Assign mass fraction to a region in the domain for particle tracking
 

Hello everybody,

I am simulating a stirred tank with Impeller using MRF in steday state. After that, I do transient study for injecting massless particles to keep track of flow pathline. At this point, my simulation works fine. BUt I want to go further. Now, I want to define a shperical region with a specific mass fraction assigned to that. (I want the species to be the same as my fluid inside the vessel, because I don't want to make any interaction with the flow, since its not a combustion problem with reacting material, it's all about particle tracking).
I think I must use "patch" to define a spheical region? But I am not sure if the FLUENT has the capability to assign mass fraction to that region...? I noticed that for species definition, Fluent only has some predefined materials, I cannot find mine...
I really appreciate if anyone could help me to better understanding of the capability of FLUENT in this case...
Thanks a ton!
Sahar