Lets have a look onto the planar chromatogram left: It shows the beauty of a visualization by fluorescence. It shows the power of spectral information. But in case of wider ranging concentrations one will immediately reach limits especially when these spot lines even only partially overlap.
There is of course full spectral signal power available. There is time to enhance even very weak peak signals which is not available with elution chromatography as in HPLC or with forced flow HPTLC.

Figure 1:

The main error source for a false
Rf value = s / f. Reason: the mobile phase compresses already sorbed mobile phase traces in the layer which results in a wrong upper limit of the mobile Phase front “mP”. The more volatile the mobile phase or the more saturated the linear HPTLC chamber gas volume is, the more falsified is the visible position of the mP front line. What can help: adding a test substance with NO any sorption power in the stationary phase. Optimal is a sprayed line of such a substance solution exactly positioned onto the starting line for the samples. Text error: lokking = looking

Figure 2:

HPTLC can be understood as an elution chromatography procedure based on time: residence time in the mobile phase = tm and residence time in BOTH phases = tms.

If the mobile phase would be evaporated at the “end line” E, the mobile phase would still move. The substances “1” and “2” would continue to move. But the relation of the mobile front path and the substance moving path length remain constant despite the fact, that the mobile phase speed decreases from second to second in case we have standard HPTLC, that is no forced flow HPTLC.

Figure 3:

One must very carefully look onto the positions of a start line, a mobile front line and where and how the substance “x” has been sampled.
Figure 3 shows Rf_direct versus Rf _lin and                   Rf_linear values
and where the f and s data must be measured. NOTE: In order to let the substance “x” start exactly at the center of the circular run the sample solution is injected into the layer just together with the mobile phase. In circular runs the phase flow volume is constant .
 Text error: Direkt = direct

Figure 4:

In anti-circular HPTLC there is a quite complex Rf formula necessary to express LINEAR Rf data. Thus Rf linear - the second line formula under Rf direct = (s / f)_ac looks complicated. But NOTE: If an anti-circular run is done twice, the resulting Rf linear values are LINEAR and equal with Rf direct = (s / f)_ac.
Important is now the right part of figure 4: Here one sees the correlation of Rf values and chromatography related HPTLC-k-values. Whilst the linear (standard) HPTLC is only “good” in k ranges from 0.1 to 5, the circular technique helps to get data between 0.1 and 500, and the anti-circular technique is powerful in the
k-range 0.01 to 5. Remember: HPTLC-k-values are NOT numerically equal to HPLC-k-values.

Figure 5:

From figure 2 it follows that Rf = tm/tms. As discussed on GC and HPLC pages the residence time in the mobile phase is equal for all substances and thus has no chromatography information impact. The non adjusted (raw) tms value is only useful for chromatography until we know the value of tm, as tms = ts + tm and only ts is chromatography related. Thus the Rf data as tm / tms are really not a qualified value, just a not to easy measurable “number”. Therefore figure 5 is IMPORTANT :   k_HPTLC = 1/Rf - 1

Figure 6:

This figure prepares the comparison of the HPTLC related k value with the HPLC related k-value. In isocratic runs the partition coefficient (large) K is fundamentally equal in PLC and HPLC. Reason: the substance nicotine in a HPLC column packed with silicagel and in the HPTLC layer consisting of exactly the same silicagel “does not know” if confronted with the mobile phase methanol. For nicotine the HPLC column stationary phase is a round rod and in HPTLC it is a flat open rod - or layer. Thus the base for the relation of HPLC-k and HPTLC-k values is the EQUAL value of the partition coefficient K. Look onto the “triangel” comparison model given in fig. 6.

Figure 7:

The “triangle” comparison model is used again comparing now HPLC-k data with HPTLC k-data.
It is simple to understand :
The analyst works with TWO compounds, (this is always a good idea: chromatography separates and thus needs as a minumum two substances). As the relation of k_a to k_b in HPTLC is exactly equal the relation of k_a to k_b in HPLC, the HPTLC-k-value can be used as HPLC-k-value by an inter method standard substance. This is comparable to the use of the inner standard technique in any mode of quantitative chromatography either GC, HPLC or HPTLC.

Figure 8:

Now we can help HPTLC by HPLC or HPLC by HPTLC in case of equal phase compositions. But still there is a critical point to be considered: Qualitative HPTLC data can easily be falsified by the gas phase effect in classical chambers. The gas phase is absent in all HPLC runs and is present in all HPTLC runs besides in forced flow techniques. Thus the strong effect of the gas phase MUST be brought under control. How this becomes possible is intensively discussed in a series on ‘HPTLC errors’ published (and continued at the time of this site production) in Journal of Planar Chromatography - Modern TLC (JPC) - issue 102 (2005), Vol 18, 118-126 and 50-55.

Figure 9:

HPPLC - high pressure planar liquid chromatography - can solve special ‘large k’ problems. The linear single column HPLC cannot. Special problems in liquid chromatography arise with substances of large k-values. If k is larger than 100, the elution time is larger than 100 times the dead time. Those substances elute - if ever - from a column in large dilution. Therefore their detectability is poor. In addition the analysis time is very long, mobile phase consumption is high. Circular high pressure planar chromatography is an answer. ASK us using interchromforum@t-online.de .