Microwave Catalytic Esterification Of Fatty Acid Analysis

A new rapid microwave-assisted procedure for the digestion of fresh tissue, transmethylation of lipids, and the extraction of fatty acid methyl esters(FAMEs) in one step is described. The tissues or isolated lipids are irradiated by microwave with mixed acid-catalyzed transesterification reagents, including boron trifluride (BF3)-methanol : benzene : hexane : 2,2-dimethoxypropane (DMP) (37:30:28:5, by vol.) and methanol : benzene : hexane : DMP : H2SO4 (37:30:26:5:2, by vol. ), or mixed base-catalyzed transesterfication reagent of sodium methoxide-methanol : benzene : hexane (30:50:20, by vol.). The simultaneous digestion and lipid transmethylation takes place in a single phase in period of microwave irradiation. This procedure takes just 5 to 10 minutes. After cooling at room temperature two phases were formed; the upper one (hexane phases) contains the FAMEs ready for GLC analysis. The method allows the rapid determination of the fatty acid composition of lipids from plant tissues containing high proportions of triacylglycerols (oil seeds), water (leaves), or both (oil fruits) and dried biological tissues.The optimum conditions for derivatization, such as the derivatization-solvent composition, the time consuming of microwave-irradiation and water content, were studied in detail. The experimental results were compared with those obtained by traditional technique, and the advantages of this new method are obviously of being rapid, efficient, accurate and solvent saving.On the other hand, the strong organic bases are used to transesterify lipids to FAMEs and convert free fatty acids (FFAs) to their salts. The salts are subsequently decomposed at the high temperature of a GC injection port to form their FAMEs. Therefore, the new procedure allows a simple one-step determination of both esterified and FFAs. The tetramthylammonium(TMAH) is widely used in analysis of fatty acids, it, however, makes degradation of polyunsaturated fatty acids (PUFAs). This paper discussed the differences between TMAH and TMSH (trimethylsulforium), which takes advantages over TMAH. Using TMSH, few loss of Eicosapentaenoic (EPA) and Docosahexaenoic (DHA) of fish oil sample was observed, but a great loss of EPA and DHA took place with TMAH.In separation of FAMEs, a non-polar phase (HP-5, capillary column) and two polar phase (HP-20M, capillary column and 10%DEGS, packed column) were carried out. Excellent results have been achieved.The equivalent chain-lengths values (ECL) and gas chromatograhy-mass spectrometry (GC-MS) were utilized to identify the FAMEs. The experimental data of ECL, a kind of special retention index used in analysis of FAs, were similar to the standards value of literature. Being the most efficient method of qualitative analysis, GC-MS was also carried out to identify FAMEs in this paper, and with using ECL and GC-MS together, the majority of FAs, isomer included, were identified.In a word, making use of microwave irradiation, the esterification of fatty acids of sample is rapid and complete and does not result in significant degradation of polyunsaturated fatty acids and loss of short-chain fatty acids.