NH4NO3
From Tardis Wiki, the free Doctor Who reference
NH4NO3 was, according to Ian Chesterton, a chemical formula. A vial of whatever compound it denoted was found by Ian and Barbara Wright in Darrius's laboratory on Marinus. (TV: The Keys of Marinus)
Behind the scenes
Though not identified as such on-screen, in the real world, NH4NO3 is "ammonium nitrate."
