Pressure eluent incompressibility

If the mobile phase is a liquid, and can be considered incompressible, then the volume of the mobile phase eluted from the column, between the injection and the peak maximum, can be easily obtained from the product of the flow rate and the retention time. For more precise measurements, the volume of eluent can be directly measured volumetrically by means of a burette or other suitable volume measuring vessel that is placed at the end of the column. 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phase is compressible, however, the volume of mobile phase that passes through the column, measured at the exit, will no longer represent the true retention volume, as the volume flow will increase continuously along the column as the pressure falls. This problem was solved by James and Martin [3], who derived a correction factor that allowed the actual retention volume to be calculated from the retention volume measured at the column outlet at atmospheric pressure, and a function of the inlet/outlet pressure ratio. This correction factor can be derived as follows. 

Valayudhan et al. (37) found that R v linearly increases with the flow rate but Fa shows slight nonlinearity at higher flow rates. This phenomenon is independent of the viscosity of applied eluents (methanol, ethanol, and heptane). The pressure drop linearly increases with the migration distance and the time of development (See Fig. 3). It depends on the viscosity of the eluent, the particle size of the sorbent layer, and the external pressure on the layer surface. Within experimental error their incompressible model is in agreement with experiments, and the velocities of the fronts are (37) ... 

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