Diffusion problem

Hi friends

I want to model a fluid (Air) from one side and gases (3 species i.e xenon,radon,helium) to diffuse from the bottom.
the problem is when i select air for the domain for fluid flow and then want to select these gases from Material selection list then Air is overridden by new material. what should i do for selection of material for diffusion ?
is it not need to select material from the material list for diffusion ?