Plate Concept Applied to TLC

Plate concept and its terms as developed for column chromatography can be applied to thin layer chromatography after slight modifications. As described in the retardation factor R_f is given by the equation R_f = d_s/ d_m. In order to adapt, thin layer chromatography with the plate concept, a chromatogram may be visualized as a plot, shown in Figure, where the spot diameter may be considered as width in Eq. Further, d_s and d_m may be defined in terms of retention time of solute (t_r) and retention time of mobile phase (t_m), respectively.

Figure:  Schematic representation of thin layer chromatogram with plot

For the mobile phase, t_m may be defined as the distance divided by its linear velocity, u

tm    =   d_s /u

The solvent does not reach the similar point until the mobile phase has travelled the distance dm. Therefore,

t_r      =     d_m /u

Substituting a values of t_r and t_m in given equation     

t_r    =  t_m (1 + k′), we get

k′ =   (d_m   - d_s)/ d_s

The capacity factor k′ can also be expressed in terms of the retardation factor R_f as

k′         =          (1 - d_s/d_m) / (d_s/d_m)           =   (1 - R_f)/ R_f

This approach can be used for the method development in column chromatography. It is not easier to calculate capacity factor by thin layer chromatography but it is more rapid than obtaining it from column chromatography experiment. Also, this can be used for calculating the plate height by first determining the number of plates as follows:

N = 16 (d_s / w) ²

Where w is the spot width as shown in Figure. Thus, the plate height H is represented by

H   =   d_m /N