Effect of alcohol chain length, concentration and polarity on separations in high-performance liquid chromatography...

The effect of alcohol chain length, concentration and polarity on separation in high-performance liquid chromatography using beta-cyclodextrin-bonded silica is discussed. The results show that retention times cannot be predicted merely from the polarity of the binary mobile phase. Although organic modifiers with the same physico-chemical properties and from the same solvent group were used, the retention times obtained using binary mobile phases having the same polarity, were different. It was also observed that normal-chain carbon alcohols gave retention times shorter than those obtained with a branched-chain alcohol (n-propanol vs. isopropanol), and the longer the alcohol chain the shorter the retention times. A plot of ln k' vs. alcohol volume fraction for benzene, toluene, ethylbenzene, propylbenzene, butylbenzene, 1-phenylhexane and 1-phenyloctane gave a linear relationship in methanol, ethanol and propanol (except for 1-phenylhexane). A non-linear relationship was obtained for all the solutes in isopropanol, tert.-butanol and 1-butanol, in the alcohol volume fraction studied.