Relevance of the calculation of the diffusion coefficient in a capillary electrophoresis experiment.

This paper analyzes the role of the diffusion coefficient in the movement of analytes that can reversibly react with a selector given a product in the presence of drift. The problem mimics the movement of enantiomers in a capillary electrophoresis experiment. As is well known, the signal in the capillary must be sharp enough to make a good determination of the effective mobility of the analytes being analyzed. The essence of the technique is based on fast interconversion rates. Therefore, the effective diffusion coefficient must be negligible during the experiment. In the present work, an exact expression for both the apparent mobility and the diffusion coefficient is obtained. This is done by writing the rate equations governing the process and solving them using the generating function technique. The effective mobility coincides with the Wren and Rowe equation, whereas the diffusion coefficient allows us to determine the values of the parameters to be taken into account so that this quantity is minimal or close to zero. On the other hand, the numerical solution of the kinetic equations and Monte Carlo simulations allow us to follow the signal in the capillary and to determine its space-time evolution.

Xenon Trapping in Metal-Supported Silica Nanocages.

Xenon (Xe) is a valuable and scarce noble gas used in various applications, including lighting, electronics, and anesthetics, among many others. It is also a volatile byproduct of the nuclear fission of uranium. A novel material architecture consisting of silicate nanocages in contact with a metal surface and an approach for trapping single Xe atoms in these cages is presented. The trapping is done at low Xe pressures and temperatures between 400 and 600 K, and the process is monitored in situ using synchrotron-based ambient pressure X-ray photoelectron spectroscopy. Release of the Xe from the cages occurs only when heating to temperatures above 750 K. A model that explains the experimental trapping kinetics is proposed and tested using Monte Carlo methods. Density functional theory calculations show activation energies for Xe exiting the cages consistent with experiments. This work can have significant implications in various fields, including Xe production, nuclear power, nuclear waste remediation, and nonproliferation of nuclear weapons. The results are also expected to apply to argon, krypton, and radon, opening an even more comprehensive range of applications.

Structural evolution of two-dimensional silicates using a "bond-switching" algorithm.

Silicates are the most abundant materials in the earth's crust. In recent years, two-dimensional (2D) versions of them grown on metal supports (known as bilayer silicates) have allowed their study in detail down to the atomic scale. These structures are self-containing. They are not covalently bound to the metal support but interact with it through van der Waals forces. Like their three-dimensional counterparts, the 2D-silicates can form both crystalline and vitreous structures. Furthermore, the interconversion between vitreous to crystalline structures has been experimentally observed at the nanoscale. While theoretical work has been carried out to try to understand these transformations, a limitation for ab initio methods, and even molecular dynamics methods, is the computational cost of studying large systems and long timescales. In this work, we present a simple and computationally inexpensive approach, that can be used to represent the evolution of bilayer silicates using a bond-switching algorithm. This approach allows reaching equilibrium ring size distributions as a function of a parameter that can be related to the ratio between temperature and the energy required for the bond-switching event. The ring size distributions are compared to experimental data available in the literature.

Preconcentration and post-column fluorescent derivatization for the environmental water monitoring of an antihelmintic macrocyclic drug used in livestock.

In this paper, a green analytical methodology based on fluorescence derivatization is proposed for the anti-helminthic drug monitoring ivermectin as environmental emergent contaminant. After sample clean-up, ivermectin was converted into a highly fluorescent derivative through a catalytic oxidation process followed by dehydration and tautomerization. Under optimal experimental conditions, a linear response was obtained for ivermectin within the range 0.38-600 µg L-1, with detection and quantification limits of 0.11 and 0.38 µg L-1, both values are lower than other previously reported. This method has been applied for ivermectin determination in environmental water samples at trace levels, showing its potential for contamination monitoring.

Caracterización de aceites de girasol con cantidades variables de ácido oleico / Characterization of sunflower'oil with variable quantity of oleic acid

El aceite de girasol se puede obtener a través de semillas de diversas variedades, las cuales presentan distintas características de calidad. Su valor nutritivo y aptitud culinaria se relaciona con la composición de ácidos grasos y la concentración de tocoferoles. Los ácidos grasos instaurados ayudan a prevenir diversas patologías hepáticas biliares, la osteoporosis, enfermedades cardiovasculares y ciertos cánceres en sistemas de tejidos celulares. El contenido en ácidos grasos saturados en el aceite de girasol es bajo, lo cual es deseable para uso alimenticio. Los tocoferoles tienen actividad antioxidante que permite retardar el enranciado del aceite. Son además, precursores de la vitamina E, indispensable para la nutrición humana. Se puede establecer dentro del aceite de girasol, tres grandes grupos de aceites en función de las proporciones de ácido oleico y ácido linoleico: alto oleico, medio oleico y tradicional. En este trabajo se buscó establecer una relación entre el contenido de ácido oleico y los índices utilizados convencionalmente para su identificación. El objetivo fué caracterizar la calidad de aceites de girasol de diferentes niveles de ácido oleico utilizando distintas metodologías de análisis físicos y químicos. Se extrajo con solventes el aceite de semillas de girasol alto oleico, medio oleico y tradicional, provenientes de un solo genotipo de semilla para cada variedad. A partir de los aceites alto oleico y tradicional obtenidos, se realizaron mezclas de diferentes proporciones de cada uno. En todos los casos, se utilizaron aceites sin refinar. Se determinó en primer lugar la composición ácida y contenido de tocoferoles de las muestras y luego los índices de acidez, de refracción, de yodo y punto de humo. Tanto el índice de acidez, el de refracción como el de yodo presentaron buena correlación con el porcentaje de ácido oleico, encontrándose similitud en los valores para aceite medio oleico y la muestra conteniendo el mismo porcentaje de ácido oleico. El punto de humo arrojó valores situados por debajo de los esperados para estos tipos de aceites. En caso de los tocoferoles, no se estableció similitud en su contenido comparando el aceite medio oleico con la mezcla de igual porcentaje de ácido oleico...