# Plate Theory and Extensions - The Compressibility of the Mobile Phase > Page 19

# The Compressibility of the Mobile Phase

By measuring the retention volume of a solute, the distribution coefficient can be obtained. The distribution coefficient, determined over a range of temperatures, is often used to determine the thermodynamic properties of the system; this will be discussed in The Thermodynamics of Chromatography

. Thermodynamic studies are used largely as a diagnostic
tool to investigate the nature of the distribution. Thus, an accurate
measurement of (V'_{r}) can be extremely important and those factors
that reduce the measurement accuracy need to be examined. The retention
volume of a solute (from equation (13)) is given by,

V_{r = }V_{m }+ KV_{S Thus,
}

If the mobile phase is a liquid, and can be considered incompressible, then the volume of the mobile phase eluted between the injection point to the peak maximum will be given by the product of the flow rate and the retention time. If the mobile phase is compressible, however, the volume of mobile phase that passes through the column, measured at the exit, will no longer represent the true retention volume, as the volume flow will increase continuously along the column as the pressure falls. James and Martin [3], derived a correction factor that expressed the true retention volume in terms of the retention volume measured at the column outlet at atmospheric pressure and a function of the inlet/outlet pressure ratio.