Adsorption, by contrast with absorption, consists of the retention of gaseous substances by solid adsorbents which, in most cases, do not chemically combine with the gases. Instead, adsorptive forces hold the gases or vapors which can subsequently be removed unchanged. Any solid substance adsorbs a small amount of most gases; but to be useful as an adsorbent, a substance must have a large surface area and be able to concentrate a substantial amount of gas in a small volume of adsorbent.

FIG. 5.10.3 Standard and midget impingers.

FIG. 5.10.2 Gas-absorbing vessels.

FIG. 5.10.3 Standard and midget impingers.

Activated carbon or charcoal and activated silica gel are most widely used for this purpose. A small quantity of either adsorbent placed in a U-tube or other container through which air is passed quantitatively removes many vapors and gases from a large volume of air. These gases can then be taken to the laboratory where desorption removes the collected substances for analysis. Desorption commonly involves heating the adsorbent and collecting the effluent gases or eluting the collected substances with a suitable organic liquid.

For most organic vapors, subsequent analysis by GC, infrared (IR), or UV spectroscopy is most convenient. For some purposes, either silica gel or activated carbon is used; but the use of silica gel is not recommended because it also adsorbs water vapor, and a short sampling period in humid air can saturate the silica gel before sufficient contaminant is adsorbed. Because charcoal does not adsorb water, it can be used in humid environments for days or even weeks if the concentration of the contaminant is low.

The ease of sampling using adsorbents is offset somewhat by the difficulty of quantitatively desorbing samples for analysis. When published data are not available to predict the behavior of a new substance, the environmental engineer should perform tests in the laboratory to determine both the collection efficiency and the success of desorption procedures after sample collection.

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