Identification of monoacylglycerol regio-isomers by gas chromatography-mass spectrometry

Monoacylglycerols (MAGs) are lipids found in trace amounts in plants and animal tissues. While they are widely used in various industrial applications, accurate determination of the regio-specific distribution is hindered by the lack of stable, commercially available standards. Indeed, unsaturated beta-MAG (or Sn-2 MAG) readily undergoes isomerization into alpha-MAG (acyl chain is attached to the Sn-1 or the Sn-3 position). In the present study, we describe structural elucidation of alpha- and beta-regio-isomers of monopalmitoyl-glycerol (MAG C16:0) as model compounds in their silylated forms using gas chromatography-mass spectrometry (GC-MS) with electronic impact (EI) ionization. MS fragmentation of alpha-MAG C16:0 is characterized by the loss of methylene(trimethylsilyl)oxonium (103 amu) and the consecutive loss of acyl chain yielding a fragment ion at m/z 205. The fragmentation pattern of beta-MAG C16:0 shows a series of diagnostic fragments at m/z 218, 203, 191 and 103 that are not formed from the alpha-isomer and hereby enable reliable distinction of these regio-isomers. Possible fragmentation scenarios are postulated to explain the formation of these marker ions, which were also applied to characterize the regio-isomer composition of a complex mixture of MAG sample containing n-3 long-chain polyunsaturated fatty acids.