Analytical Sciences

Effect of the Net Surface Charge Density of Heptakis-6-bromo-6-deoxy-β-cyclodextrin Bonded Silica Gels on the Retention Behaviors of Neutral Cresol Isomers in HPLC

Hiroyuki NAKAGAWA, Shinya KITAGAWA, and Hajime OHTANI

Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan

The effect of the surface charge density of heptakis-6-bromo-6-deoxy-β-cyclodextrin (β-CD-BR) bonded silica gels, which was used as the stationary phase of a packed capillary column for HPLC, was investigated concerning the retention behaviors of neutral cresol isomers. On the whole, the retention factors of the cresol isomers increased with an increase in the pH values of the mobile phase, although they were slightly smaller at pH 6.1 than at pH 4.7. An investigation on the retention variation using a van't Hoff plot revealed that the increase in the retention factor (k) at a higher pH region could be mainly attributed to the increase in ΔS, while a partial decrease in k around pH 5 - 6 was caused by a decrease in the -ΔH/T value. On the other hand, a measurement of the electroosmotic flow velocity under various pH of the mobile phase solutions revealed that the retention variations of the neutral cresol isomers were strongly correlated with the surface charge on the packing materials. The positive charge of secondary ammonium functional groups to bind β-CD-BR inhibit the insertion of the cresol isomers into the cavity of β-CD-BR while reducing the retention factor, whereas the negative charge of silanol group enhanced it through a local change in the mobile phase composition.

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