Trimethylsilyl ether derivatives of sterols were prepared according to ISO 12228: 1999. The final eluted fractions (25 mL)
containing cholesterol and cholestanol were rotary-evaporated to approximately 1 mL under vacuum at 50 C, and then the concentrated solution was transferred to a reaction vial, which was dried by a gentle nitrogen flow, then added with 100 lL N-Methyl-Ntrimethylsilylheptafluorobutyramide: 1-methylimidazole (95:5, v/v). After that, the vial was sealed and heated at 105 C for 15 minutes, and then cooled to room temperature for GC analysis.