Separation of dimer acids using enhanced-fluidity liquid chromatography

The polymerization products of C18 fatty acids are complex mixtures of high dimeric, trimeric and higher molecular weight liquid acids. The current separation method for these mixtures involves a lengthy normal-phase HPLC gradient and the use of primary standards that are prepared via a semi-preparative HPLC. In this study, enhanced-fluidity liquid mobile phases, methanol/CO2 and methanol/CHF3, were studied as a possible alternative methodology for the separation of dimer and trimer acids. This study demonstrated that enhanced-fluidity liquid chromatography provides several advantages over normal-phase HPLC with conventional solvents. These advantages included faster speed of analysis, lower consumption of organic solvents, and much improved sample throughput. The best separation of dimer acids was achieved on a silica column using a steep methanol concentration gradient. Methanol/CO2 and methanol/CHF3 mixtures showed comparable elution strength using this steep gradient on the silica stationary phase. However, these polar compounds showed a normal-phase type retention on five reversed-phase columns which indicated an unusual retention mechanism.