The H versus L Curve

Collection of experimental data in TLC is much more difficult than in HPLC where a single column can supply much information whereas, the development of many TLC plates is needed for the same purpose.

The relationship between H and L has been checked with small particles carefully sorted; the results are shown in Figure 12. The scattering of experimental points on both sides of the theoretical straight line is the consequence of the lack of reproducibility from one plate to another and mainly from the spot shape. The original spot is too large for its contribution to the final spot width to be neglected. It may be corrected but it must be remembered that a Gaussian profile of the injection spot is almost impossible to achieve. Be that as it may, the H versus L line is well predicted by theory.

We used data from Halpaap and Krebs to check our own results. No information was provided on solutes and particle size but the knowledge that ordinary TLC plates are made from 11-15 |im particles and HPTLC plates from 5-7 |im particles, and that lipophilic dyestuffs are high molecular weight molecules of Dm = 6 x 10~6 cm2 s_1, was used to plot H versus L as shown in Figure 13. The fit is not perfect but the main feature is well predicted by theory.

Brinkman has carried out an experimental verification of our conclusions. He performed TLC of solutes of very different Dm such as chloroanilines and dyes-tuffs. With chloroanilines, he observed a very steep increase in H with development length. To quote Brinkman: 'a similar steep increase is absent in conventional TLC'. With the dyestuffs the difference between the curves is not so large.

As predicted, the efficiency is better with HPTLC plates over a small development length.

In the same way Tyihak found a striking agreement between observed and theoretical curves and checked experimentally that TLC plates are more efficient than HPTLC plates for a 10 cm development distance.

A careless comparison between experimental and theoretical curves could lead to some fallacies. Experimental plots exhibit a minimum plate height, o

Figure 12 Variation of plate height with development length. Mixture of bis chlorophenyl triazene isomers; solvent CCl4. Silica 7 |im particle diameter. ■, RF = 0.28; O, RF = 0.58. (Reprinted with permission from Guiochon G and Siouffi A (1978) Journal of Chromatographic Science 16: 470-481.)

Figure 13 Plots of H versus L from data of Halpaaf H and Krebs KF (1981) JournalChromatography209: 129-147. Dyestuffs: O, violet (Dm = 6x 10"6cm2s"1; RF = 0.75; C = 0.10); □, green (Dm = 6x 10"6cm2s"1; RF = 0.45; C = 0.20); A, blue (Dm = 4x10"6cm2s"1; RF = 0.17; C = 0.20).

Experimental points and curves were calculated with d = 70 cm s"1, y = 0.75, A = 0.80 and dp = 15 ^m for TLC plates. (A) A = 1.0 and dp = 7 ^m for HPTLC plates. (B) Curves: 1, blue; 2, green; 3, violet. The lower curves in (B) correspond to the assumption that ysDs = 0.5 ymDm and all others correspond to ysDs = ymDm. (Reprinted with permission from Journal of Chromatography.)

Figure 13 Plots of H versus L from data of Halpaaf H and Krebs KF (1981) JournalChromatography209: 129-147. Dyestuffs: O, violet (Dm = 6x 10"6cm2s"1; RF = 0.75; C = 0.10); □, green (Dm = 6x 10"6cm2s"1; RF = 0.45; C = 0.20); A, blue (Dm = 4x10"6cm2s"1; RF = 0.17; C = 0.20).

Experimental points and curves were calculated with d = 70 cm s"1, y = 0.75, A = 0.80 and dp = 15 ^m for TLC plates. (A) A = 1.0 and dp = 7 ^m for HPTLC plates. (B) Curves: 1, blue; 2, green; 3, violet. The lower curves in (B) correspond to the assumption that ysDs = 0.5 ymDm and all others correspond to ysDs = ymDm. (Reprinted with permission from Journal of Chromatography.)

which is not predicted by theory, and measured H are higher than calculated. Currently available spectro-photodensitometers are inadequate to detect carefully a spot of very short migration because their response time is too large, the light spot is too large, and the scanning speed too quick. Halpaap and Ripphahn have corrected the data they published. In Figure 14 we assumed an independent contribution to the spot width. Evidence suggests that experimental data cannot fit theoretical calculations with very short development length because of the lack of a special instrument. This misadaptation of detecting device is a further explanation for the poor efficiency often observed.

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