They did very careful distillation of the water (and confirmed it with mass spectroscopy). Check out thunderf00t's video which is linked elsewhere in this threa

They did very careful distillation of the water (and confirmed it with mass spectroscopy). Check out thunderf00t's video which is linked elsewhere in this thread, it's one of the first things he thought to test (and basically his main contribution to the final results: he spent a month doing pretty painstaking distillation, even after it was already demonstrated not to matter).