Column Regeneration

One of the more widely used cyclodextrin columns are derivatized with trifluoroacetic acid. It follows, that, in the presence of water, the ester will be hydrolyzed and the stationary phase may loose its chiral selectivity for certain separations. This will occur if the carrier gas is not completely dry and/or if the samples injected on the column contain water. In general, loss of column efficiency will be a symptom of general column degradation and regeneration will not be possible and the column will need to be replaced. In contrast, a loss of chiral selectivity accompanied by little or no loss of efficiency usually indicates that the column can be regenerated.

Columns can be regenerated using the apparatus shown in figure 50. The column is first heated to 150 C for 30-60 min. and the apparatus shown in figure 50 is then assembled. Fused silica tubing is quite flexible and can be allowed to pass through the joint between the oven door and the oven wall, and the oven door carefully closed without damaging the column. The injection port is connected to vial containing trifluoroacetic anhydride by means of a 2 m length of capillary tubing. The connecting tubing is arranged to dip into the head-space above the trifluoroacetic anhydride.

Courtesy of ASTEC Inc.

Figure 50. Column Generation Procedure

The vial is then connected to the capillary column with a second piece of capillary tubing. All connections can be made with Teflon tubing. The end of the capillary column is then connected to a beaker containing a solution methyl red in methanol and the carrier gas from the column allowed to bubble through the methyl red solution.

The inlet pressure to the system is adjusted until a good flow of bubbles passes through the indicator solution. After about 20-30 min. the indicator changes from yellow to red. (The excess anhydride in the exit gas should be adsorbed by passing it through a 1 M solution of caustic soda). The oven is then programmed up to 150 C at 5 C per min. and held at 150 C for 30 min. After the column has been regenerated, the tubing is disconnected, the apparatus dissembled and the column is reassembled in the normal manner. Finally, the column is again programmed from 40 C to 150 C at 5 C/min. and held at 150 C for 45 min. The column should now be regenerated and ready for use.