Page 215 - Fundamentals of Air Pollution

II. Sampling Systems for Gaseous Pollutants 181

for volatile hydrocarbons by the physical adsorption of the individual hy-
drocarbon molecules on active sites of the sorbent (2). Collection efficiency
drops drastically when the active sites become saturated.
Sample stability becomes increasingly important as the time between
sampling and analysis increases. Effects of temperature, trace contami-
nants, and chemical reactions can cause the collected species to be lost
from the collection medium or to undergo a transformation that will prevent
its recovery. Nearly 100% recovery is also required because a variable recov-
ery rate will prevent quantification of the analysis. Interference should be
minimal and, if present, well understood.

II. SAMPLING SYSTEMS FOR GASEOUS POLLUTANTS

Gaseous pollutants are generally collected by the sampling systems
shown in Fig. 13-l(a-d). The sampling manifold's only function is to trans-
port the gas from the manifold inlet to the collection medium in an unaltered
state. The manifold must be made of nonreactive material. Tests of material
for manifold construction can be made for specific gases to be sampled. In
most cases, glass or Teflon will not adsorb or react with the gases. No
condensation should be allowed to occur in the sampling manifold.
The volume of the manifold and the sampling flow rate determine the
time required for the gas to move from the inlet to the collection medium.
This residence time can be minimized to decrease the loss of reactive species
in the manifold by keeping the manifold as short as possible.
The collection medium for gases can be liquid or solid sorbents, an
evacuated flask, or a cryogenic trap. Liquid collection systems take the
form of bubblers which are designed to maximize the gas-liquid interface.
Each design is an attempt to optimize gas flow rate and collection efficiency.
Higher flow rates permit shorter sampling times. However, excessive flow
rates cause the collection efficiency to drop below 100%.

A. Extractive Sampling
When bubbler systems are used for collection, the gaseous species gener-
ally undergoes hydration or reaction with water to form anions or cations.
For example, when SO 2 and NH 3 are absorbed in bubblers they form HSO 3~
+
and NH 4 , and the analytical techniques for measurement actually detect
these ions. Table 13-1 gives examples of gases which may be sampled with
bubbler systems.
Bubblers are more often utilized for sampling programs that do not
require a large number of samples or frequent sampling. The advantages
of these types of sampling systems are low cost and portability. The disad-
vantages are the high degree of skill and careful handling needed to ensure