The complexity of the system increases with the number of solvents used and, of course, their relative concentrations. The process can be simplified considerably by pre-conditioning the plate with solvent vapor from the mobile phase before the separation is started. Unfortunately, this only partly reduces the adsorption effect, as the equilibrium between the solvent vapor and the adsorbent surface will not be the same as that between the liquid solvent and the surface. It is clear that by forming a gradient by the frontal analysis of the mobile phase and carefully choosing the solvent mixture, very delicate pseudo-gradients can be created, which, in no small measure, accounts for the great versatility, popularity, and success of TLC.

Chromatography Nomenclature

Chromatography nomenclature has evolved over the years but it was not until the late 1950s that the various terms used for the characteristics of a chromatogram were rationalized.

A summary of the nomenclature is shown diagramatically in figure 4. The various terms are defined as follows.

The baseline is any part of the chromatogram where only mobile phase is emerging from the column.

The peak maximum is the highest point of the peak.

The injection point is that point in time/position when/where the sample is placed on the column.

The dead point is the position of the peak-maximum of an unretained solute.

The dead time (to) is the time elapsed between the injection point and the dead point.

Figure 4. The Nomenclature of a Chromatogram.

The dead volume (Vo) is the volume of mobile phase passed through the column between the injection point and the dead point.

Thus, Vo = Qto where (Q) is the flow rate in ml/min.

The retention time (tr) is the time elapsed between the injection point and the peak maximum. Each solute has a characteristic retention time.

The retention volume (Vr) is the volume of mobile phase passed through the column between the injection point and the peak maximum.

Thus, Vr = Qtr where Q is the flow rate in ml/min.

Each solute will also have a characteristic retention volume.

The corrected retention time (t'r) is the time elapsed between the dead point and the peak maximum.