Chiral separation by high-speed countercurrent chromatography.

Various parameters involved in the chiral separation of (+/-)-DNB-amino acids were investigated using N-dodecanoyl-L-proline-3,5-dimethylanilide as a chiral selector (CS) and two-phase solvent systems composed of hexane/ethyl acetate/methanol/10 mM hydrochloric acid at various volume ratios. The results indicated that increasing the concentration or net amount of the CS in the stationary phase improves both separation factor (alpha) and peak resolution (Rs). The hydrophobicity of the solvent system also increases the alpha value of the racemate while it affects the peak resolution differently according to the partition coefficient of the racemate. Overall results indicated that the best separation of a racemate will be achieved by applying a high CS concentration in the organic phase while adjusting the hydrophobicity of the solvent system so that the partition coefficient of the racemate falls between 0.6 and 0.8. The peak resolution will be further increased by using a longer and/or greater internal diameter coiled column.