# Plate Theory and Extensions - The Compressibility of the Mobile Phase > Page 26

Integrating
between (P_{i}) and (P_{o})

(25)

Substituting for (P_{r})
in equation (24) from equation (25),

= V_{r(o)}

Thus, the correction factor for retention data in GC is .

This factor
must be used in all retention volume measurements in GC. It should be noted
that the factor depends on the packing being *homogeneous* and the *flow
impedance being constant* along the column. From the inverse relationship _{},
equation (23) can be used to show how the mobile phase velocity changes along a
column for different inlet/outlet pressure ratios.