# Dispersion in Chromatography Columns - Effect of Mobile Phase Compressibility On the HETP Equation for a Packed GC Column > Page 39

The resistance
to the mass transfer term for the stationary phase must be considered in
isolation. The experimentally observed plate height (variance per unit length)
resulting from a particular dispersion process [*e.g*., (h_{s}),
the resistance to mass transfer in the stationary phase] will be the sum of the
local plate height contributions (h'); *i.e.*,

_{}

Consequently, substituting for (h') the expression for the resistance to mass transfer in the stationary phase will be,

_{}

or
_{} (24)

Now, it can be shown that, (see equation 23 in Plate Theory and Extensions),