Peak recycling and the creation of the “infinite column”

This article from Knauer discusses the circumstances when peak recycling can be used for scaling up an analytical method to a preparative method. In effect, by using the length of the column bed multiple times, peak recycling elongates the separation length of your column to a potentially “infinite” length.

Scaling up from analytical columns to preparative columns can be economically impractical, due to the small particle size of the analytical column media. In addition, using small particle media in the preparative column can create back-pressure and flow rate limitation issues. One way around this is to use larger particle-sized media and peak recycling, to feedback peaks into the column that are not achieving baseline separation.

Peak recycling can be performed using “time-based recycling” whereby the peaks of interest are collected at a fixed elution time window and then reinjected back onto the column. The problem with this method is that shifts in retention time within each separation can cause differences in the purity of the collected fractions. 

Automated peak recycling uses software to detect the peaks of interest by their peak height and/or slope. Thus overcoming any retention time shift problems. A motorized valve, T-fitting and control software with peak detection are all that are required to offer a simple solution to increase the separation performance without the need for larger investments. Please note that peak recycling only works with an isocratic separation method since the composition of the eluent needs to stay constant during multiple repetition cycles.

This article discusses the difference between time-based recycling and automated peak recycling and uses tocopherol (Vitamin E) and its stereoisomers to demonstrate how this can be done.