Comparative studies of instrumental and bioassay methods for the analysis of herbicide residues

A study of the quantitative analysis of herbicide residues by both chemical and bioassay methods in soils is presented. Field and laboratory residue trials were carried out with a representative member of the following groups of herbicides: ureas, triazines, diphenylethers, phenoxyacetic acids, and dithiophosphates. Representative samples were taken at different time intervals, and degradation curves were established both by chemical methods and by two types of bioassay. Chemical analysis either separated active ingredient and metabolites by chromatographic techniques or compresied total residues. Bioassays were performed using either monocotyledons and dicotyledons or algae. The results obtained by chemical and bioassay analysis for the degradation rates of chlorotoluron, ametryn, 2,4-D and C 19490 showed a correlation coefficient of 0.914, indicating that the two methods gave almost identical results. Especially with the highly adsorbed urea and triazine herbicides, the uptake of biologically active material by test plants was slightly less than the solvent-extractable parent compound plus its metabolities, and so the absolute level of residues obtained by bioassay was lower. In the case of fluorodifen, the correlation between the methods was not established. The bioassay showed higher residues and slower degradation than chemical analysis. Various factors which could explain this anomalous result are discussed.