In a case of suspected poisoning, the preservative used for vitreous during autopsy is:

Fluoride should be added to urine, CSF, and vitreous humour if alcohol estimation is required, and also to samples for analysis for cocaine, cyanide and CO. Sodium or potassium fluoride should be used for preserving blood. Ref: The essentials of forensic medicine and toxicology by Dr K S Narayan Reddy, 27th edition, Page 111.