Conclusion

In HPLC the analyst carefully selects optimum mobile phase velocity to obtain the lowest HETP and the highest plate count. In TLC the situation is more difficult to handle. Most often, mobile phase velocity is not considered with much attention and this may lead to erroneous conclusions based solely on observed RF values. Adsorption or vaporization can dramatically affect retention and it is of the greatest importance to standardize experimental conditions to get reproducible results. A glance at a TLC figure caption is often deceptive since it is rare that information is gathered on the time taken by the solvent front to ascend the plate, the position of the spot above the solvent level (z0), etc.

Since the mobile phase velocity is not constant the classical HETP equations (van Deemter equation in GC or Knox's equation in HPLC) do not apply. In TLC the HETP equation is of the H versus development length type.

With TLC, low HETP may be generated but the final performances do not compare with HPLC. Nanoplates coated with small particles can provide good performances with short development

20

Figure 14 Variation of plate height with development length. Solute: Desaga yellow (M=225); Dm = 1.37x 10-9m2s-1; Rf = 0.80. Solvent: toluene; 6 = 77. Packing: silica (dp = 11 |im).

Curve 1 Theoretical curve calculated with y = 0.7, A' = 2.48x 10-3, A = 1, B=2.8x10-6, C' = 0.11 x10-4, C = 0.03.

Curve 2 curve calculated by adding a constant equipment contribution to the spot variance (ceq = 4.3 x 10 3 cm2). (Reprinted with permission from Guiochon G and Siouffi A (1978) Journal of Chromatographic Science 16: 470-481.)

distances. The analyst's task is to consider the characteristics of solutes to separate (particularly the diffusion coefficient) and the particle size of the available plates. Then he or she can select the correct plate and the correct development distance to achieve the separation if the plate count is reached under those conditions. It must be remembered that in this article we have only considered the chromatographic experiment. We have not considered extra band broadening from injection (which is important in TLC) or detection devices.

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