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selenium stock into beakers containing 0.50 L dilution water. Test solutions were stirred and <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />1 <br /> <br /> <br />transferred to exposure beakers within 30 minutes of preparation. <br />Analytical procedures <br />Dissolved selenium concentrations in the dietary study were measured weekly (four <br />occasions). Concentrations in renewal-acute tests were measured on one occasion at the <br />beginning of the exposure period. Selenium concentrations nearest the median lethal <br />concentration were also measured 24 h after renewal to quantify any change in bioavailability of <br />dissolved selenium. Dissolved selenium concentrations in algae and rotifer batch cultures were <br />measured on one occasion. Preparation ofbatch-culture medium required pasteurization at <br />approximately 70°C for 1 hour to prevent contamination of cultures with undesirable algae or <br />other biological organisms. To determine if pasteurization changed dissolved selenium <br />procedure was completed. <br />concentrations, the highest and lowest concentrations were measured before and after the <br />On each sampling occasion, three 250 ml samples were collected from each exposure <br />concentration. Samples were obtained from different exposure chambers. Samples were placed <br />in acid-washed polyethylene bottles, and held at 4°C until analyzed by Paragon Analytics, Inc. <br />(Fort Collins, Colorado). <br />Selenium concentrations in algae, rotifers, and razorback sucker larvae were also <br /> <br />Razorback sucker larvae were collected at the end of the dietary study. Samples were placed in <br />determined. Duplicate samples of algae and rotifer were collected weekly (four occasions). <br />acid-washed polyethylene vials, and held at -4°C until analyzed at analytical facilities at <br />7 <br />