Boundary conditions: Third order or Robin kind

Hi everyone, i want to simulate a mass transfer problem in Matlab. I have a ODE system, but one of the boundary conditions belongs at Robin kind. I dont know how to program this boundary condition, Matlab only let me use first or second order condition. The condition is:

 D*(dS/dz)=k*(Sb-S)

Where D is constant

S is concentration (depend variable)

Z is space variable (independ variable)

k is constant

Sb is concentration that vari with time (not depend of space variable)

[Hooman Ghaaemzadeh]

Did you find how to program boudary condition like this? May you help me

Answer 1

[Walter Roberson]

Possibly this might help: http://www.mathworks.com/help/pde/ug/elliptic-pdes.html