During the development of pharmaceutical molecules, compounds at the g-to-kg scale are purified using batch reverse phase (RP) chromatography. Fast project timelines and low quantities of available material limit the extent of process development and optimization, thus analytical HPLC methods are sometimes directly scaled up to the semi-preparative separation process. However, in comparison to analytical HPLC where peak resolution is most important, the main objectives for preparative chromatography are productivity and yield.
The conventional scale-up approach attempts to meet these objectives by overloading semi-preparative columns. However, due to the strongly non-linear retention behaviors of the compounds under this condition, the peak resolutions reduce significantly, even under isocratic elution conditions. In this work, we discuss an innovative approach that will increase the process throughput of batch separations without extensive optimization. A negative solvent gradient increases the separation factor α between the compounds of interest and reduce the purification time of the RP separation. The purification and re-equilibration time is further reduced by stepping up the organic solvent content to the initial high value when the separation of the desired component is almost completed. Now, all long-retained impurities are lumped and quickly eluted from the column. The effectiveness of this concept was demonstrated. For example, two diastereoisomers were purified using a solvent gradient ranging from 100 to 50% organics. The column load increased by three folds in comparison to the best isocratic condition. Thus, the production rate was improved by 33% and the organic solvent consumption was reduced to 50%.