Partition effect of polyethylene glycol in partition thin-layer chromatography
1973 Volume 22 Issue 3 Pages 306-311
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1973 Volume 22 Issue 3 Pages 306-311
In column chromatography, the following Martin equation is well satisfied: 1/Rf=1+K (Ls/Lm), where, Ls/Lm is the volume ratio of stationary phase to mobile phase, and K is the partition coefficient of solute. On the other hand, the above relation has not been fully examined in thin-layer chromatography (TLC) because of the difficulty in measurements of the values of Ls and Lm.
In the present work, a series of TLC experiments has been carried out in order to examine the applicability of the Martin equation in TLC. The values of Ls and Lm were determined gravimetrically by using mechanically strong “sintered plates”, which were prepared by sintering the mixture of glass powder and adsorbent (silica gel or kieselguhr) on glass plates at 450 to 750°C.
The outline of the TLC procedure was as follows. The silica gel sintered plates or the kieselguhr sintered plates were heated at 110°C for 1 hr; after being cooled in a desiccator they were coated with liquid polyethylene glycol(PEG) of various molecular weights(62600) by dipping the plates into PEG-acetone solutions. Samples of 2, 4-dinitrophenylhydrazones of aromatic aldehydes were applied on the plates and developed ascendingly with isopropyl ether. The spots were detected by spraying 5% KOH-ethanol solution.
Rf values decreased with the increase in PEG amount, and plots of 1/Rf against Ls were found to be linear.
The values of Kt, the partition coefficient of the sample solutes between PEG and isopropyl ether calculated from Ls, Lm and Rf values in TLC experiments, were only 1155% of the values of K, the partition coefficient measured by the batch method.
The difference between the values Kt and K can be explained by (1) the alteration of the solubility of PEG by mutual interaction of PEG and the adsorbent, and (2) incomplete partition equilibrium of the sample solutes between PEG and the mobile phases.
For practical use the effectiveness of a stationary phase can be estimated from the value of θ (=Kt/K), which is tabulated in Table V. As the values of θ increased with the molecular weight of PEG, it can be realized that the use of PEG of large molecular weight, such as PEG 400 or 600, is effective in partition TLC.
In the course of the investigation, the authors also found out that adsorption sites in adsorbent were destructed by coating with PEG. In TLC with silica gel adsorbent, for example, its adsorption chromatographic behavior disappeared gradually and partition chromatographic behavior appeared with the increase of the amount of PEG coated.
