This page concentrates on a classical systematic error in HPTLC:
the falsified quality value “Rf”. By the Rf value HPTLC users express qualitative results.
The linear Rf value for substance “x” = s / f See figure 1 below.
f = length of the mobile phase path developed from start to front.
s = the path length the substance x was moved until the HPTLC run stopped.
The main reason to concentrate here in all details on the Rf value error is the fact, that its correction allows to use HPTLC-k-values which are comparable with HPLC-k-values. If the mobile and stationary phases in HPLC are exactly equal in HPTLC, the HPLC-k-value corresponds with the HPTLC-k-value. Although both differ by value it becomes evident that the two liquid chromatography techniques can support each other qualitatively. Due to the very limited separation efficiency of both if compared to modern micro gas chromatography a both sides support of these sister techniques is good to avoid serious systematic qualitative errors. As the life combination of HPLC with HPTLC is a next and powerful bridge to gap error trouble, an equal “language” for both techniques is mandatory. For this the Rf value fails.
For qualitative HPTLC and qualitative HPLC isocratic separation techniques are preferable. In this case qualitative HPLC data are time or volume based. The best quality value is k = residence time in the stationary phase over the residence time in the mobile phase: k = ts / tm .
It is not immediately visible, that HPTLC data are also time based. But this is shown in the figures below.
Especially helpful in HPTLC is the comparison and the simultaneous use of linear, circular and anti circular HPTLC. Circular techniques are not available in HPLC. NOTE that HPLC is blind to detect substances which chemisorb or very strongly adsorb in the stationary phase, that is for substances with HPLC-k-values of over 1000. Especially circular HPTLC with enlarged separation power for substances with higher
HPTLC-k-values can help HPLC users. Substances with very large HPLC-k-values remain often fixed in the HPLC column, that is: remain invisible. HPTLC allows to make substances with extreme large k-values visible Whilst a HPTLC information about substances with extreme k-values changes HPLC quantitation techniques towards quantitative inner standard modes, anti circular HPTLC data may help to avoid HPLC errors hidden under peaks with very small k-values.
Circular HPTLC techniques are important for HPLC users .
NOTE: µ-PLC IS CIRCULAR. A complete top level HPTLC working place needs quite some table space, lots of mobile phase volume - far above what modern micro - HPLC needs today and it costs a fortune. In using µ-PLC instead of instrumentalized HPTLC all is available for a HPLC ./. Planar Chromatography combination but at one to two orders of magnitude smaller in space, mobile phase consumption and investment.