Equation (23) allows the ratio  to be calculated along the length of the column for different inlet/outlet pressure ratios. Using equation

(23), the values of  at different points along the column were

calculated and the results shown as curves relating  to fraction of the column length are shown in Figure 4.

Figure 4. Graph Showing Change of Pressure Along a GC Column for Different Inlet/Outlet Pressure Ratios

At low inlet/outlet pressure ratios (i.e., g < 2), the pressure falls almost linearly from the inlet to the outlet of the column. At high inlet/outlet pressure ratios, (i.e., g > 2), the relationship is no longer linear and the pressure falls very rapidly in the latter half of the column. It is obvious that the average flow rate can not be used for measuring retention volumes and a correction factor must be employed.