Validation of a one-step extraction/methylation method for determination of fatty acids and cholesterol in marine tissues

A simple and fast direct extraction/methylation with methanolic hydrogen chloride was validated for determination of fatty acids (FA) in marine tissues. Three parameters: reaction time, temperature and presence of non-polar solvent, were studied by an experimental 23 full factorial design. The method was validated for five different types of samples; cod liver (high lipid content >60%, mainly triacylglycerol), cod muscle (low lipid content, ≈1%, mainly phospholipids), cod plasma (lipid content, ≈2%, mainly lipoprotein complex, high water amount), cod testis (lipid content ≈3%, high levels of cholesterol), and herring muscle (lipid content ≈7%). The one-step procedure for extraction/methylation of wet tissues was compared with the traditional procedure of extraction of the lipids by the Folch method (chloroform/methanol, 2:1, v/v), followed by methylation. The two methods gave similar FA profiles. The one-step extraction/methylation procedure gave a higher recovery of the total FA than the traditional procedure. Problems with carry-over peaks of cholesterol from previous samples were avoided by application of extra long GC temperature programs. The cholesterol decomposed to some degree under the preceding methanolysis step, giving several peaks in the chromatograms. The decomposition peaks were identified by mass spectrometry as cholestdienes originating from dehydration of cholesterol, a metylether of cholesterol and a cholesteryl chloride. These cholesterol artefacts can be used for quantitative determination of cholesterol in the samples. Standard samples of cholesterol were determined with high accuracy, (R2 > 0.99), and cholesterol in cod plasma was compared with good agreement (R2 = 0.97) to an enzymatic method.