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                       TLC and HPTLC

35 Rf falsified (front)

Rf falsified

Figure 35 : Rf values are most often systematically wrong.
Main reason: the mobile phase evaporates immediately into the gas phase around the plate. Within seconds it adsorbs on the stationary phase when the plate either is in a classical chamber or in any newer including the latest electronically controlled TLC chamber. In case the vapor phase around the plate is not mechanically quite fast circulated, the gas phase composition is non uniform, it has a gradient. As the stationary phase wetting produces in most cases sorption heat, there is a further vapor phase movement normally not uniform over the full gas phase volume. This causes sorption gradients in the stationary phase above the true front line. The moving liquid makes the sorbed liquids pushing forward and soon mimics a combined liquid bed the front. Thus mP - see above - makes the visible front much higher than the correct front line - better to say: front bow.. Some users even overdo the falsification by an extra sorption of phase vapor thus increasing the amount of pre sorbed parts in the layer. They understand the separation process now as running FASTER. But the Rf values are now even more falsified. They can by no means used for identification. There is no way to transform those Rf values CORRECTLY into k-values - see figures 23 to 26, 28 to 34. k-values are fundamental chromatography values. This quality is lost in classical - and modern - chamber techniques.

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