Small correction... 400ppm (parts per million) mean 400 parts / 1 million parts = 400/1000 000 = 0.00004
(mod edit, no it isn't, it's 0.0004)
In percentage that is 0.00004 × 100 = 0.04%
(mod edit, maths wrong again but at least the final percentage is right :)
(mod edit, come on you...