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Seven pairs of cis-trans isomers of bis-steroidal tetraoxanes have been examined by both normal-phase (NP) and reversed-phase (RP) planar chromatography. Unmodified silica gel was used with ethyl acetate-toluene and ethyl acetate-petroleum ether as mobile phases in typical normal-phase systems. CN-silica with the mobile phases methanol-water and acetone-water and RP-18 silica with water-organic modifier (methanol, acetone, or dioxane) mobile phases were used as reversed-phase systems. For the RP systems a good linear correlation was established between R M values and amount ([%] ν/ν ) of organic modifier in the mobile phase (usually r > 0.99). It was found that under both NP and RP conditions cis isomers were more weakly retained than the corresponding trans isomers. The only exception to this was the chromatographic behavior of C(24) methyl esters. It was established that increasing the polarity of substituents at C(24) and C(24a) led to stronger retention in NP systems, i.e. weaker retention in RP systems. Highly selective separation was achieved in all the chromatographic systems investigated. Possible separation mechanisms are discussed on the basis of the results obtained.

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The retention behavior of 29 biologically active compounds, mixed 1,2,4,5-tetraoxanes, has been investigated by thin-layer chromatography on unmodified silica, CN-silica, and RP-18-silica. The binary mobile phases ethyl acetate-petroleum ether and ethyl acetate-toluene were used under normal-phase conditions and water-organic modifier (methanol, acetone, dioxane) under reversed-phase conditions. For normal-phase conditions R F values were plotted against the amount [%, v/v ] of more polar component in the mobile phase. For reversed-phase conditions R M values were calculated and plotted against the amount [%, v/v ] of organic modifier in the mobile phase. On the basis of the results obtained the effects of the structures of the compounds on their retention were studied. Retention mechanisms are discussed for all the separations.

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JPC - Journal of Planar Chromatography - Modern TLC
Authors:
Sandra Gaica
,
Dejan Opsenica
,
Bogdan Šolaja
,
Živoslav Tešić
, and
Dušanka Milojković-Opsenica

The thin-layer chromatographic behavior of cholic acid and fourteen of its synthetic derivatives has been studied in six reversed-phase and three normal-phase systems. Four significantly different adsorbents were used — RP-18-silica, CN-silica, polyacrylonitrile adsorbent, and unmodified silica gel. Reversed-phase chromatography was performed with water-organic modifier (methanol, dioxane, or acetone) binary mobile phases of widely variable composition. An approximately linear relationship was obtained between R M values and the amount of organic modifier in the mobile phases investigated. The effect of mobile-phase composition on retention has been considered and the selectivity of the chromatographic systems is also discussed. Separation mechanisms are proposed on the basis of the results obtained. A positive solvation effect was proposed as a predominant factor determining retention under normal-phase conditions whereas hydrophobic interactions of the substances with non-polar parts of the adsorbents were assumed to be predominant under reversed-phase conditions.

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