Cyclodextrins in Capillary Electrophoresis

In the pharmaceutical industry the importance of capillary electrophoresis is continuously growing. It is a technique which is increasingly used for determination of the optical purity of intermediates and final products. Many different optically pure compounds

Figure 12 Capillary GC analysis on cyclodextrin stationary phases. Experimental conditions: column: (A) 15 m x0.32 mm ID Chiraldex® B-PH ((S)-hydroxypropyl-^-cyclodextrin (Astec)); (B) 15mmx0.32mm ID Chiraldex® B-DA (dipentylated ^-cyclo-dextrin (Astec)); carrier gas: helium (linear velocity 25cms~1); temperatures: column 150-200°C (3°Cmin~1), injector 210°C, detector 210°C; detection: FID; injection: 1 |L split (split ratio 1/100).

The different types of cyclodextrins, which are most frequently used are:

• (2-hydroxy)propylated ^-cyclodextrin;

• (2-hydroxy)propylated y-cyclodextrin;

• carboxymethylated ^-cyclodextrin.

Figure 12 Capillary GC analysis on cyclodextrin stationary phases. Experimental conditions: column: (A) 15 m x0.32 mm ID Chiraldex® B-PH ((S)-hydroxypropyl-^-cyclodextrin (Astec)); (B) 15mmx0.32mm ID Chiraldex® B-DA (dipentylated ^-cyclo-dextrin (Astec)); carrier gas: helium (linear velocity 25cms~1); temperatures: column 150-200°C (3°Cmin~1), injector 210°C, detector 210°C; detection: FID; injection: 1 |L split (split ratio 1/100).

can be used to generate the required chiral environment. Cyclodextrins of course are also very suitable as an electrolyte additive.

Figure 13 Capillary GC analysis on cyclodextrin stationary phases. Experimental conditions: column: (A) 15 m x0.32 mm ID Chiraldex® B-TA (trifluoroacetyl-^-cyclodextrin (Astec)); (B) 15 m x 0.32 m ID Chiraldex® G-TA (trifluoroacetyl-y-cyclodex-trin (Astec)); carrier gas: helium (linear velocity 25 cm s~1); temperatures: column 150-200°C (2°Cmin~1), injector 210°C, detector 210°C; detection: FID; injection: 1 |uL split (split ratio 1/100).

Figure 13 Capillary GC analysis on cyclodextrin stationary phases. Experimental conditions: column: (A) 15 m x0.32 mm ID Chiraldex® B-TA (trifluoroacetyl-^-cyclodextrin (Astec)); (B) 15 m x 0.32 m ID Chiraldex® G-TA (trifluoroacetyl-y-cyclodex-trin (Astec)); carrier gas: helium (linear velocity 25 cm s~1); temperatures: column 150-200°C (2°Cmin~1), injector 210°C, detector 210°C; detection: FID; injection: 1 |uL split (split ratio 1/100).

Figure 14 Capillary electrophoresis using a derivatized cyclodextrin as electrolyte additive. Experimental conditions: equipment: P/Ace system 5500 (Beckman); capillary: 50 ^m ID uncoated fused silica; total length: 57 cm; length to detector: 50 cm; electrolyte: 20 mM Heptakis (2,3,6-tri-0-methyl)P-cyclodextrin 10 mM disodium hydrogen phosphate solution adjusted to pH 2.2 with phosphoric acid; analysis: temperature 25°C, voltage # 20 kV, inject sample 2 s, detection UV (220 nm).

Figure 14 Capillary electrophoresis using a derivatized cyclodextrin as electrolyte additive. Experimental conditions: equipment: P/Ace system 5500 (Beckman); capillary: 50 ^m ID uncoated fused silica; total length: 57 cm; length to detector: 50 cm; electrolyte: 20 mM Heptakis (2,3,6-tri-0-methyl)P-cyclodextrin 10 mM disodium hydrogen phosphate solution adjusted to pH 2.2 with phosphoric acid; analysis: temperature 25°C, voltage # 20 kV, inject sample 2 s, detection UV (220 nm).

The usefulness of cyclodextrins as an electrolyte additive is illustrated in the following example. A substance containing three optical centres, which means eight possible isomers, had to be separated. HPLC experiments on different types of chiral stationary phases did not succeed in a complete resolution of the mixture. The result of a capillary electrophoresis experiment using Heptakis (2,3,6-tri-O-methyl) ^-cyc-lodextrin as electrolyte additive is illustrated in Figure 14.

Compared with HPLC, in capillary electrophoresis many more parameters can be varied to improve separation. Therefore, most of the methods developed on one of the commonly used chiral stationary phases can be replaced by a capillary electrophor-esis methods, using cyclodextrins or another chiral auxiliary as electrolyte additive.

See also: N/Chromatography: Liquid: Ion Pair Liquid Chromatography; Mechanisms: Chiral; III/Chiral Separations: Amino Acids and Derivatives; Capillary Electrophoresis; Cellulose and Cellulose Derived Phases; Chiral Derivatization; Gas Chromatogrpahy; Ion-Pair Chromatography; Ligand Exchange Chromatography; Liquid Chromatography; Molecular Imprints as Stationary Phases; Protein Stationary Phases; Synthetic Multiple

Interaction (' Pirkle') Stationary Phases; Thin-Layer

(Planar) Chromatography.

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