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 designed by the U.S. Environmental Protection Agency (EPA) to test whether laboratories were performing necessary basic hydrolysis in their sample preparation for acid herbicide analysis. This was accomplished by the inclusion of the butyl ester as well as the acid form of 2,4-dichlorophenoxy acetic acid (2,4-D). The results reported for WS 41 did show a significantly lower percentage of labs passing the PE acceptance criteria, suggesting that many labs do not hydrolyze before analysis, but were clustered around a higher than expected value. An in-house study of the WS 41 ampules indicated that the acid and ester concentrations of WS 41 had changed since the time of manufacture and helped explain the bias in reported laboratory results. Extended storage stability studies of acid-catalyzed methylation and transesterification were conducted to investigate the stability of an acid herbicide standard made to mimic WS 41. These results are presented together with recommendations for PE sample preparation.