Determination of total polar compounds (TPC) in oil
The previously reported procedure (Hwang et al., 2022) was followed to determine TPC with some modifications. Oil sample (1.0 g) was dissolved in hexane (2 ml) and added into a glass column filled with a silica gel (about 12 g, 70-230 mesh) slurry in hexane. Ethyl acetate in hexane (3% in volume, 200 ml) was used to elute the non-polar fraction and then ethyl acetate (100 ml) for the polar fraction. After the separation of two fractions was confirmed with thin-layer chromatography (TLC, silica), the solvent in the polar fraction was removed by a rotary evaporator. The polar fraction was dissolved in a small amount of acetone, transferred to a 100 ml round bottom flask, which was kept in a desiccator and weighed beforehand, subjected to a rotary evaporator and then a vacuum pump, and weighed. TPC analysis was conducted in triplicate and reported as the average weight percentage of the polar fraction in the starting oil.