Simulation of the thermal transfer during an eutectic melting of a binary solution

The main objective of this paper is to present a model for the heat transfer in the case of the melting of saline binary solution. This model is applied to calorimetry in order to determine the kinetics of the eutectic melting. The investigated cell containing the solution is a cylinder of a few mm³ in volume. By simulation, we could replicate the shape of the experimental thermogramms. The validation of the model permits determining some parameters which are inaccessible due to the small size of the cell, like the space-time evolution of the temperature inside the differential scanning calorimetry (DSC) sample.     

Le dosage de l'oxygene au moyen de particules chargees dans le silicium et le germanium. Mises au point relatives aux reactions parasites et a l'isolement du fluor-18

Résumé Le dosage de l'oxygène par radioactivation avec les particules chargées alphas et hélions-3 a été appliqué au silicium et au germanium. Des précisions ont été apportées aux conditions opératoires en analyse non destructive qui est utilisable à l'heure actuelle en toute sécurité pour le silicium. Une méthode nouvelle d'isolement du fluor-18 est proposée en présence des activités étrangères créées à partir de la matrice dans l'irradiation du germanium.     

Characterisation of hydrolysable tannins from leaves of Betula pubescens by high-performance liquid chromatography-mass spectrometry

A high-performance liquid chromatography-electrospray ionisation mass spectrometry (HPLC-ESI-MS) method, assisted by diode array detection, for the characterisation of individual hydrolysable tannins in birch leaves was developed. With the method, it was found that birch (Betula pubescens) leaves contained an exceptionally complex mixture of hydrolysable tannins; 14 gallotannins and 20 ellagitannins were identified. The developed HPLC-ESI-MS method allows the qualitative and quantitative determination of individual gallotannins and ellagitannins directly from crude birch leaf extract. This is important in studying ecological functions of these phenolic compounds, especially their role in the resistance of birch leaves against insects.     

Theoretical study of the internal elimination reactions of xanthate precursors

The gas‐phase internal elimination (E_i) reaction of ethyl xanthate (CH₃‐CH₂‐S‐CS‐O‐CH₃) has been investigated by means of Hartree–Fock, second‐order Møller–Plesset, and density functional theory (DFT) using the Becke three‐parameter Lee–Yang–Parr (B3LYP) functional and the modified Perdew–Wang one‐parameter model for kinetics (MPW1K). Considerable differences between the ground‐ and transition‐state geometries and the calculated activation energies are observed from one approach to the other, which justifies first a careful calibration of the methods against the results of benchmark CCSD(T) calculations. Compared with these, DFT calculations along with the MPW1K functional are found to be an appropriate choice for describing the E_i reaction of xanthate precursors. The precursor conformation and the transition states involved in the internal conversion of xanthate precursors of cyano derivatives of ethylene, and of cis‐ and trans‐stilbene, are then characterized in detail by means of this functional. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 2023–2031, 2003     

Quantification of docetaxel and its main metabolites in human plasma by liquid chromatography/tandem mass spectrometry

Docetaxel is an antineoplastic agent widely used in therapeutics. The objective of this study was to develop and validate a routine assay, using liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS), for the simultaneous quantification of docetaxel and its main hydroxylated metabolites in human plasma. A structural analogue, paclitaxel, was used as the internal standard. Determination of docetaxel and four metabolites (M1, M2, M3 and M4) was achieved using only 100 microL of plasma. Liquid-liquid extraction was used for sample preparation, with extraction efficiency of at least 90% for all analytes. Detection used positive-mode electrospray ionization in selected reaction monitoring mode. The lower limit of quantification (LLOQ) was 0.5 ng/mL for all analytes. The assay was linear in the calibration curve range 0.5-1000 ng/mL and acceptable precision and accuracy (<15%) were obtained with concentrations above the LLOQ. This method was sufficiently selective and sensitive for quantification of metabolites in plasma from cancer patients receiving docetaxel chemotherapy, and is suitable for routine analyses during pharmacokinetic studies.     

