Relative gas-liquid chromatographic retention factors of trimethylsilyl ethers of diradylglycerols on polar capillary columns

Abstract

Gas-liquid chromatography (GLC) on polar capillary columns provides a highly reproducible resolution and quantitation of molecular species of diradylglycerols when analyzed as the trimethylsilyl (TMS) ethers. In the absence of peak collection and determination of fatty acids or mass spectrometry, peak identification is obtained on the basis of relative retention times of reference standards or of relative retention times calculated from the additive contributions of component fatty chains. Unlike simple esters, complex mixtures of diradylglycerols present special problems in GLC peak identification, which must be attended to by auxiliary separations prior to GLC analyiss. In the present study the positional sn-1,2(2,3)- and X-1,3-isomers were resolved by borate thin-layer chromatography (TLC) while the alkenylacyl-, alkylacyl- and diacylglycerols were separated as their TMS ethers by normal-phase high-performance liquid chromatography. The diradylglycerol nature of the sample was further verified by GLC determination of the carbon number distribution, which must be consistent with the composition of the fatty chains of the sample. Under these conditions the identification and quantitation of the molecular species on the polar capillary columns was always consistent with the total fatty acid composition of the sample, as well as with the fatty acid composition of any argentation TLC fractions isolated from some of the samples prior to the polar capillary GLC. Due to the great complexity of the natural diradylglycerol mixtures some peak overlaps occurred, which were reflected in their relative retention times. Nevertheless, a determination of diradylglycerol peak identity from relative retention times proved very satisfactory provided the above described procedures were employed.