TABLE OF CONTENTS Thin-layer chromatography can be defined as a differential migration process where sample components are retained to differing degrees in a thin layer of sorbent as a solvent or solvent mixture moves by capillary action through the layer. The retention of analytes will depend on the interactions that occur with the liquid phase on the surface and contained within the porous physical structure of the sorbent, or there may be a direct interaction with the molecular structure of the sorbent. Also retention will be affected by the nature of the moving solvent and any additives contained in it. The types of interactions involved may be electrostatic, hydrogen-bonding, ion-exchange, size exclusion, and other van der Waals' forces. Sorbents are chosen to be inert to any reaction with the solvent or solvent mixture used for development of the chromatogram (often described as the mobile phase, but strictly the term only applies under development conditions), the solute, or the "liquid" retained in the sorbent (called the stationary phase).
As the environment in the development chamber can constantly change during chromatography due to charging conditions in the sorbent, the migration rate of the solvent front does not occur in a linear fashion. As a general rule the movement of the liquid front as it migrates through the inert sorbent by capillary action is denoted by the following equation:
| Where | Z f is the distance from the immersion line to the solvent front |
| k is... |
