Cleaning of the resin

For cleaning the resin, the column is inserted into the glass continuous extractor (Fig. 18-1). This arrangement has several advantages over batch extraction. The extraction may be carried out in a protected, if necessary an inert gas atmosphere a small solvent volume is sufficient for exhaustive extraction the all glass system eliminates any possible contamination from plastic surfaces contacting solvents. 

The resin is extracted with 200 mL of refluxing solvent for at least 12 h. A 1 1 mixture of methanol and dichloromethane is an excellent eluent. However, the high concentration of organically bound chlorine makes it a poor choice, if the resin is to be used for analyses of chlorinated hydrocarbons. In that case the acetonitrile-water azeotrope (16.3% water) should be used despite lower recoveries of aliphatic hydrocarbons measured with surrogate standard additions. After cooling, extraction is resumed with fresh solvent for another 12 h period. 

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A practical consideration in using XAD-8 resin should be noted. Despite exhaustive cleaning of the resin before use, the resin bleeds on the order of 3 mg C/L in 0.1 N NaOH (5). A major component of the bleed is acrylic acid, which has a low capacity factor on XAD-8 at pH 2, and is separated to a large extent from the sample during successive reconcentration steps. However, a small amount of contamination from resin bleed may be present in the final product. 

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