Role played by basic hydrolysis in the validity of acid herbicide data: Recommendations for future preparation of herbicide performance evaluation standards

Water Supply (WS) 41 herbicide performance evaluation (PE) ampules were des igned by the U.S. Environmental Protection Agency (EPA) to test whether lab oratories were performing necessary basic hydrolysis in their sample prepar ation for acid herbicide analysis. This was accomplished by the inclusion o f the butyl ester as well as the acid form of 2,4-dichlorophenoxy acetic ac id (2,4-D). The results reported for WS 41 did show a significantly lower p ercentage of labs passing the PE acceptance criteria, suggesting that many labs do not hydrolyze before analysis, but were clustered around a higher t han expected value. An in-house study of the WS 41 ampules indicated that t he acid and ester concentrations of WS 41 had changed since the time of man ufacture and helped explain the bias in reported laboratory results. Extend ed storage stability studies of acid-catalyzed methylation and transesterif ication were conducted to investigate the stability of an acid herbicide st andard made to mimic WS 41. These results are presented together with recom mendations for PE sample preparation.