Detailed description of Purge and trap

Compounds with relatively large air/water partition constants (e.g. chlorinated solvents like dichloroethylene, tetrachloromethane, and others) can be extracted from water by bubbling clean air through the sample. The air is subsequently passed over a filter which contains a sorbent (typically a heat resistant, non-polar polymer like Tenax®) that traps the extracted pollutants but not the water vapor. The sorbent is then transferred to a little oven in which the compounds are thermally desorbed into a small gas volume by strong heating. This gas volume is then directed into a gas chromatograph for further separation of the analytes and subsequent detection/quantification. The sorbent can usually be used again in the next extraction.

For an estimation of the volume of air that is needed to extract a desired fraction of pollutant (keep in mind effects of temperature and salt) a simple partition equation can be used in which the entire volume of the sample is equilibrated with the entire volume of air used to purge the sample. However, this equation makes the simplifying assumption that there is only "one" equilibration step. In reality, the total volume of air is split into many small bubbles which pass through the water over a certain amount of time (depending on the flow rate). The air therefore gets into contact with the water sample in many consecutive steps (i.e., in small volume increments). Clearly, the concentration decreases during the purging and hence the amount of analyte that is transferred to each gas bubble. Note that splitting the volume in many bubbles results in a more efficient extraction than the equilibration with the total air volume in one step. The accurate description of the purging process requires a partial differential equation (see script for details).

The mass of sorbent that is required in order to trap the purged analytes and the temperature that must be applied subsequently to desorb the analytes can also be calculated if the respective sorbent/air partition coefficients are known (see Box 10 and Box 11 ).