Browsing by Author "Silva, Francisco A."
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- An analytical solution for the analysis of zero-length-column experiments with heat effectsPublication . Silva, José A.C.; Silva, Francisco A.; Rodrigues, AlírioAn analytical solution for the analysis of zero-length-column (ZLC) experiments with heat effects is developed. The model is an extension of the original one developed by Eic and Ruthven with the inclusion of the energy balance. Two additional parameters are obtained, beta = (DeltaH/C-p) (partial derivativeq/partial derivativeT)\ (c0),(T0) and alpha = (ha/C-p)(r(c)(2)/D-c). A criterion for negligible heat effects, 3L beta/alpha < 0.1, is derived from the analytical solution based on ZLC operating parameters. ZLC desorption curves in nonisothermal operation are discussed. The model reduces to the original solution of isothermal operation developed by Eic and Ruthven when heat effects are negligible. ZLC experiments with heat effects are analyzed, and trends are in good agreement with theory. Because of its simplicity, the model is a valuable tool for the analysis of ZLC experiments with heat effects.
- NMR studies on the antiradical mechanism of phenolic compounds towards 2,2-diphenil-1-pycridrazyl radicalPublication . Silva, Artur; Santos, Clementina M.M.; Cavaleiro, José; Tavares, Hilário; Borges, Fernanda; Silva, Francisco A.Phenolic compounds are the most abundant c1ass of natural antioxidants. These types of compounds are ubiquitous in fruits and vegetables and in plant-derived beverages, such tea and wine, being important constituents of human diet. Recent studies have pointed out particular interest on phenolic type compounds (e.g. flavonoids, catechins, cinnamic and benzoic acid derivatives) with respect not on1y to the organoleptic characteristics of foodstuffs (e.g. flavour, bitterness, astringency) but also to their potential benefits in deleterious oxidative radicalar processes related with human disease (e.g. cancer, atherosc1erosis).l,2 The antioxidant activity of phenolic compounds could be related with their antiradicalar activity and/or with the ability to act as metal ions chelators. Although there is general agreement that catecholic compounds possess radical scavenging properties till now its mechanism of action is not fully understood. The. determination of antiradicalar potency of antioxidants is usually performed using "Trap Assays" from which a screening of their scavenging activity towards different radical species could be obtained. The DPPH method is a non-enzymatic assay widely used for this purpose, in which the reactivity of the tested compounds towards a stable free radicaI2,2-diphenyl-l-picrylhydrazyl (DPPHe ) is measured.3 Following our interest on the antioxidant behaviour of phenolic compounds4 ,5 the mechanism of the antiradicalar activity of ortho-dihydroxy cinnamic acids and 2-styry1chromone derivatives towards DPPHe was studied by NMR spectroscopy.