Implementing the Sampling Plan: Gases

Gases

Typical examples of gaseous samples include automobile exhaust, emissions from industrial smokestacks, atmospheric gases, and compressed gases. Also included with gaseous samples are solid aerosol particulates.

Sample Collection 

The simplest approach for collecting a gas sample is to fill a container, such as a stainless steel canister or a Tedlar/Teflon bag, with a portion of the gas. A pump is used to pull the gas into the container, and, after flushing the container for a predetermined time, the container is sealed. This method has the ad- vantage of collecting a more representative sample of the gas than other collection techniques. Disadvantages include the tendency for some gases to adsorb to the container’s walls, the presence of analytes at concentrations too low to detect with accuracy and precision, and the presence of reactive gases, such as ozone and nitro- gen oxides, that may change the sample’s chemical composition with time, or react with the container. When using a stainless steel canister many of these disadvan- tages can be overcome with cryogenic cooling, which changes the sample from a gaseous to a liquid state.

Due to the difficulty of storing gases, most gas samples are collected using ei- ther a trap containing a solid sorbent or by filtering. Solid sorbents are used to col- lect volatile gases (vapor pressures more than approximately 10–6 atm) and semi- volatile gases (vapor pressures between approximately 10–6 atm and 10–12 atm), and filtration is used to collect nonvolatile gases.

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 volatile compounds, and 2–500 m3 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 breakthrough volume and is normally reported with units of m3/(g_pack), where g_pack is the grams of sorbent.

The short-term exposure of humans, animals, and plants to gaseous pollutants 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 O₃, CO, SO₂, NH₃, H₂O₂, and NO₂ by in situ sampling is important.

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.