Figure 5 Graphs of Log Retention Time against Inlet/Outlet Pressure Ratio for Compressible and Incompressible Mobile Phases
It should be noted, however, that the effect of increasing the flow rate above the optimum will progressively denigrate the column efficiency, whether the mobile phase is a liquid or a gas.
The compressibility of the mobile phase in GC has interesting implications for the use of pressure or flow programming. The net effect of pressure programming on elution time can be evaluated as follows.
Rearranging
equation (2), 
(4)
Therefore, during a range of values for (g) that occur during a pressure or flow program, over a time interval (Dt), the contribution of the column flow to the retention volume (DVr(0)) will be given by,
(5)
Taking (Dt) as unit time (1 second)