Electric-field-controlled water and ion permeation of a hydrophobic nanopore

The permeation of hydrophobic, cylindrical nanopores by water molecules and ions is investigated under equilibrium and out-of-equilibrium conditions by extensive molecular-dynamics simulations. Neglecting the chemical structure of the confining pore surface, we focus on the effects of pore radius and electric field on permeation. The simulations confirm the intermittent filling of the pore by water, reported earlier under equilibrium conditions for pore radii larger than a critical radius R(c). Below this radius, water can still permeate the pore under the action of a strong electric field generated by an ion concentration imbalance at both ends of the pore embedded in a structureless membrane. The water driven into the channel undergoes considerable electrostriction characterized by a mean density up to twice the bulk density and by a dramatic drop in dielectric permittivity which can be traced back to a considerable distortion of the hydrogen-bond network inside the pore. The free-energy barrier to ion permeation is estimated by a variant of umbrella sampling for Na(+), K(+), Ca(2+), and Cl(-) ions, and correlates well with known solvation free energies in bulk water. Starting from an initial imbalance in ion concentration, equilibrium is gradually restored by successive ion passages through the water-filled pore. At each passage the electric field across the pore drops, reducing the initial electrostriction, until the pore, of radius less than R(c), closes to water and hence to ion transport, thus providing a possible mechanism for voltage-dependent gating of hydrophobic pores.     

Synthesis of (-)-matairesinol, (-)-enterolactone,and (-)-enterodiol from the natural lignan hydroxymatairesinol

[reaction: see text] We describe here a four-step semisynthetic method for the preparation of enantiomerically pure (-)-enterolactone starting from the readily available lignan hydroxymatairesinol from Norway spruce (Picea abies). Hydroxymatairesinol was first hydrogenated to matairesinol. Matairesinol was esterified to afford the matairesinyl 4,4'-bistriflate, which was deoxygenated by palladium-catalyzed reduction to 3,3'-dimethylenterolactone. Demethylation of 3,3'-dimethylenterolactone and reduction with LiAlH(4) yielded (-)-enterolactone and (-)-enterodiol, respectively.     

Synthese d'oligosaccharides sur polymere support—IV : Reactions sur polymere “popcorn”

The different steps leading to oligosaccharide synthesis on polymeric support have been studied in the case of a functionalized ”popcorn” polystyrene: anchoring of the first glucidic group, unblocking of a selectively protected hydroxyl group, glycosylation and cleavage of the glucide support bond. The first glucidic unit is attached by a benzoic ester bond cleaved by methanolysis (Zemplén's method); β-benzoylpropionic ester was used as temporary protecting group. The synthesis of benzyl - 2 - acetamido - 4,6 - di - O - acetyl - 3 - O - (2 - acetamido - 3,4,6 - tri - O - acetyl - 2 - deoxy - β - D - glucopyranosyl) - 2 - deoxy - α - D - glucopyranoside is described as an example.     

Measurement of <Superscript>36</Superscript>Cl in nuclear wastes and effluents: Validation of a radiochemical protocol with an in-house reference sample

Summary ³⁶Cl is a beta-emitter with a very low specific activity. It is produced during the irradiation of nuclear fuel, in the reactor core of power plants, from neutron capture by stable ³⁵Cl that may be present at trace level in any part of the irradiated material. Due to its long half-life (T_1/2 = 3.01 ^. 10⁵ y), ³⁶Cl may be significant in impact assessment studies of disposal sites of nuclear wastes. Considering these different elements, the National Radioactive Waste Management Agency (Andra-France) requests information on the ³⁶Cl content of the waste packages destined to be stored at Andra sites. As for other halogens, the measurement of ³⁶Cl is a difficult analytical task in view of its potential losses during the different chemical steps and also because of the lack of international certified reference material needed to validate the chemical and measurement procedures. This paper describes the methodology processed to constitute an in-house solid reference sample with a known content of stable and radioactive chlorine. The original radiochemistry developed to extract ³⁶Cl from solid samples and purify it before a liquid scintillation counting is explained. The comparison of the results given by this radiochemical protocol and other methods allow its validation. The replication of the measurements on the constituted reference materials gives a repeatability around 8% at a confidence level of 95% that is very close to the calculated combined uncertainty value.     

Investigation of norflurazon pesticide photodegradation using plasma desorption time-of-flight mass spectrometry analysis

We have previously demonstrated that PD-TOFMS (plasma desorption time-of-flight mass spectrometry) analysis is a powerful technique for the in situ analysis of pesticides deposited or adsorbed on solid materials. With the aim of producing reproducible data on the modification of a pesticide under controlled photodegradation conditions, we have now undertaken a study where both the substrate and the pesticide are well characterized. This is the case for norflurazon deposited onto an aluminium substrate, in particular regarding the reproducibility of preparation of the samples and the change with time of their chemical composition. Degradation parameters have been derived from the variation in yield of ions representative of the molecule and of its breakdown products and, particularly, from the time required for 50% dissipation of their initial concentration (DT50). DT50 values ranging between 1 and 10 h have been found. An interpretation of the degradation process is proposed from the decay of other ions. As expected, the degradation is faster when the UV sunlight is unfiltered (a factor of 3.8 for the molecule, and around 5 for the breakdown products).