The determination of glyphosate in soils with moderate to high clay content

Abstract An improved method for the quantitative analysis of the herbicide glyphosate [N‐(phosphonomethyl)glycine] in soils containing moderate to high clay content is described. Critical evaluations of previously published methods have indicated that recoveries of glyphosate from soils with high clay content are often low. Where acceptable recovery estimates have been reported, these methods also report increased interferences and rarely include soils with clay content exceeding 30%. The proposed method was developed and characterized using six soils of different clay content (25–87% clay), with other physical and chemical properties as described. Recoveries of glyphosate from the soils were determined after duplicate extractions with 0.1 M potassium hydroxide. Clean‐up of soil extract solutions was by cation‐exchange column chromatography. Subsequent quantitation was by HPLC with post‐column oxidation, followed 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 glyphosate kg −1 ) with all recoveries being greater than 80%. Detection limit for glyphosate in soil was 0.04 mg kg −1, and instrument response was linear for solutions up to 50.0μg glyphosate ml −1. Reproducibility relative confidence interval, for a single sample analysis, was determined as.