Effect of Mobile Phase Compressibility On the HETP Equation for a Packed GC Column

 As the pressure falls along the column length, the velocity changes and, as the solute diffusivity depends on the pressure, the diffusivity of the solute will also change. The multi-path term, which contains no velocity or gas pressure dependent parameters, will be unaffected and the expression that describes it the same. The other terms in the  HETP equation, however, all contain parameters that are affected by gas pressure (solute diffusivity and mobile phase velocity) and, therefore, need to be modified to accommodate the compressibility of the mobile phase.

 Reiterating the HETP equation for a packed column,

     where  f₁(k")  =      and       f₂(k") =