Page 115 - phytochemistry general program

Adsorption chromatography .5

The phenomenon of adsorption forms the basis of separation in both liquid solid
chromatography (LSC), and gas solid chromatography (GSC). Adsorption
chromatography is the oldest and most popular of the various chromatographic

types.

In adsorption chromatography, the intermolecular forces between the surface
atoms of a solid and an adjacent molecule, such as water, can lead to its
adsorption and the formation of an adsorbed phase (adsorbed water, in this case).

The intermolecular forces which are normally responsible for adsorption in
chromatographic systems can be classified as follows:

a) Dispersion (London) forces.

b) Dipole (orientation, induction) forces.

c) Hydrogen bonds.

d) Weak covalent bonding (e.g. acid base interaction and complex formation).

Adsorption chromatography has a solid stationary phase and a liquid or gaseous
mobile phase. Each solute has its own equilibrium between adsorption onto the
surface of the solid and solubility in the solvent, the least soluble or best adsorbed
ones travel more slowly. The result is a separation into bands containing different
solutes. The solvent that is put into a column is called the eluent, and the liquid

that flows out of the end of the column is called the eluate.

The process is a competition between the solute and the solvent for the surface
sites. Adsorption chromatography gives best results when applied to certain kinds

of samples, or when a certain kind of separation selectivity is required.

The basis of selectivity in adsorption chromatography can be understood in terms
of the adsorption process. Retention of a solute in LSC requires displacement of

an equivalent number of adsorbed solvent molecules.

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