OVERPRESSURED LAYER CHROMATOGRAPHY IN COMPARISON WITH THIN-LAYER AND HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY FOR THE DETERMINATION OF COUMARINS WITH REFERENCE TO THE COMPOSITION OF THE MOBILE-PHASE

The retention behaviour of fifteen closely related coumarins in normal -phase overpressured layer chromatography (OPLC) was studied with the aim of comparing the retentions with those in normal-phase thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC) when optimization of the mobile phase was carried out according to th e PRISMA system. The mobile phase optimization was carried out on TLC plates in unsaturated chambers. The resulting mobile phases were trans posed to off-line, non-equilibrated OPLC and further to HPLC. The rete ntion in TLC was measured at 37 selectivity points and in OPLC and HPL C at 13 points. Capacity factors (k') and separation factors (alpha) w ere calculated in order to study the retention behaviour in the differ ent systems. Two- and three-dimensional evaluations of k' against sele ctivity points showed similar retention behaviours for the coumarins i n TLC, OPLC and HPLC. The ct values for TLC, OPLC and HPLC showed simi lar patterns in the three-dimensional evaluations. The retention behav iour at different solvent strengths was also examined. According to qu adratic regression, k' showed a dependence on the change in solvent st rength. OPLC, which can be considered as a ''planar column'' technique , and TLC are closely related methods, whereas HPLC shows a different behaviour in the elution process with regard to solvent strength.