INTRODUCTION

Chromatographic separations are carried out where interference takes place during the analysis of a mixture of compounds. Separation is performed by passing a mobile phase over the mixture to be resolved put on a stationary phase, until resolution occurs. Partition, adsorption, and absorption are the factors, which influence resolution. Accordingly, chromatography is classified into the following varieties:

Mode of resolution	Technical type	Mobile phase	Stationary phase
Partition	Paper, column (liquid) Gas	Liquid Gas	Liquid Liquid
Adsorption	Column (liquid) Gas	Liquid Gas	Solid Solid
Absorption	Ion-exchange	Liquid	Solid

In partition chromatography, the chromatographic system is composed of a mobile phase and a stationary phase supported by adsorption on an inert support. The more a component of a mixture is soluble in the mobile phase than in the stationary one, the faster its movement in the chromatogram than another one less soluble in the mobile and more soluble in the stationary phase, leading to separation (Fig. 1). The partition (distribution) coefficients of the components of the mixtures between the mobile and stationary phases control separation.

Fig. 1: A schematic diagram of a partition chromatogram .

(extent of solubility is represented by the depth of the component in the phase).

The chromatographic system in the case of adsorption chromatography is composed of a mobile phase and a stationary phase, the adsorbent. The more adsorbed a component of a mixture to the adsorbent, the less its movement in the chromatogram than another one less adsorbed, leading to separation (Fig. 2).

Fig. 2: A schematic diagram of an adsorption chromatogram.

(extent of adsorption is represented by the depth of the component in the adsorbent phase).

Cassidy’s rules of adsorption:

These rules govern the adsorption process, consequently separation:

1. Greater adsorption to a surface occurs from that system which produces the greater lowering of interfacial tension.

2. The extent of adsorption is usually greater from solvents in which the adsorptive is less soluble.

3. The more polar the substance, the more extensive it s adsorption at a polar surface, other things being equal.

4. The less polar a substance, the more extensive its adsorption at a non-polar surface, other things being equal.

5. Extent of adsorption changes in a regular manner along a homologous series.

In ion-exchange chromatography, the system is composed of a mobile phase and a stationary one – the adsorbent (a resin). Separation is achieved by ion-exchange. The exchanged ion is retained by the absorbent while the unexchanged passes through. Chemical affinities of the resin toward the different ions have a rule in separation.