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Abstract
Overpressured‐layer chromatography (OPLC) has been widely recommended in many scientific publications as a special thin‐layer chromatographic technique with forced flow of mobile phase that largely out‐performs classical thin‐layer chromatography (TLC) with its spontaneous low‐pressure capillary flow. Some authors even claim that OPLC can be regarded as a successful bridge between TLC and high‐performance liquid chromatography (HPLC). The essential difference between OPLC and TLC is the nature of the force that, in each of these two modes of planar chromatography, pushes the mobile phase through the pores of the stationary phase. It was the objective of this study to compare the impact of capillary and forced flow on analyte retention and separation quality. As test analytes, we selected three different hydrocarbons, tetralin, phenanthrene, and anthracene; these are, respectively, mono‐, bi‐, and tricyclic aromatic compounds with no functionality and, hence, are unable to participate in lateral interactions that might eclipse the basic effects of retention. In the most advanced OPLC systems, development of the thin‐layer chromatograms resembles that in HPLC as closely as possible (e.g., the stationary phase bed is preconditioned with the mobile phase and the samples are applied on‐line to the sorbent layer, without interrupting the flow of mobile phase and without drying the initial spots). With the Cobrabid OPLC apparatus used in this study, however, the only possibility was to develop dry layers with the dried spots of samples applied off‐line. Therefore, the only difference between development of TLC and OPLC chromatograms was, in fact, the pressure (and consequently the flow rate) of the mobile phase. Surprisingly enough, values of the retardation factor (R F) obtained for our test analytes by OPLC were always substantially lower than those obtained by TLC, which, under the conditions of our experiment, was proof of the poorer selectivity of OPLC compared with TLC. Two physical explanations (either alternative, or complementary) are offered to explain how the elevated pressure of the mobile phase in OPLC results in much lower numerical values of R F than in TLC.
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Acknowledgment
The authors wish to thank Merck KGaA (Darmstadt, Germany) for the silica gel plates used in this study.