1. National Health and Environmental Effects Research Laboratory, Toxicity Assessment Division, US Environmental Protection Agency, Research Triangle Park, North Carolina 27709
2. Oak Ridge Institute for Science Education, Oak Ridge, Tennessee 37803
3. National Health and Environmental Effects Research Laboratory, Mid-Continent Ecology Division, US Environmental Protection Agency, Duluth, Minnesota 55804
4. Analytical Chemistry Core, National Health and Environmental Effects Research Laboratory, US Environmental Protection Agency, Research Triangle Park, North Carolina 27709