Unmodified and Modified Cyclodextrins as Mobile Phase Additives

Among the three cyclodextrins (a, ft, y), only ft-CD and its derivatives have been used for successful resolution of various racemates by TLC. In an aqueous solution ft-CD is represented as a truncated cone with different sized mouths (0.60-0.65 nm); the height of the cavity is 0.78 nm. The 2- and 3-hydroxyl groups are oriented towards the outside and are responsible for the aqueous solubility properties of this oligosaccharide. The hydrogen atoms and glycosidic oxygen groups are located inside the molecule, forming the relatively hydrophobic cavity that interacts with organic optical isomers to form diastereomeric inclusion complexes. Under reversed-phase conditions, the combination of hydrophobic and steric interactions with hydrogen bonding between the chiral solutes and the 2- and/or 3-hydroxyl groups may be the cause of enantioselectivity.

Aqueous-organic solutions (i.e. methanol-water or acetonitrile-water mixtures) are usually used as eluents. ft-CD is enantioselective in TLC only at the high concentrations reached by adding large amounts of urea, which increases the aqueous solubility of ft-CD more than ten-fold. However, urea tends to compete with solutes for the preferred location in the hydrophobic cavity, thus decreasing the separation factor. Chemical modification with hy-droxypropyl, hydroxyethyl or methyl groups has been used to increase the solubility of ft-CD and its complexes in water, eliminating the need to use urea. Optimization of the enantioselectivity can be achieved by modifying the concentration and nature of organic solvent, pH and buffer concentration of the eluent.

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Solar Panel Basics

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