I stored parthenolide and dehydroleucodine in the freezer at -20C. These compounds were stored as dry powders after pruification from Ethyl Acetae/Hexanes column chromatography. After a year of storage, I tried to redissolve the powder in ethyl acetate. a clear plastic film with an white opaque center precipitated from the solution. It does not dissolve in organic and other solvents (DCM, Hexanes, Ethyl Acetate, ethanol, methanol, DMSO, water). 1. I filtered the precipitate and solvent through a 0.2uM PTFE filter. The clear, plastic film reemerged. 2. I tried to purify on a silica column with ethyl acetete/hexanes. solution was clear initially, but clear plastic film with opaque white center precipitated. 3. I tried dissolving in benzene and adding activated charcoal. This I filtered through Celite. This will slow down the process of forming the precipitate, but some precipitate still forms. Has anyone had sesquiterpene lactones polymerize on them, and what have you done to stop the reaction