We show that the I10 line in ZnO photoluminescence is associated with Sn impurities. The evidence comes from material implanted with radioactive 117Ag, which decays through 117Cd and 117In to stable 117Sn, and 121Ag which decays through the same chain to stable 121Sb. Supporting evidence is provided by ZnO:Sn prepared by the in-diffusion of stable Sn. The I2 and I9 lines are also shown conclusively to be due to In, confirming earlier identifications in the literature. We also observe shallow bound exciton emission at 3.3643(3) eV due to Sb impurities produced at the end of the decay chain of 121Ag.