A weaker solvent was constantly added at the middle of
the two
columns to set up a two-step solvent gradient in twin-column recycling
chromatography so that the eluotropic strength of the downstream liquid
was reduced. Therefore, the back edge of the band moved faster than
the front edge during the circulation of the solute band, initiating
special gradient compression to effectively counteract the band spreading.
Herein, modified recycling chromatography was applied to the simultaneous
purification and concentration of minor impurities in the bulk drug.
Taking orlistat as an example, the target impurity with only 0.7%
content in orlistat bulk drug was isolated, giving a purity of 96%.
Its concentration was enhanced by 9.4 times. Investigations on operating
conditions verified that decreasing the eluent eluotropic strength,
increasing the solvent gradient, extending the column switching interval,
and enhancing the number of switches within proper ranges were beneficial
to the improvement of separation.