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%.