Considerations necessary in gathering occurrence data for selected unstable compounds in the USEPA Unregulated Contaminant Candidate List in USEPA Method 526.

U.S. Environmental Protection Agency (EPA) Method 526 was developed for the analysis of target analytes that are subject to degradation by hydrolysis. Two technical hurdles that had to be overcome were preservation of the target analytes and selection of a suitable solid-phase extraction material. The target analytes were diazinon, disulfoton, fonofos, terbufos, prometon, 1,2-diphenylhydrazine, nitrobenzene, acetochlor, 2,4,6-trichlorophenol, 2,4-dichlorophenol, and cyanazine. Diazolidinyl urea was used for the first time as a microbial inhibitor in an EPA drinking water method. Experiment confirmed antimicrobial agents containing copper or mercury salts increased hydrolysis degradation rates. Trisodium ethylenediaminetetraacetic acid salt was added to chelate metal ions that may increase hydrolysis rates. A pH 7 buffer of tris(hydroxymethyl)aminomethane (Tris) and Tris hydrochloride was used to minimize rates of hydrolysis. The use of ascorbic acid prevented degradation of 2,4-dichlorophenol, terbufos, fonofos, diazinon, and disulfoton due to residual chlorine. Samples were extracted using a styrene divinylbenzene solid-phase material and analyzed by capillary column gas chromatography/mass spectrometry. A 21-day storage stability study, together with precision and accuracy studies, showed that this method has suitable sensitivity, accuracy, precision, and ruggedness for use in the EPA's Unregulated Contaminant Monitoring Rule drinking water occurrence survey.