle interactions are computed from the integrated force-distance curve on a pair of particles approaching each other in an isotropic polymer medium. Two distinct contributions to the polymer-particle interaction potential are identified: a damped oscillatory component that is due to chain density fluctuations and a steric repulsive component that arises from polymer confinement between the surfaces of approaching particles. Significantly, in systems where particles are in a dense polymer melt, the latter effect is found to be much stronger than the attractive polymer bridging effect. The polymer-particle interaction potential and the van der Waals potential between particles determine the equilibrium particle structure. Under thermodynamic equilibrium, particle aggregation is observed and there exists a fully developed polymer-particle network at a particle volume fraction of 11.3%. Near-surface polymer chain configurations deduced from our simulations are in good agreement with results from previous simulation studies.     

The preparation, assay and certification of aqueous ethanol reference solutions

Internationally, certified ethanol reference materials are required to calibrate breathalysers and blood-alcohol measurement instruments. The CSIR National Metrology Laboratory of South Africa provides certified aqueous ethanol solutions with traceability to the SI. Ethanol solutions in the concentration range 10 mg/100 g to 20 g/100 g are prepared gravimetrically by mixing ethanol and reagent quality water. To verify the concentration of the ethanol it is oxidized to acetic acid with potassium dichromate in the presence of sulphuric acid. The unreacted potassium dichromate is back-titrated with sodium thiosulphate. The method utilizes gravimetry and titrimetry, which are both primary methods. This paper addresses aspects of ethanol-certified reference materials that have not been previously published: traceability, stability of unpreserved ethanol solutions, homogeneity, quality control measures and the effect of reproducibility on the measurement uncertainty. Presented at -- “BERM-10” -- April 2006, Charleston, SC, USA     

Luminescence in colloidal Mn-doped semiconductor nanocrystals

Recent advances in nanocrystal doping chemistries have substantially broadened the variety of photophysical properties that can be observed in colloidal Mn²⁺-doped semiconductor nanocrystals. A brief overview is provided, focusing on Mn²⁺-doped II–VI semiconductor nanocrystals prepared by direct chemical synthesis and capped with coordinating surface ligands. These Mn²⁺-doped semiconductor nanocrystals are organized into three major groups according to the location of various Mn²⁺-related excited states relative to the energy gap of the host semiconductor nanocrystals. The positioning of these excited states gives rise to three distinct relaxation scenarios following photoexcitation. A brief outlook on future research directions is provided.     

The conditions for quantitative precipitation of phosphate as ammonium 12-molybdophosphate

The radionuclide phosphorus-32 has been usud to measure the effects of (I) temperature, (2) time of standing, (3) stirring, (4) the molybdate-phosphate ratio, (5) hydrochloric, sulphuric, nitric and perchloric acids, and (6) ammonium nitrate on the efficiency of precipitation of ammonium l 2-molybdophosphate,The precipitate is formed quantitatively after 30 min at any temperature between 50° and 80° followed by 30 min at room temperature' provided that the liquid is stirred at 15-min intervals. Twice the stoichiometric amount of nitromolybdate reagent is sufficient for quantitative precipitation. Nitric add is essential ; perchloric acid does not interfere, but hydrochloric and sulphuric acids do. Ammonium nitrate does not affect the efficiency of the precipitation,     

“Fracture” phenomena in shearing flow of viscous liquids

In start-up of steady shearing flow of two viscous unentangled liquids, namely low-molecular-weight polystyrene and -D-glucose, the shear stress catastrophically collapses if the shear rate is raised above a value corresponding to a critical initial shear stress of around 0.1–0.3 MPa. The time dependence of the shear stress during this process is similar for the two liquids, but visualization of samples in situ and after quenching reveals significant differences. For -D-glucose, the stress collapse evidently results from debonding of the sample from the rheometer tool, while in polystyrene, bubbles open up within the sample, as occurs in cavitation. Some similarities are pointed out between these phenomena and that of lubrication failure reported in the tribology literature.