The over pressured layer separation chamber eliminates the vapor phase above the layer by contacting the layer surface with a plastic membrane pressurized on its opposite surface by hydraulic pressure of up to 50 bar. The mobile phase is then forced through the layer at a constant velocity by a mechanical pump.On high performance thin-layer plates a minimum plate height of about 10–15 m at an optimum linear velocity of 0.4–0.6 mm/s can be obtained [35,38,39]. The plate height is reasonably constant andindependent of the migration distance, favoring the use of layers ofa smaller particle diameter for difficult separations. A typical separation is shown in Fig. 10. Precoated thin-layer plates are usedfor the separation but require additional treatment to seal the edges