Cyclodextrins (CDs) have been most frequently utilized as selectors for chiral and achiral separations by capillary zone electrophoresis (CZE) and HPLC. [1][2][3][4][5][6][7][8][9][10][11] The fit between the CDs and the solutes is an important factor for their separation, because the formation of the inclusion complex is a crucial step for separation using CDs. The hydroxyl groups of the CDs are considered to play an important role in the formation and stabilization of the inclusion complexes. It is well known that a chemical modification of the hydroxyl groups brings about changes in the shapes and sizes of their cavities, in their hydrogen-bonding abilities and in other physical properties, as well as affecting their molecular-recognition abilities. In previous works, we found that selective methylation of the hydroxyl groups of the CDs produced remarkable enantioselectivity changes using CZE and HPLC. [1][2][3]10,11 In this work, we investigated the separation of the mono-, diand trichlorophenols by HPLC using selectively methylated β-CD (2-and 3-monomethylated and 2,3-dimethylated) bonded stationary phases. We examined the preliminary separation of these chlorophenols by capillary electrochromatography (CEC) using a capillary packed with the native β-CD stationary phase.
ExperimentalThe preparation of β-CD-bonded stationary phases has been described in detail elsewhere.11 The mono-and dimethylated β-CDs are denoted by prefixing the native β-CD with MM-and DM-, respectively, and the stationary phases by suffixing with -silica. The amounts of the β-CD derivatives immobilized to the silica surface were evaluated by elemental analysis: 84 µmol g -1 for β-CD-silica, 83 µmol g -1 for 2MM-β-CD-silica, 88 µmol g -1for 3MM-β-CD-silica and 80 µmol g -1 for 2,3DM-β-CD-silica. The HPLC apparatus, as previously described, 10 was used under the following conditions: column, 15 cm × 4 mm i.d. at 20˚C; flow rate, 1.0 ml min -1 ; detection at 280 nm; sample volume, less than 2.5 µl.The CEC measurements were carried out using a Model 270A automatic capillary electrophoresis system (Applied Biosystems) equipped with a variable-wavelength detector operated at 230 nm: fused-silica capillary, 48 cm (total length) and 22 cm (packing length) × 100 µm i.d. and 375 µm o.d. at 30˚C; detection window, 23 cm from inlet. Samples were electrokinetically injected into the capillary (5 V for 1 s).The mobile phases used were mixtures of triethylammonium acetate buffer and methanol (or acetonitrile) (v/v 1:9 -9:1), and were filtered through a membrane filter followed by degassing in an ultrasonic bath for about 1 min before use. The concentration of the sample solutes was ca. 1.0 mM.
Results and Discussion
Separation of chlorophenols on β-CD-bonded stationary phases by HPLCAt first, the retentions of fourteen chlorophenols on four β-CD-bonded stationary phases were investigated by changing the methanol content of the mobile phase, 10 mM triethylammonium acetate (TEAA) (pH 5.0)-methanol (v/v 9:1 -5:5).The retention increased as the methanol c...