Optimization and validation of a method for the determination of caffeine, 8-chlorotheophylline and diphenhydramine by isocratic high-performance liquid chromatography. Stress test for stability evaluation

The optimization of a HPLC method for caffeine, 8-chlorotheophylline and diphenhydramine separation with UV detection at 229 nm is described. The conditions studied included: stationary phase, compositions of mobile phases with pH modulators. Optimal conditions were: SymmetryShield RP8 column and acetonitrile–(0.01 M H₃PO₄–triethylamine, pH 2.8) (22:78, v/v). Validation was performed using standards and a pharmaceutical preparation containing the compounds described above. Results from both standards and samples show suitable validation parameters. The pharmaceutical grade substances were tested by factors that could influence the chemical stability. These reaction mixtures were analyzed to evaluate the capability of the method to separate degradation products. Degradation products did not interfere with the determination of the substances tested by the assay.     

An improved configuration of a moving wire transport detector

An improved design of the transport detector is described that uses a pre-oxidized titanium ribbon as the transport medium. The titanium ribbon has a high loading capacity that permits a large proportion of the total column eluent to be taken into the sensing system. The solute is sensed by pyrolysis and the subsequent detection of pyrolysis products by a miniature argon detector. The pyrolyzer and sensor system is designed to ensure that all the pyrolysis products enter the detector with minimum dilution and band dispersion. As a result, the sensitivity of the detector (or minimum detectable concentration) has been reduced by approximately two orders of magnitude compared with the original design. The sensitivity of the system described to sucrose is 8 x 10(-8) g/mL, which is similar to the sensitivity of the fixed-wavelength UV detector to benzene (approximately 5 x 10(-8) g/mL). It would appear that the new design has potentially a sensitivity at least an order of magnitude lower than that reported here.     

Analytical applications of membrane extraction in chromatography and electrophoresis

An overview of the analytical applications of membrane-based systems for sample enrichment in chromatography and capillary electrophoresis is presented. A brief introduction to the different types of membranes and the main forces related to the transport through them is also given.     

Electrochemical determination of the stability of complexes formed by proton-ionizable ligands of 3,5-disubstituted 1H-pyrazole with phenethylamine

The application of two extreme models for diffusion in two-component systems to electrochemically determine equilibrium constants is discussed. The application of cyclic voltammetric, diferential pulse and rotating-disc electrode voltammetric data to elucidate the stoichiometry and formation constant of complex species by applying a generalization of the molar-ratio method is described. Molar-ratio experiments permit the distinction between the limiting diffusive regimes. The values of the equilibrium stability constants for complexation of phenethylamine and phenethylammonium ions by a 26-membered dioxotetraester crown of 3,5-disubstituted 1H-pyrazole as free ligand 1[L] and as dipyrazolate anion 1'[L(2-)]2Na(+), respectively, were determined.     

Room temperature phosphorescence of 1-naphtalenacetamide included in beta-cyclodextrin in presence of 1,3-dibromopropane

The spectral characteristics of the inclusion of 1-naphthalenacetamide in beta-cyclodextrin have been studied by luminescence methods. The addition of bromoalcohols or bromoalkanes produces a decrease of the fluorescence of the inclusion complex, giving rise to a new band corresponding to phosphorescence emission. The emission intensity is much higher in the presence of bromoalkanes than in the presence of bromoalcohols. The deoxygenation of the solutions is achieved by flowing nitrogen, by the addition of sodium sulphite and by a combination of both methods, which results in the most effective approach. The phosphorescence emission is produced in the turbid suspension formed in the presence of the bromoalkane. The formation of microcrystals seems necessary to obtain phosphorescence emission from the ternary system.     

Scattering from particles on surfaces: visibility factor and polydispersity

We have developed an experimental method based on the visibility factor of light-scattering minima to obtain size-polydispersity information from contaminants upon a flat substrate. We verify the method by using double-interaction-model calculations and use this technique to examine experimentally the radial variation of a micrometer-sized fiber and the size polydispersity of spherical particles upon a substrate.     

Pulsed-field gradient nuclear magnetic resonance study of transport properties of fluid catalytic cracking catalysts

Pulsed-field gradient nuclear magnetic resonance (PFG NMR) has been applied to study molecular diffusion in industrial fluid catalytic cracking (FCC) catalysts and in USY zeolite for a broad range of molecular displacements and temperatures. The results of this study have been used to elucidate the relevance of molecular transport on various displacements for the rate of molecular exchange between catalyst particles and their surroundings. It turned out that this rate, which may determine the overall rate and selectivity of FCC process, is primarily related to the diffusion mode associated with displacements larger than the size of zeolite crystals located in the particles but smaller than the size of the particles. This conclusion has been confirmed by comparative studies of the catalytic performance of different FCC catalysts.     

Characterization of a 20 kHz sonoreactor. Part II: analysis of chemical effects by classical and electrochemical methods

A new electrochemical redox probe has been investigated in order to characterize the local production of radicals during the cavitation events. The results have been compared with those obtained with Fe(CN)6(3-)/Fe(CN)6(4-) (electrochemical probe for local mechanical effects) and classical chemical methods such as iodide and Fricke dosimeters (chemical probes for global effects).     

Design of pi-conjugated organic materials for one-dimensional energy transport in nanochannels

Various end-substituted distyrylbenzenes have been synthesized to serve as guest molecules in inclusion compounds to promote efficient energy transport along one-dimensional channels. Their optical and photophysical properties have been characterized at both experimental and theoretical levels. All molecules display a large transition dipole moment between the ground state and lowest excited state and hence a short radiative lifetime (on the order of 1-2 ns). They also exhibit a large spectral overlap between the emission and absorption spectra, which enables efficient energy transport between molecules arranged in a head-to-tail configuration in nanochannels. Hopping rates on the order of 10(12) s(-1) are calculated at a full quantum-chemical level; this is much larger than the radiative lifetimes and opens the way for energy migration over large distances. Changes in the nature of the terminal substituents are found to modulate the optical properties weakly but to impact significantly the energy transfer rates.     

Nanostructured Ti-W mixed-metal oxides: structural and electronic properties

In this article, the structural and electronic properties of Ti-W binary mixed oxide nanoparticles are investigated by using X-ray diffraction, Raman, X-ray absorption spectroscopies (XAS; near edge XANES and extended EXAFS), UV-vis spectroscopy, and density functional calculations. A series of Ti-W binary oxide samples having W content below 20 atom % and with particle size between 8 and 13 nm were prepared by a microemulsion method. The atoms in these nanoparticles adopted the anatase-type structure with a/b lattice constants rather similar to those of the single TiO(2) reference and with a c cell parameter showing a noticeable expansion upon doping. Within the anatase structure, W occupies substitutional positions, while Ti atoms only suffer minor structural perturbations. A change in the W local order at first neighboring distance is observed when comparing samples having a W content below and above 15 atom %. Charge neutrality is mostly achieved by formation of cation vacancies located at the first cation distance of W centers. Upon addition of W to the TiO(2) structure, the Ti charge is not strongly modified, while changes in the W-O interaction appear to drive a modest modification of the W d-electron density throughout the Ti-W series. A combination of these geometrical and electronic effects produced Ti K- and W L(I)/L(III)-edge XANES/EXAFS spectra with distinctive features. UV-vis spectra show a nonlinear decrease of the band gap in the Ti-W solid solutions with a characteristic turning point at a W content of ca. 15 atom %. The relationship between local/long-range order and electronic parameters is discussed on the basis of these experimental results.