Recycle shaving

FIGURE 10-3. Recycle-shaving technique. Mobile phase 10% CH2CI2 in isooctane (50% water saturated). Flow rate 2 mL/min. Detector UV, 254 nm, 1.0 AUFS. Sample mixture of aromatics. Column Porasil A, 2 mm ID x 122 cm. (Note Actual separation will depend upon quality of mobile phase and column packing.)... 

F. The example of recycle shaving of the four-component mixture is shown in Figure 10-3. Compare your chromatogram to this figure. 

The first application which comes into mind when thinking about liquid foams is in cleaning processes, such a shampoo or shaving foams. More important, however, are liquid foams in mineral froth flotation and, a closely related process, the de-inking of recycled paper. Other uses include fire fighting. In food we often find foams such as whipped cream or egg whites. 

If we have to increase load higher, say to obtain a 50-fig sample for NMR analysis, we can use the shave/recycle technique to be described in the true preparative section. These runs must be made isocratic and column overload occurs at 100-150ng for most compounds. If this much material is needed, it is better to switch to a semipreparative column, which can easily produce milligram quantities in a single pass. 

By using solvent polarity techniques to increase k we can push the load to 20 mg. Going isocratic and using shave/recycle, the load can be increased to 100 mg with column overload occurring at 200-300 mg injections. 

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. 

Figure 11.3 Shave recycle, (a) Analytical (b) k increase (c) overload (d) preparative shave/re cycle.

A commercial customer had four compounds to recover from a synthesis mixture they separated as two pairs of compounds. They injected and collected each pair together, then diluted each pair with water, reinjected onto the reverse-phase column, and ran shave/recycle. Using this technique they purified 50gm of each compound in the two injections of the pair fractions on the 3-in column. 

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). 

This experiment is intended to first demonstrate the additivity of plates between columns. Recycle is then investigated from the theoretical viewpoint and from the practical standpoint of shaving to collect fractions. 

This experiment will use the recycle mode to shave the peaks for purity as one would do in a preparative situation. 

Is the shaving-recycle technique useful in preparing pure fractions Refer to your data. 

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.]... 

Fig. 5.9 Closed-loop recycling chromatography with peak shaving.

Fig. 5.10 Development of concentrations during closed loop recycling chromatography operation with peak shaving.

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