Absorption

Most air sampling for gases and vapors involves absorbing the contaminant in a suitable sampling medium. Ordinarily, this medium is a liquid, but absorption can also take place in solid absorbents or on supporting materials such as filter papers impregnated with suitable absorbents. Carbon dioxide, for example, is absorbed in a granular bed of alkaline material, and sulfur dioxide is frequently measured by the absorption of reactive chemicals placed on a cloth or ceramic support. Environmental engineers also detect a number of gases by passing them through filter papers or glass tubes containing reactive chemicals. The reaction produces an immediate color change that can be evaluated by eye to measure the concentration of a substance.

Most commonly, however, gases and vapors are absorbed when they are passed through a liquid in which they are soluble or which contains reactive chemicals that combine with the substance being sampled. Many absorption vessels have been designed, ranging from simple bubblers, made with a piece of tubing inserted beneath the surface of a liquid, to complex gas-washing devices, which increase the time the air and liquid are in contact with each other (see Figure 5.10.2).

The impinger is probably the most widely used device. It is available in several sizes and configurations. An im-pinger consists of an entrance tube terminating in a small orifice that causes the velocity of the air passing through the orifice to increase. When this jet of air strikes a plate or the bottom of the sampling vessel at an optimal distance from the orifice, an intense impingement or bubbling action occurs. This impingement results in more efficient absorption of gases from the airstream than takes place if the air is simply bubbled through at low velocity. The two most frequently used impingement devices are the standard impinger and the midgetimpinger (see Figure 5.10.3). They are designed to operate at an airflow of 1 cubic foot per minute (cfm) and 0.1 cfm, or 28.3 and 2.8 lpm, respectively. Using such devices for sampling periods of 10 or 30 min results in a substantial amount of air passing through the devices, thus permitting low concentrations of trace substances to be determined with improved sensitivity and accuracy. Many relatively insoluble gases, such as nitrogen dioxide, are not quantitatively removed by passing through an impinger containing the usual sampling solutions.

The most useful sampling devices for absorbing trace gases from air are those in which a gas dispersion tube made of fritted or sintered glass, ceramic, or other material is immersed in a vessel containing the absorption liq-

FIG. 5.10.2 Gas-absorbing vessels.

uid (see Figure 5.10.2). This device causes the gas stream to be broken into thousands of small bubbles, thus promoting contact between the gas and the liquid with resulting high-collection efficiencies. In general, fritted absorbers are more applicable to sampling gases and vapors than impingers and are not as dependent on flowrates as impingers. Fritted absorbers are available from scientific supply companies and come in various sizes suitable for many sampling tasks. Prefiltering the air prior to sampling with a fritted absorber is advisable to prevent the gradual accumulation of dirt within the pores of the frit.

The use of solid absorbents is not widely practiced in ambient air sampling because the quantity of absorbed gases is usually determined by gravimetric means. With the exception of carbon dioxide, few atmospheric gases lend themselves to this type of analysis.

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