Free chlorine is widely used as a disinfectant in households and water treatment facilities to inactivate viruses. Accumulated evidence has shown that viruses have a wide range of resistance to free chlorine. However, very little is known about molecular features, particularly viral protein structures, that drive virus inactivation by free chlorine. This project aims to optimize and apply a tandem mass tag (TMT) labeling technique to investigate the reactivities of viral proteins during the free chlorine treatment of viruses.