NORMAL-PHASE SEPARATION EFFECTS WITH LIPIDS ON A SILVER ION HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY COLUMN

The separation of saturated fatty acids (as the methyl esters and as t riacylglycerols) of differing chain lengths by silver ion chromatograp hy (ChromSpher Lipids; containing silver ions; Chrompack, Middelburg, Netherlands) was studied. The retention characteristics (isocratic sol vent system of acetonitrile in hexane) of saturated fatty acid methyl esters and homogeneous, saturated triacylglycerols were found to be in versely related to the number of carbon atoms in the fatty acid(s). Th us methyl stearate eluted first and methyl acetate eluted last; for tr iacylglycerols, tripalmitoylglycerol eluted first while triacetin was strongly retained (11 min vs. 70 min). While mixtures of fatty acid me thyl esters and of triacylglycerols could be separated, resolution was found to improve at higher concentrations of acetonitrile, a characte ristic of normal-phase separations. In the absence of carbon-carbon do uble or triple bonds, the effects of the more polar silanol groups of the column packing tended to override the silver ion contributions (in teraction of silver ions with the unpaired electrons of the carbonyl o xygens), resulting in separations similar to those observed in silica thin layer and normal-phase high-performance liquid chromatography.