The simplest method of collecting a sample of air for analysis is to fill a bottle or other rigid container with it or to use a bag of a suitable material. Although sampling by this method is easy, the sample size is distinctly limited, and collecting a large enough sample for subsequent analysis may not be possible.
Bottles larger than several liters in capacity are awkward to transport; and while bags of any size are conveniently transported when empty, they can be difficult to handle when inflated. Nevertheless, collecting several samples in small bags can prove more convenient than taking more complex sampling apparatus to several sampling sites. If analyzing the contaminant is possible by GC procedures or gas-phase infrared spectroscopy, samples as small as a liter or less can be adequate and can be easily collected with bags.
Several methods exist for filling a rigid container such as a bottle. One method is to evacuate the bottle beforehand, then fill it at the sampling site by drawing air into the bottle and resealing it (see Figure 5.10.1). Alternatively, a bottle can be filled with water, which is then allowed to drain and fill with the air. A third method consists of passing a sufficient amount of air through the bottle using a pumping device until the original air is completely displaced by the air being sampled.
Plastic bags are frequently filled by use of a simple hand-operated squeeze bulb with valves on each end (see Figure 5.10.1) that are connected to a piece of tubing attached to the sampling inlet of the bag. In most cases, this procedure is satisfactory; but the environmental engineer must be careful to avoid contaminating the sampled air with the sampling bulb or losing the constituent on the walls of the sampling bulb. Problems of this kind can be avoided when the bag is placed in a rigid container, such as a box, and air is withdrawn from the box so that a negative pressure is created, resulting in air being drawn into the bag.
Selecting bag materials requires care; some bags permit losses of contaminants by diffusion through the walls, and others contribute contaminants to the air being sampled.
A number of polymers have been studied, and several are suitable for many air sampling purposes. Materials suitable for use as sampling bags include Mylar, Saran, Scotchpak (a laminate of polyethylene, aluminum foil, and Teflon), and Teflon.
Even when a bag is made of inert materials, gas-phase chemical reactions are always possible, and after a period of time, the contents of the bag are not identical in composition to the air originally sampled. Thus, a reactive gas like sulfur dioxide or nitric oxide gradually oxidizes, depending on the storage temperature. Generally, analyses should be performed as soon as possible after the samples are collected. Losses by adsorption or diffusion are also greater with the passage of time and occur to some extent even when the best available bag materials are used.
The use of small bags permits the collection of samples to be analyzed for a relatively stable gas, such as carbon monoxide, at a number of locations throughout a community, thus permitting routine air quality measurements that might otherwise be inordinately expensive.
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