Chemical lipid extraction or mathematical isotope correction models: should mathematical models be widely applied to marine species?

Abstract

Chemical lipid extractions, as means of standardizing sample preparations, have been identified as important for comparability of studies. Unfortunately, these methods are expensive, because of the costly chemicals and the need to analyse two sets of samples, one for δ(13) C values (treated) and another for δ(15) N values (untreated). To avoid this, studies have suggested mathematical solutions to the problem. Our study intends to (i) determine the applicability of the five most common mathematical correction models and (ii) which of the widely applied chemical extraction methods is the most suitable for a variety of marine organisms. Muscle, heart and liver samples were collected from eight different species. The tissues were treated with Bligh and Dyer, Folch and Soxhlet extraction methods and analysed in a Europa 20-20 mass spectrometer. Predicted lipid-extracted δ(13) C values were calculated from untreated tissue values using the five most common mathematical models. The results indicated that the mathematical methods could not be accurately applied to any of the eight species used in this study, highlighting current issues with accepted isotope methodologies. The Folch chemical extraction removed the highest amount of lipid, suggesting it is the most suitable delipidation method. Analysing two samples, one treated one not, remains the best method to obtain accurate δ(13) C isotope values of muscle tissue. By using this approach every study will obtain two datasets, eventually providing a suitable collective dataset for determining how isotopic signatures are affected by delipidation and potentially producing better mathematical correction models in future.