Use of microwave irradiation for rapid transesterification of lipids and accelerated synthesis of fatty acyl pyrrolidides for analysis by gas chromatography-mass spectrometry: study of fatty acid profiles of olive oil, evening primrose oil, fish oils and phospholipids from mango pulp

Abstract

Transesterification of lipids using 14% boron trifluoride in methanol is widely used in lipid biochemistry to study fatty acid profiles of various lipids by gas chromatography-mass spectrometry (GC/MS). Synthesis of fatty acyl pyrrolidides from fatty acid methyl esters is also important in GC/MS analysis for unambiguous positional assignment of double bonds in unsaturated fatty acids. Under conventional heating (boiling water bath), transesterification step requires 10 min for phospholipids and 40 min for triglycerides. However under low power (60 W) microwave irradiation, transesterification of phospholipids and triglycerides can be achieved in 50 s and 60 s, respectively. The products (fatty acid methyl esters) formed under microwave irradiation were identical to the products formed under conventional reaction conditions as evidenced by similar mass spectral fragmentation patterns and gas chromatographic retention times. Similar acceleration of reaction rate was also observed during the synthesis of fatty acyl pyrrolidides from fatty acid methyl esters using pyrrolidine and glacial acetic acid under microwave irradiation. This reaction which requires 1 h of heating at 100°C in an oil bath by the conventional method is completed with slightly better yields in 12 min under microwave irradiation. Therefore, microwave acceleration of these widely used derivatization procedures, which are essential pre-steps for gas chromatography-mass spectrometry, is most appropriate for the rapid analysis of labile samples such as free unsaturated fatty acids or unsaturated fatty acid containing phospholipids and triglycerides.