Kinds of Samples 



The kind of tissue sampled and the sampling unit (i.e., bdividual 

 organisms vs. composites of several organisms) greatly influence the 

 sensitivity, precision, and representativeness of an exposure assess- 

 ment. The issues of composite sampling and sample preparation tech- 

 niques are addressed in the following sections. 



Composite Sampling-An alternative to the analysis of tissue from 

 individual organisms is the analysis of composite samples. Composite 

 tissue sampling consists of mixing tissue samples, each called a sub- 

 sample, from two or more individual organisms typically of a single 

 species collected at a particular site and time period. The mixture is 

 then analyzed as a single sample. The analysis of a composite sample 

 therefore provides an estimate of an average tissue concentration for 

 the individual organisms that make up the composite sample. Com- 

 posite sampling is a cost-effective strategy when the individual chemi- 

 cal analyses are expensive but the cost of collecting individual samples 

 is relatively small. The collection of composite samples is required in 

 cases where the tissue mass of an individual organism is insufficient for 

 the analytical protocol. 



Bioaccumulation surveys designed to support exposure assessments 

 may use a composite sampling strategy. Current risk assessment 

 models used by EPA are based on estimates of long-term average 

 exposure. Estimates of the mean concentrations of contaminants in 

 edible tissue samples from harvested organisms are used as estimates 

 of the exposure concentrations for human consumers of fish and 

 shellfish. Composite sampling of the tissue from selected organisms is 

 a method for preparing a sample that will represent an average con- 

 centration. The collection of replicate composite tissue samples at 

 specified sampling locations will result in a more efficient estimate of 

 the mean (i.e., the variance of the mean obtained with replicate com- 

 posite samples is smaller than that obtained with the collection of 

 replicate samples of individual organisms). 



One major disadvantage of composite sampling is the inability to 

 directly estimate the range and the variance of the underlying popula- 

 tion of individual samples. Such information is extremely useful in 

 bioaccumulation monitoring programs as an early warning signal of 

 increasing levels of contamination. For example, only a few individuals 

 within a sample may contain high contaminant concentrations. Mixing 

 these individuals with less contaminated organisms in a composite 

 sample at a given station may dilute the contaminants and mask a 

 potential problem. In exposure assessment, the patchy distribution of 

 highly contaminated fish or shellfish may indicate the spatial distribu- 

 tion of sources of contaminants. 



The benefits of compositing individual samples from a single station 

 within a given sampling period often outweigh the disadvantages just 

 discussed. In such cases, Rohde (1976) and Tetra Tech (1986b) provide 

 a method for calculating the variance of the underlying population (X) 

 of individual samples when the variance of the composite samples (Z) 

 is known: 



Var X = n (Var Z) (3) 



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