Lipid Extraction and Sample Preservation Techniques for Stable Isotope Analysis and Ecological Assays

Lipid extraction is an important component of many ecological and ecotoxicological measurements. For instance, percent lipid is often used as a measure of body condition, under the assumption that those individuals with higher lipid reserves are healthier. Likewise, lipids are depleted in ¹³C compared with protein, and it is consequently a routine to remove lipids prior to measuring carbon isotopes in ecological studies so that variation in lipid content does not obscure variation in diet. We provide detailed methods for two different protocols for lipid extraction: Soxhlet apparatus and manual distillation. We also provide methods for polar and non-polar solvents. Neutral (non-polar) solvents remove some lipids but few non-lipid compounds whereas polar solvents remove not only most lipids but also many non-lipid compounds. We discuss each of the methods and provide guidelines for best practices. We recommend that for stable isotope analysis, researchers test for a relationship between the change in the carbon stable isotope ratio and the amount of lipid extracted to see if the degree of extraction has an impact on isotope ratios. Stable isotope analysis is widely used by ecologists, and we provide a detailed methodology that minimizes known biases.