Utilities are required to measure TOC concentrations in treatment plant influents and waters that contain a variety of dissolved molecules and organic particles. Uncertainties about the analytical accuracy of TOC measurements are the result of inadequate proficiency evaluation standards. The three primary research goals of the project were to: (1) develop total organic carbon (TOC) standards that can be used with all TOC methodologies to evaluate performance; (2) determine the ability of different TOC methodologies to accurately measure TOC; (3) test the ruggedness of the proficiency...