Because information on the effects of tissue preparation methods on 

 the results of chemical residue analyses is limited, it is recommended 

 that a pilot survey be performed to establis|i consistent, reliable 

 methods. Relevant protocols for sample storage and preparation are 

 available in a bioaccumulation monitoring guidance document issued 

 by the EPA Section 301(h) (Clean Water Act) program (Tetra Tech 

 1986e) and in the EPA Interim Methods for the Sampling and Analysis 

 of Priority Pollutants in Sediments and Fish Tissue (U.S. EPA 1981). 

 Because many decisions about sample preparation depend on the 

 specific objectives of the study, no single protocol for sample prepara- 

 tion covers all of the possible approaches. For example, samples are 

 usually blotted dry before being weighed to obtain an estimate of wet 

 weight. However, when bivalve molluscs are being prepared for 

 analysis, it may be desirable to retain excess water for later analysis. 



In general, field studies to support exposure assessment should focus 

 on the kind of tissue that is most commonly consumed (e.g., fillet). 

 Analysis of raw edible tissues is recommended to provide data on the 

 concentrations of contaminants initially present in tissues that are 

 normally consumed. Eventually, it may be possible to mathematically 

 account for cooking effects in the exposure assessment. At present, 

 however, data on cooking effects are highly variable. 



Replicated measurements of contaminant concentrations in tissue 

 samples are needed to perform uncertainty analysis (e.g., charac- 

 terizing the precision of the estimates of mean contaminant concentra- 

 tions). Replicated data are also needed for many statistical tests of 

 spatial and temporal trends. Sample replication is recommended here 

 for all bioaccumulation measurements to be used in exposure assess- 

 ments. Guidance on selection of a sample replication scheme is 

 provided in Appendix E. In most cases, at least five replicate samples 

 of individual fish (or shellfish) are required to provide minimal statis- 

 tical power (e.g., ability to discriminate a treatment difference equal 

 to 200 percent of the overall mean among treatments). Increases in 

 sample replication beyond about 10 individual replicates clearly do not 

 provide sufficient benefits in statistical power to justify added costs of 

 sampling and analysis (Appendix E). Greater power can be achieved 

 in a cost-effective manner by composite sampling if information on 

 contamination of individual organisms is not needed (Appendix D). 



Criteria for selection of method detection limits for analytical 

 protocols may be based on risk assessment models explained below 

 (see Risk Characterization). For example, the analytical chemistry 

 methods may be chosen to enable detection of a chemical concentra- 

 tion associated with a specified minimum risk level defined as accept- 

 able by risk managers. Other factors may dictate choice of a lower 

 detection hmit. For example, routine analytical methods may attain 

 much lower Hmits than required by the specified minimum detectable 

 risk level. Also, lower detection limits may be desired if an objective of 

 the study is to develop baselme bioaccumulation data as well as health 

 risk data. In some cases (e.g., 2,3,7,8-tetrachlorodibenzo-/7-dioxin, ben- 

 zidine, dieldrin, N-nitrosodimethylamine), the minimum detection 



Sample Replication 



Selection of Analytical 

 Detection Limits and Protocols 



47 



