Principles and Practice of Chromatography - The Development Process > Elution Development > Page 7

Figure 1. The Elution of a Solute Through a Chromatographic System

A net amount of solute must now leave the stationary phase and enters the mobile phase to re-establish equilibrium. Thus, the solute moves through the chromatographic system as a result of solute entering the mobile phase at the rear of the peak and returning to the stationary phase at the front of the peak. However, that solute is always transferring between the two phases over the whole of the peak in an attempt to attain or maintain thermodynamic equilibrium. Nevertheless, the solute band progresses through the system as a result of a net transfer of solute from the mobile phase to the stationary phase in the front half of the peak. This net transfer of solute is compensated by solute passing from the stationary phase to the mobile phase at the rear half of the peak.

Equilibrium processes between two phases is complicated, but a simplified explanation is as follows. The distribution of kinetic energy of the solute molecules contained in the stationary phase and mobile phase is depicted in Figure 2A and 2B.

Solute molecules leave the stationary phase when their kinetic energy is equal to or greater than the potential energy of their interaction with the stationary phase. The distribution of kinetic energy between the molecules dissolved in the stationary phase at any specific temperature T, can be considered to take the form of a Gaussian curve as shown in Figure 2A.

Figure 2. Energy Distribution of Solute Molecules in the Stationary and Mobile Phase