Recycling Development

The basic procedure is as follows. The two valves are set so that solvent from the reservoir is pumped through the injection valve to the column and the column outlet valve is set to the fraction collector. After the sample has been placed on the column, the column outlet valve is set to pass the column eluent back to the pump and the solvent selection valve set to the column eluent. Thus the mobile phase is continuously circulated through the column, through the detector, back to the pump and then back to the column. As a result the column is used over and over again many times, and each time the sample passes through the column, the resolution is improved. Unfortunately, the resolution is not necessarily proportional to the number of cycles, as significant peak dispersion can occur each time it passes through the pump. Nevertheless, there is a substantial net gain in resolution on each cycle. This procedure can be very time consuming if long retention times are involved, but has the great advantage of solvent economy.

The recycling procedure, in effect, artificially increases the column length and actually trades in time for solvent economy. An example of the recycling process is demonstrated in the separation of the Warfarin enantiomers shown in figure 20. The separation was carried out on a CYCLOBOND 1 2000 column, 25 cm long, 1 in, I.D., using a mobile phase consisting of methanol/acetic acid/triethylamine : 100/0.3/0.2 v/v/v, at a flow rate of 12 ml/min.

Courtesy of ASTEC Inc.
Figure 20 The Separation of the Warfarin Enantiomers by Recycling