Thelomium-847
Thelomium-847 was a chemical compound with a half-life of one hundred years.
The scientists of Project Proteus used thelomium-847 as a stabilizing agent in the formula they created to enhance a Changeling's mimetic ability. Consequently, trace amounts of the chemical were present in all of the Changelings that accepted these enhancements from Vadic. In 2401, Doctor Beverly Crusher took advantage of this fact to track Vadic and her personnel remotely. (PIC: "Dominion")