# Dispersion in Chromatography Columns - Effect of Mobile Phase Compressibility On the HETP Equation for a Packed GC Column > Page 36

Consider a point (x) along the column,

** **

**
**_{}

** **

The HETP equation, as derived by Van Deemter, is seen to apply only to a particular point distance (x) from the inlet of the column where the pressure is constant.

Now, applying the gas laws

P_{x}u_{x}
= P_{o}u_{o}
or _{}

where (P_{x}) is the pressure at point (x) along the column,

(u_{x}) is the linear velocity of the mobile phase at point (x),