How to get specie mass fraction at a boundary face
 

In Fluent I could use C_YI(c,t,i)to get mass fraction of a cell in a domain,but can I use F_YI(f,t,i)to get a specie mass fraction at a boundary face?

in Fluent User's Guide 29.1.2 Node Values,it reads: Various boundary conditions impose values of field variables at the domain boundaries, so grid node values on these boundary zones are defined explicitly.

but in Fluent User's Guide 5.4 Face Variables it provide the Flow Variable Macros to access flow variables at a boundary face.

when the boundary condition is not imposed by default,can I use F_(f,t,i) to get a specie mass fraction at the boundary?