Preparative Chromatography - Recycling Development > Page 42

The column was 30 cm long, 2 in. I.D., packed with 10 mm particles carrying Vancomycin as the chiral stationary phase. The mobile phase was ethanol and the flow rate 100 ml/minute. The sample load was 400 mg dissolved in 5 ml of ethanol.

It is seen in figure 21 that after the first cycle, there is very little resolution of the enantiomers, but an impurity is separated on the front of the composite peak. This peak is diverted to waste a procedure that is termed peak shaving (from the main peak). During the second cycle, the separation of the enantiomers is beginning, although the isomers are insufficiently resolved for peak collection to be initiated. During the third cycle, the first major peak is 'shaved' from the composite peak. It is important to note that as the overloaded peak is asymmetrical and tails, the first peak will be collected virtually pure. The second peak will remain contaminated with a small amount of the first peak. After the fourth cycle is complete, the trace of the first isomer is shaved from the major peak and passed to waste. The remainder of the peak is then collected with little or no contamination. The separation is fairly rapid, the four cycles are completed in less than 25 min. and, thus, the output of the system would be nearly 1200 mg per hour. The recovery of each enantiomer was 98% and had an enantiomeric excess of 98%.

In addition, this procedure can be fully automated, providing the recycling and collection program is controlled by the detector signal. Alternatively the separation can be programmed on the basis of time but the reference points must be re-adjusted if required from the detector signal after each cycle. The purity of the products is confirmed by the chromatograms of each major fraction which are shown in figure 22.