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196 Modern Analytical Chemistry
breakthrough volume
The volume of sample that can be passed is the grams of sorbent.
through a solid sorbent before the The short-term exposure of humans, animals, and plants to gaseous pollutants
analytes are no longer retained.
Solid sorbent sampling is accomplished by passing the gas through a canister
packed with sorbent particles. Typically 2–100 L of gas is sampled when collecting
3
volatile compounds, and 2–500 m when collecting semivolatile gases.* A variety of
inorganic, organic polymer and carbon sorbents have been used. Inorganic sor-
bents, such as silica gel, alumina, magnesium aluminum silicate, and molecular
sieves, are efficient collectors for polar compounds. Their efficiency for collecting
water, however, limits their sorption capacity for many organic compounds.
Organic polymeric sorbents are manufactured using polymeric resins of
2,4-diphenyl-p-phenylene oxide or styrene-divinylbenzene for volatile compounds,
or polyurethane foam for semivolatile compounds. These materials have a low affin-
ity for water and are efficient collectors for all but the most highly volatile organic
compounds and some low-molecular-weight alcohols and ketones. The adsorbing
ability of carbon sorbents is superior to that of organic polymer resins. Thus, carbon
sorbents can be used to collect those highly volatile organic compounds that cannot
be collected by polymeric resins. The adsorbing ability of carbon sorbents may be a
disadvantage, however, since the adsorbed compounds may be difficult to desorb.
Nonvolatile compounds are normally present either as solid particulates or
bound to solid particulates. Samples are collected by pulling large volumes of gas
through a filtering unit where the particulates are collected on glass fiber filters.
One of the most significant problems with sorbent sampling is the limited ca-
pacity of the sorbent to retain gaseous analytes. If a sorbent’s capacity is exceeded
before sampling is complete then a portion of the analyte will pass through the can-
ister without being retained, making an accurate determination of its concentration
impossible. For this reason it is not uncommon to place a second sorbent canister
downstream from the first. If the analyte is not detected in the second canister, then
it is safe to assume that the first canister’s capacity has not been exceeded. The vol-
ume of gas that can be sampled before exceeding the sorbent’s capacity is called the
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breakthrough volume and is normally reported with units of m /(g pack ), where g pack
is more severe than that for pollutants in other matrices. Since the composition of
atmospheric gases can show a substantial variation over a time, the continuous
monitoring of atmospheric gases such as O 3 , CO, SO 2 , NH 3 , H 2 O 2 , and NO 2 by in
situ sampling is important. 10
Sample Preservation and Preparation After collecting the gross sample there is
generally little need for sample preservation or preparation. The chemical composi-
tion of a gas sample is usually stable when it is collected using a solid sorbent, a fil-
ter, or by cryogenic cooling. When using a solid sorbent, gaseous compounds may
be removed before analysis by thermal desorption or by extracting with a suitable
solvent. Alternatively, when the sorbent is selective for a single analyte, the increase
in the sorbent’s mass can be used to determine the analyte’s concentration in the
sample.
7 3 Solids
C.
Typical examples of solid samples include large particulates, such as those found in
ores; smaller particulates, such as soils and sediments; tablets, pellets, and capsules
used in dispensing pharmaceutical products and animal feeds; sheet materials, such
as polymers and rolled metals; and tissue samples from biological specimens.
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*1 m is equivalent to 1000 L.