Ambient air monitoring is a priority in the ongoing efforts to contain and manage the oil spill from the Deepwater Horizon in the Gulf of Mexico.
The following provides information about the kinds of airborne contaminants that are currently being monitored in the Gulf, how they are collected and how they are analyzed. For a given project, a suitable sampling plan – developed by environmental or industrial hygiene professionals – will determine which methods are most applicable to the project objectives.
Volatile Organic Compounds (VOCs)
VOCs are loosely defined as organic compounds with boiling points up to 250°C. Due to their presence in crude oil, VOCs of particular interest in the Gulf include benzene, toluene, ethylbenzene, xylenes and naphthalene.
Stainless steel canisters (“Summa” or equivalent) may be used to collect air samples, either as instantaneous (“grab”) or time-integrated samples.
Pre-cleaned, certified, and evacuated canisters are provided by the analytical laboratory and are available in a range of sizes. 6L canisters are often used for ambient air monitoring, while smaller MiniCans may be used for personal monitoring applications.
Canisters may be batch or individually certified as clean, depending on the project data quality objectives.
EPA Method TO-15 is the most utilized method for the analysis of VOCs in ambient air. This technique analyzes “whole air” samples collected in canisters using cryogenic concentration gas chromatography/mass spectrometry (GC/MS). Contaminants can be identified and quantified down to the low parts per billion (ppbV) level using this analytical method.
For more information on canister sampling…
Air Phase Petroleum Hydrocarbons
In addition to the individual target compounds associated with crude oil, the various hydrocarbon fractions that may be present in the vapor phase can be quantified. A method developed by the Massachusetts Department of Environmental Protection (MassDEP) for air-phase petroleum hydrocarbon (APH) analysis was most recently updated in 2009.
Based on EPA TO-15, this method also utilizes stainless steel canisters for sample collection.
Like EPA TO-15, MassDEP APH utilizes GC/MS instrumentation for analysis and has comparable sensitivity. The benefit provided by this method is that, in addition to the target compounds reported via EPA TO-15, it reports three hydrocarbon ranges: C5-C8 aliphatics, C9-C12 aliphatics, and C9-C10 aromatics.
Since the analysis utilizes GC/MS, non-petroleum related peaks (e.g. chlorinated solvents, alcohols, etc.) may be subtracted out of the hydrocarbon ranges, thus yielding a more accurate representation of the petroleum product.
Polycyclic Aromatic Hydrocarbons (PAHs)
PAHs are of concern in the Gulf since they are present in crude oil, particularly as it “weathers” with exposure to the marine environment. They are also products of incomplete combustion and may be present due to the oil burning that was conducted earlier in the cleanup activities.
Since PAHs are only semi-volatile in ambient conditions, as air pollutants in the Gulf they will be mostly present in the aerosol phase. Therefore, they must be collected on solid sorbent media, rather than as whole air samples. PAHs are usually collected using a polyurethane foam and XAD resin cartridge/tube (PUF/XAD-2) with a quartz fiber filter.
PAHs currently being monitored by the EPA include benzo(a)pyrene, benzo(a)anthracene, benzo(b)fluoranthene, benzo(k) fluoranthene, chrysene, dibenz(a,h)anthracene, indeno(1,2,3-cd)pyrene, and naphthalene.
EPA TO-13A cites a 7-day holding period for collected samples before extraction must occur. Therefore, it is recommended that samples be shipped on ice via overnight courier to the laboratory immediately after collection.
EPA TO-13A is the most appropriate choice for the identification and quantification of PAHs in ambient air. As written, high volume air samplers are used to draw air through the aforementioned sampling tube at a rate of 200-300L/min for 24 hours. Many practitioners also utilize a low volume modification where air is pulled through a smaller PUF/XAD-2 tube at up to 5L/min using a personal sampling pump.
Once sampling has been completed, the PUF/XAD/filter combo undergoes solvent extraction followed by evaporative concentration. The concentrated sample extract is analyzed by gas chromatography/mass spectrometry (GC/MS), often in Selected Ion Monitoring (SIM) mode to reduce matrix interferences. Reporting limits between 0.5 – 5.0 ug/sample are achievable using this technique. The resulting concentration is dependent upon the total volume of air pulled through the sampling media.
Hydrogen Sulfide (H2S)
H2S and other reduced sulfur compounds (mercaptans, sulfides, and disulfides) are among a number of odorous compounds that may be associated with crude oil. Although the crude oil released from the Deepwater Horizon is thought to be “sweet” crude, and therefore contain less than 0.5% sulfur, sulfur-like odors have been reported in the Gulf, prompting sampling for this class of compounds.
Unlike the previously discussed VOCs, H2S is not stable in a stainless steel canister. H2S and other reduced sulfur compounds should instead be collected using a Tedlar bag, which is constructed of an inert polyvinyl fluoride material. Sample bags should also be equipped with inert polypropylene fittings.
Due to the highly reactive nature of reduced sulfur compounds, Columbia Analytical recommends the use of a vacuum box or lung sampling device to fill the Tedlar bag, rather than drawing the air sample directly through a sampling pump. The use of these accessories allows the samples to be collected without the air passing through the pump, which reduces the potential for contamination of the pump and subsequent samples.
There is a 24-hour holding time for this analysis, so it is important the samples are sent to a laboratory via overnight courier for morning delivery. The bags should only be half to two-thirds full to allow room for expansion during air shipment.
H2S and other reduced sulfur compounds are identified and quantified by ASTM D5504-08, which specifies the use of gas chromatography with sulfur chemiluminescence detection (GC/SCD). The sulfur chemiluminescence detector is one of the most selective sulfur detectors available, with a sulfur-to-carbon selectivity of >107, which means that the response of the reduced sulfur compounds is not affected by the co-elution of hydrocarbons in the sample matrix. The typical reporting limit for hydrogen sulfide via ASTM D5504-08 is approximately 5.0 ppbV.
For more information on Hydrogen Sulfide bag sampling…
Other Semi-Volatile Organic Compounds (SVOCs)
In addition to PAHs, there may be other SVOCs of interest related to the oil spill and the cleanup activities. The EPA, for example, has monitored 2-butoxyethanol, 1-1(1-methyl-2butoxy-ethoxy)-2-propanol and propylene glycol, which are associated with the oil dispersants being applied.
Sampling and Analytical Approach:
EPA TO-17 can be used to identify and quantify compounds associated with the dispersants such as 2-butoxyethanol, as well as hydrocarbons up to C20. These less volatile, higher boiling point compounds cannot effectively be sampled and analyzed by EPA TO-15, which specifies the collection of a whole air sample with the use of a stainless steel canister.
Instead, the EPA TO-17 method specifies sampling using a solid sorbent packed in a thermal desorption tube using a personal sampling pump.
Once in the lab, the tubes are rapidly heated, and compounds of interest are desorbed off the tube and into the gas chromatograph/mass spectrometer (GC/MS) for analysis. Reporting limits between 1.0-10 ng/sample are achievable with this technique; the resulting concentration is dependent upon the total volume of air pulled through the sampling media.
Proper sorbent selection and sample collection technique is crucial. The sorbent selection is influenced by many factors such as the chemicals of interest, anticipated moisture levels, and desired reporting limits. Please consult with the laboratory prior to sampling to ensure valid sample collection.