I'm wondering if any expert can help me out on this, but recently I started a project involving extracting cortisol in water samples using SPE, basically running sample through C18 waters column to concentrate the cortisol, eluted it out using ethyl acetate then evaporate the solvent in Speedvac. However, after 30 min or so there's unexpected white precipitate in the microcentrifuge tubes containing my cortisol sample, and it won't dissolve in the ELISA buffer (Cayman chemicals, they won't tell what's in the buffer, but my best guess would be some harmless salt). It's unlikely to be the ethyl acetate eating the plastics since the tubes containing only ethyl acetate evaporated just fine based on my trial. Any ideas? Thanks for your help and any input would be appreciated!