Recycle/peak shaving technique

FIGURE 6-12. The peak-shaving technique using recycle. In the heavily loaded case, the first peak may have a much steeper front edge than illustrated. Also remember that since more mass can be removed from the leading peak during peak shaving, it is always desirable to have the component of interest elute first from the column (if possible). 

The recycling techniques have been used for preparative chromatography using lower-resolution columns packed with larger particle sizes. The application is usually a binary separation. In this case, it is possible to extract the sides of the bands during each run to obtain the pure compounds. This technique is called peak shaving. The center of the band is recycled as usual. Thus the load on the column continuously decreases, and the width of the band can be controlled. With this method it is possible to obtain high-resolution prepara-... 

An additional example of the technique of recycling, which includes a procedure called peak shaving, is the preparative separation of the enantiomers of 5-methyl-5-phenylhydantoin. The recycling and peak shaving procedure is depicted in figure 12.8. The column was 30 cm long, 2 in. I.D., packed with 10 m 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. 

A technique called shave/recycle, mentioned earlier, allows separtion of a pair of close resolving peaks. To use shave/recycle, it is necessary to plumb the HPLC system so that the output from the detector can be returned to the HPLC pumps through small diameter tubing and switching valves (Fig. 11.2). Twenty-thousandths tubing is used to connect the detector output to valve 2, the waste recycle valve 0.02-in tubing connects from valve 2 to valve 1, the solvent select valve and, finally, a third valve 3, the collect valve, can be placed in the waste line from valve 2. 

C. Install two 2 mm ID x 61 cm large-particle, totally porous columns (Porasil A) and inject a sample load so that you obtain an R value equal to 0.7. Using the refractometer as a guide, shave the first section of the peak and recycle the overlapped portion. Continue the shave-recycle sequence for purity. [Note Before attempting this experiment, Chapter 10 (Experiment 3) should be performed to develop the technique of recycle.]... 

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