Strontium isotope was a material used to develop Sentox nerve gas.
At some point prior to Day 5, Terra-Dyne acquired over a hundred pounds of raw strontium isotope for their production of Sentox for the military. After receiving an encrypted chip from James Nathanson, Chloe O'Brian discovered Terra-Dyne's name and their link to the gas. She passed the information on to Jack Bauer, who investigated Terra-Dyne's operation and uncovered Christopher Henderson's involvement in the Sentox nerve gas conspiracy. ("Day 5: 3:00pm-4:00pm")