Stability against CID was investigated in a vacuum. Specifically, X@Ag54 was introduced into an ESI-mass spectrometer, where it collided with argon (Ar) to undergo CID. The stability of each cluster was examined by evaluating the chemical composition and ion intensity of the generated fragment ions. Mass spectra were obtained following the collision of [S@Ag54 + SO3 t Bu] 2+ and [I@Ag54 + SO3 t Bu + 2AgS] 2+ with Ar at different collision energies (Figure and 5B, Table ).