Solvent strength, selectivity and retention mechanism studies on polybutadiene-coated alumina columns in reversed-phase liquid chromatography

Abstract

The effects of mobile phase composition and temperature on retention and methylene group selectivity were investigated for polybutadiene-coated alumina columns (PBD columns) using retention data for selected aromatic and aliphatic compounds including n-alkylbenzenes (C 7C 10) and n-alkanes (C 5C 7) collected on two different types of PBD columns and a silica-based C 18 column using hydroorganic mobile phases containing acetonitrile (MeCN) or methanol (MeOH). A detailed comparison of solvent strength differences between MeCN and MeOH reveals the limitations of using the slope from a plot of log k′ vs. volume fraction of organic solvent as a measure of solvent strength in reversed-phase liquid chromatography (RPLC). Part of the difference in strength between MeCN and MeOH observed for both the PBD and C 18 columns, particularly in the region of mid-range composition where the differences are generally greater, can be explained by the much greater tendency for MeOH to associate with water. The greater difference in MeCN and MeOH observed for the PBD columns is probably due to the presence of unpolymerized CC bonds on the surface, which results in a stronger displacement-like interaction with MeCN. Although these double bonds may be the reason for MeCN's greater strength on the PBD phase, they apparently have little effect on the retention mechanism. The methylene group selectivity was very similar for the PBD and C 18 columns (decreasing with increasing organic solvent), and the enthalpies of transfer were comparable. The methylene selectivity also decreased with increasing temperature for the PBD columns, another trend consistent with conventional RPLC phases. The less retentive nature of the PBD columns coupled with their similar methylene selectivity implies they might be better for the separation of a homologous series. Likewise, the greater difference in solvent strength between MeCN and MeOH on the PBD columns may be advantageous for the gradient elution of solutes with a very wide range of polarities.