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

                                        Pxux = Pouo  or 

       where (Px) is the pressure at point (x) along the column,

                 (ux) is the linear velocity of the mobile phase at point (x),