Preparation of fatty acid methyl esters for gas-chromatographic analysis of lipids in biological materials

Theoretically, preparation of fatty acid methyl esters (FAMEs) deals with reversible chemical reactions in a complex system. Methodologically, there are numerous ways, generally characterized by the type of catalysts used and steps involved. Although there are more than a half dozen common catalysts, the majority fall into either acidic (HCl, H₂SO₄ and BF₃) or alkaline types (NaOCH₃, KOH and NaOH), with each having its own catalytic capability and application limitations. In terms of steps, many conventional methods, including those officially recognized, consist of drying, digestion, extraction, purification, alkaline hydrolysis, transmethylation/methylation and postreaction work-up. Although these methods are capable of providing reliable estimates if some precautions are taken, they are cumbersome, time-consuming and cost-inefficient. A new approach has been to transmethylate lipidsin situ. Due to its simplicity, high sensitivity, comparable reliability and capability to determine total fatty acids, the method of direct transmethylation is finding a unique place in lipid determination. Regardless of which method is used, quantitative methylation requires chemists to take precautions at every step involved, particularly during FAME formation and subsequent recovery steps. Evidently, there is an urgent need for more systematic studies, guided by the chemical principle of reactions involved and physicochemical properties of regents and end products, into factors affecting these steps. Hopefully, this will lead to an improved method, which measures lipid composition in biological materials not only with high accuracy but also with high efficiency and minimum costs.