Honeycutt, ME; Mcfarland, VA; Mccant, DD
The role of lipids as the major compartment for neutral organic chemical partitioning in organism tissues has been well documented (Esser 1986; Roberts et al. 1977; Connell 1988; Schneider 1982). Lipid normalization is used in the calculation of accumulation factors (AF), which express the equilibrium distribution of neutral chemicals between sediments and aquatic biota (Ankley et al. 1992; Ferraro et al. 1990, 1991; Lake et al. 1990; McFarland et al. 1994; Young et al. 1991). A screening test used to estimate the bioaccumulation potential of neutral chemicals associated with dredged sediments relies on equilibrium partitioning to organism lipids (USEPA/USACE 1991). Additionally, the USEPA seeks to promulgate sediment quality criteria (SQC) that will require lipid normalization of data (USEPA 1993). Thus, lipid content of aquatic organism tissues is becoming an increasingly important parameter in environmental regulation involving contaminants in sediments.
No standardized method exists for lipid determinations in environmental tissue samples. Typically, analysts either reserve an aliquot of a residue-analysis tissue extract for lipid analysis, or analyze a separate tissue sample for lipids concurrently. in the former case, hexane:acetone or dichloromethane are commonly used as solvents (Ryan et al. 1985; Schwartz et al. 1993). In the latter case, the chloroform:methanol (Bligh-Dyer) method is commonly used, as it is specifically intended for lipid analysis and is routinely used to measure the lipid content of foods (Bligh and Dyer 1959). Often, the amount of tissue used in either of the above cases may vary due to differing amounts of-tissue required (or available) for chemical analysis, or to the amount of sample remaining for lipid analysis after that required for chemical analysis has been taken. Knowledge of the variability that may be introduced due to sample size or solvent used is required in order to compare lipid-normalized data obtained from different studies.
The purpose of this study was to assess these sources of variability by comparing percent lipid determinations made on different sample sizes of the same homogenized fish tissue. Three lipid extraction methods (Bligh-Dyer, hexane:acetone and dichloromethane) and six sample sizes representing a two hundred-fold range of tissue weights were compared. While other extraction methods, e.g., petroleum ether, pentane, acetonitrile, etc., are used to some extent, in our experience the three methods in the present study are the most often used, especially in the regulatory realm.