The preadsorbent zone is an inert sample application area along the bottom edge of the plate adjoining the main adsorbent zone. As the mobile phase ascends through the preadsorbent zone, it carries the sample with the solvent front presenting it to the analytical portion of the plate where separation begins. In doing this, the sample is flattened vertically which results in sample components being separated in the form of bands rather than round spots. This results in an increase in apparent resolution between sample components.
The preadsorbent zone also allows more flexibility in the sample application process. Since the samples are un-retained in the preadsorbent zone, the origin effectively becomes the interface or boundary between the adsorbent layer and preadsorbent zone. This means the samples can be applied in a less restrictive fashion saving effort and time.
The preadsorbent layer is usually thicker than the main separating layer which increases the volume of liquid sample that can be easily applied. It is also beneficial in that during the required drying time of the initial samples before development, the samples are sitting on a non-retentive surface, thus mitigating potentially irreversible adsorption effects.