A Geometrical Pore Model for Estimating the Microscopical Pore Geometry of Soil with Infiltration Measurements

This paper presents a simple geometrical pore model designed to relate characteristic pore radii of the porous network of soils with macroscopic infiltration parameters. The model composed of a stack of spherical hollow elements is described with two radii values: the pore access radius and the actual pore radius. The model was compared to cylindrical pore models and its mathematical consistency was assessed. Soil sorptivity S and the second parameter A of the Philip infiltration equation (1957), have been determined by numerically simulated infiltration. A diagram and an empirical relation have been set in order to relate the pore access and pore radii to the infiltration parameters S and A. The consistency of the model was validated by comparing the predicted sorptivity and hydraulic conductivity values, with the widely used unsaturated soil hydraulic functions (van Genuchten, 1980). The model showed good agreement with experimental infiltration data, and it is therefore concluded that the use of a model with two radii improves the relation between microscopic pore size and macroscopic infiltration parameters.     

Photochemically-Induced Fluorescence Properties of Two Benzoyl- and Phenylurea Pesticides and Determination in Natural Waters

A photo-induced fluorescence (PIF) method was developed for the determination of two benzoyl- and phenylurea pesticides, namely diflubenzuron (DFB) and fenuron (FEN). The photoconversion under UV irradiation of both pesticides into strongly fluorescent photoproducts was performed in several media (methanol, ethanol, acetonitrile, pH4 aqueous solution and pH4 water–methanol (30:70, v/v) mixture). PIF parameters were optimized. Analytical figures of merit for the PIF determination of DFB and FEN were satisfactory, with rather wide linear dynamic range (LDR) values of one to two orders of magnitude, relatively low limit of detection (LOD) values of, respectively, 9–24 ng/mL for DFB and 1–28 ng/mL for FEN, and limit of quantification (LOQ) values of, respectively, 30–80 ng/mL for DFB and 4–95 ng/mL for FEN, according to the medium. Relative standard deviation (RSD) values were in the range 1.7–5.6 %. PIF was validated by comparing its analytical performances to those of a standard UV absorption spectrophotometric method. The optimized PIF method was applied to the quantitative analysis of both pesticides in various spiked natural water samples collected in a Senegal agricultural area by the standard addition procedure prior to extraction steps in dichloromethane, with satisfactory mean recovery percentage values (97.0–105.3 for DFB and 98.3–102.8 % for FEN). An interference study of foreign species, including pesticides and inorganic ions, likely to be present in natural waters, was also carried out.     

Ultraviolet Photo-Induced Fluorescence Followed by Laser Excitation (UV-PIF-LE) for the Determination of Pesticides in Natural Waters

Abstract

A new method was developed for determining trace amounts of pesticides, using an ultraviolet (UV) lamp to obtain photo-induced fluorescent (PIF) compounds, and then using laser excitation (LE) by a tunable source to excite and simultaneously characterize their fluorescence over a short acquisition time using an intensified charge-coupled device camera. This new UV-PIF-LE method was applied to determine isoproturon, oxadiazon, and fipronil in natural waters. This approach represents an improvement on the previously published direct laser photo-induced fluorescent (DL-PIF) approach that utilized the inclusion of a supplementary UV irradiation device in the experimental setup, permitting the separation of the formation and detection steps for the PIF analytes. This evolution improves the selectivity and increases the sensitivity while maintaining a short analysis time. The UV-PIF-LE method gave very good results with satisfactory analytical performance for the determination of the selected pesticides. This approach also demonstrated good reproducibility, with relative square deviation values between 3.3% and 6.3%, for spiked river water and seawater samples with detection limits in the ng mL^?1 range.     

Study of the Histamine Electrochemical Oxidation Catalyzed by Nickel Sulfate

Thin layer technique was applied to the indirect anodic oxidation of histamine, catalyzed by NiSO₄. Coulometric determination of the electron number demonstrates that histamine complexes Ni^(II) and resulting adducts can be electrochemically oxidized in a single‐electron reaction, to lead Ni^(III)‐histamine. The system Ni^(III)/Ni^(II) acts as ‘redox mediator’ for histamine oxidation. The equilibrium constant of the Ni^(II)‐histamine complex formation was 10⁶ , value showing the strong affinity between species. High content of histamine (>0.1 M), detected in fresh fish (previously exposed to the sun) extracts, showing potentialities for future design of a sensor for direct measurement in fish.