ANALYSIS OF MONOACYLGLYCEROLS
Nomenclature and structure
Monoacylglycerols (MG) are widely used as emulsifiers mainly in food and cosmetic industries. The presence of a fatty acid at one specific position may also be used to detect a triglyceride and determine its amount. Thus, the determination of 2-monopalmitin by HPLC in infant formula after enzymatic lipase hydrolysis is used to appreciate the amount of triglycerides of human milk origin or of triglycerides of similar structure (palmitic acid in the sn-2 position is poorly hydrolyzed by pancreatic lipase in the intestine and thus is well absorbed) (Lopez-Lopez A et al., J Chromatogr B 2001, 760, 97) . The estimation of the total quantity of MG in lipid extracts can be made by the glycerol or fatty acid quantification. The fatty acid analysis gives also information on the individual molecular species present in the sample. If a previous separation by TLC is made, the estimation of the relative amount of 1- and 2-MG can be made.
Other analytical methods based on HPLC have been reported for separation and quantitative determination of mixtures of MG but a TLC followed by the analysis of fatty acids after transmethylation appears the most simple and suitable procedure in routine work.
A simple method based on solid phase extraction and gas chromatography has been developed for the determination of MG at low levels in dairy products (Fagan P et al., J Chromatogr A 2004, 1054, 251). The proposed method enabled measurement of individual MG, including isomeric forms.
Gas chromatographic retention indices of TMS derivatives of 19 species of saturated and 4 species of unsaturated monoglycerides on a non-polar stationary phase has been reported (Isidorov VA et al., J Chromatogr A 2007, 1166, 207).
An HPLC-based method for direct separation of the regioisomers and enantiomers of MG has been reported (Deng L et al., J Chromatogr A 2007, 1165, 93). The method used a tandem column system (filled with silica gel) and an evaporative light scattering detection system. The direct separation of isomers by enantioselective HPLC was also reported (Deng L et al., J Chromatogr A 2008, 1198-1199, 67).