Dosage du glyphosate par une méthode polarographique

Determination of glyphosate by polarographic method

Laboratoire des Sciences de l'Eau et de l'Environnement Faculté des Sciences de Limoges, 123 Avenue Albert Thomas, 87060 LIMOGES cedex

Résumé

Cette étude a pour objectif de valider une technique de dosage du glyphosate par polarographie. La première partie porte sur l'optimisation de la méthode : durée de dérivation, concentration et calcul de la détection limite. Les résultats montrent qu'une durée de dérivation de deux heures permet d'obtenir un bon compromis entre la rapidité de l'analyse et sa sensibilité. Dans ces conditions, le pourcentage d'erreur est évalué à 3 % et la limite de quantification dans les eaux naturelles est d'environ 40 µg.L^-1 et de 0,09 µg.L^-1 après concentration. L'influence de la Matière Organique Naturelle [Acides Humiques (AH)] a été étudiée : une légère diminution du coefficient directeur des droites d'étalonnage qui s'atténue avec une augmentation du Carbone Organique Dissout est observée. Enfin le produit commercial est utilisé pour déterminer si la présence d'adjuvants de formulation entraîne des modifications lors du dosage : celle-ci diminue légèrement la sensibilité mais aucune variation n'est observée en présence simultanée d'AH.

En conclusion, cette méthode est relativement rapide et fiable puisque deux heures suffisent pour dériver et doser le glyphosate. De plus cette technique permet de doser le glyphosate en présence d'AH et d'adjuvants de formulation. Une étude similaire pour le dosage du métabolite (AMPA) est actuellement en cours.

Abstract

Glyphosate [N-(phosphonomethyl)-glycine] is a broad-spectrum, non-selective, post emergence herbicide that is widely used in agriculture. The commercial success of glyphosate, as a highly effective herbicide, has stimulated several studies on its behaviour and persistence in soil. Microbial degradation of glyphosate produces the major metabolite aminomethyl phosphonic acid (AMPA), and ultimately, leads to the production of water, carbon dioxide and phosphate.

This study aims to validate a technique of determination of the glyphosate by polarography. The method consists in a glyphosate derivation into its derivative N- nitroso then in a dosage by polarography in differential pulse mode. The drop time was I second, the scan rate 50 mV/s and the pulse amplitude 50 mV.

The first part related to the method optimisation : duration of derivation, sample concentration and calculation of limit quantification. The results showed that a duration of two hours derivation made it possible to obtain a good compromise between the analysis and its sensitivity. Under these conditions, the result uncertainty was around to 3% and limit quantification in natural water was approximately 40 µg.L^-1 without sample concentration and 0,09 µg.L^-1 with sample concentration. The concentration was made by evaporation of one litre solution by rotary evaporator.

Then, the method was tested in presence of Natural Organic Matter (Humic Acids (HA)). A slight decrease of the directing coefficient of the calibration lines can be observed. This trend was less marked with an Organic Carbon Dissolution increase. Experiments with HA addition after and before derivation showed that the HA have no influence on derivation but on polarographic measurements.

Finally, the commercial product was tested to determine if the presence of additives of formulation involves modifications of the dosage: it slightly decreased the sensitivity but no variation was observed in simultaneous presence of AH.

Tests on natural water were carried out in the presence of natural and artificial organic matter. Result showed that the mineral substances do not have any effect on the experiment.

In conclusion, this method is relatively fast and reliable since two hours are enough to derive and quantify glyphosate. Moreover, this technique can measure the glyphosate in the presence of HA and additives of formulation. Tests in natural environment in the presence of organic and artificial matter after concentration give gratifying results. A similar study for the proportioning of metabolite (AMPA) is currently in hand.