THE DETERMINATION OF GLYPHOSATE IN SOILS WITH MODERATE TO HIGH CLAY CONTENT

An improved method for the,quantitative analysis of the herbicide glyp hosate [N-(phosphonomethyl)glycine] in soils containing moderate to hi gh clay content is described. Critical evaluations of previously publi shed methods have indicated that recoveries of glyphosate from soils w ith high clay content are often low. Where acceptable recovery estimat es have been reported, these methods also report increased interferenc es and rarely include soils with clay content exceeding 30%. The propo sed method was developed and characterized using six soils of differen t clay content (25-87% clay), with other physical and chemical propert ies as described. Recoveries of glyphosate from the soils were determi ned after duplicate extractions with 0.1 m potassium hydroxide. Clean- up of soil extract solutions was by cation-exchange column chromatogra phy. Subsequent quantitation was by HPLC with post-column oxidation, f ollowed by derivatization using OPA-MERC with fluorometric detection. No interferences were detected. Recovery estimates for each fortified sample were determined over a concentration range (0.56-11.25 mg glyph osate kg-1) with all recoveries being greater than 80%. Detection limi t for glyphosate in soil was 0.04 mg kg-1, and instrument response was linear for solutions up to 50.0 mug glyphosate ml-1. Reproducibility relative confidence interval, for a single sample analysis, was determ ined as +/- 11.6%.