Principles and Practice of Chromatography - Peak Dispersion in a Chromatographic Column > The Efficiency of a TLC Plate > Page 51

The Efficiency of a TLC Plate

TLC plate efficiency is a measure of its capacity to restrain solute dispersion and maintain narrow spots as the solutes migrate along the plate. An explicit equation that describes the dispersion in TLC has not been rigorously developed, nevertheless, high efficiencies are realized in much the same way as they are in GC and LC. Primarily, the particle size of the silica layer must be made as small as possible and the layer must be spread in a thin, homogenous film on the supporting plate. TLC plate efficiency is measured in a similar manner to column efficiency but slightly modified. It is very difficult, if not impossible, to identify the positions of the points of inflexion on a TLC spot, but if the visible edges of the spot are assumed to occur at four standard deviations of the spot distribution, then it is still possible to assess the efficiency. In general it is considered that over 95% of the material in the spot is confined within 4 standard deviations of the spot dispersion.

If the diameter of the spot (d), corresponds to four standard deviations, then applying the same rationale as with the packed column,

where (Zs) is the retention distance of the solute.

Thus,

It shout be pointed out, however, the method contains implicit assumptions that may not necessarily be valid. Besides assuming that the visible limits of the spot correspond to four standard deviations, the basic assumption that the value of (K) is constant throughout the development, is also tacitly made and this is certainly not so. In fact, this procedure would give similar errors to those that would arise from calculating the efficiency of an LC column under conditions of gradient elution. Nevertheless, the method does allow the relative performance of different plates to be accessed and in this way can be helpful.