An overview of a simple laboratory data analysis procedure to characterize and differentiate tar waste from manufactured gas plants compared to petroleum products. Presented at the American Academy of Forensic Science 2005 annual meeting in New Orleans - at the environmental sciences session which I also coordinated and moderated.
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MGP Waste Method
1. A Method to Characterize and Differentiate VOCs from MGP Wastes and Petroleum Bureau of Toxic Substance Assessment Wadsworth Center NYS Department of Health Todd R. Crawford, Michael Force, Gerald McDonald and Kenneth M. Aldous
Manufactured gas plant (MGP) wastes and many petroleum products emit volatile organic chemicals (VOCs). Limited data is available to describe the similarities and differences in the VOCs from MGP wastes and petroleum products. These VOCs are thought to consist primarily of hydrocarbons, many of which have significant impacts on air quality and human health. Growing awareness of VOC emissions has lead to inconsistent guidance and regulation being issued by many agencies; e.g. the selection or use of indicator volatile chemicals for MGP waste or petroleum, the assumption that the concentrations of contaminants in soil or water may be directly correlated to vapor concentrations, the use of indeterminate screening technology (PID or FID readings) to assess vapor concentrations, etc. New York State Department of Health conducted a study to characterize VOC emissions from MGP and petroleum, which has lead to this method with some environmental forensic applications.
The New York State Department of Health (NYSDOH) recently drafted a plan for evaluating sub-surface vapor intrusion from contaminated sites. The plan describes an approach to determine whether vapors from MGP waste are affecting air through sub-surface intrusion by, (1) identifying a target analyte list, (2) determining the environmental fate of sub-surface MGP vapors, and (3) evaluating vapor intrusion by testing soil gas and indoor air. To evaluate the first step in the plan, identifying a target analyte list, the NYSDOH Wadsworth Center analyzed headspace vapors from samples of MGP waste and some common petroleum products by gas chromatography with mass spectrometry (GC/MS) to identify the primary volatile organic compounds (VOCs). Wadsworth further characterized the samples by describing the classes of hydrocarbons in the headspace vapors, following ASTM Methods D2786 and E1618. BTSA examined that data to evaluate if it can be used to differentiate these types of materials based upon the headspace emissions. The steps in this process were: Analyze the VOCs in headspace from MGP waste and petroleum samples. Compare VOCs to determine similarities and differences. Compare chromatograms (fingerprints) for total ion chromatograms and extracted ion chromatograms. Compare proportions of hydrocarbon classes.
The majority of the chemicals identified in the headspace of the MGP waste and the petroleum product samples are similar. The relative proportions of these chemicals were different between the samples, but insufficient data was available to describe relationships that might allow the analyst to determine the type of sample contamination from the VOC data alone. The chromatograms of many common petroleum products have reproducible, recognizable features that allow an experienced analyst to distinguish, for example, automotive gasoline versus diesel fuel. The analyses by Wadsworth showed that VOC fingerprints of MGP wastes are not different but not distinctive from petroleum products; it was not possible to describe a chromatogram which is representative of MGP waste. However, when Wadsworth characterized the hydrocarbon classes, following American Society of Testing and Materials (ASTM) Standard Method E1618-94, it was found the proportions of aromatic versus aliphatic materials in MGP wastes were relatively consistent and distinctly different from the proportions of those hydrocarbons in the other common petroleum products analyzed in this study. It is possible to describe the sample as more likely to contain MGP waste than a petroleum product.
Samples were collected from sites in the state of New York with known MGP contamination. The samples consisted of solids and liquids. Samples were collected from late-2002 until early-2003. Petroleum samples used in this study, (regular gasoline, kerosene, and home heating oil), were obtained from the Wadsworth Center’s repository of petroleum samples. The sample of natural gas was collected from the Center’s natural gas service. In general, approximately 100g of solid sample was placed in a modified TCLP vessel and incubated at 50ºC for 24 hours. A 500mL volume of headspace was sampled from the headspace vessel into a 6 Liter Summa canister, which was then filled using Zero Grade nitrogen. The Summa canister was allowed to equilibrate for 24 hours at room temperature prior to analysis. 1μL of each of the liquid samples of petroleum and MGP Waste was injected into a “closed loop” laboratory grade Zero Nitrogen sampling stream connected to a pre-cleaned evacuated 6 Liter Summa canister. The canister was filled as before and allowed to sit for 24 hours to reach equilibrium. In the case of the natural gas sample, a pre-cleaned Summa canister was filled with natural gas directly from the spigot in the laboratory. A 100mL aliquot of the gas was transferred to another pre-cleaned canister and diluted to volume with nitrogen. The Summa canister samples were analyzed following EPA Method TO-15.
The headspace sampling system is shown here. The vessel on the left is a 2.5 Liter jar with a lid and 3mm PTFE-lined septum. The lid has been fitted with a Swagelok fitting which is sealed with a cap nut while it is allowed to reach equilibrium at 50ºC for 24 hours. The nut is then removed and a 1/16” outside diameter stainless steel tube which has been cut to length to give a measured flow rate, is fitted between the cap and the pre-cleaned, evacuated Summa canister. The Summa canister is filled with a known volume of the headspace vapor by opening the connection with the headspace vessel for a certain period of time, e.g. in this case, where a 100mL per minute connection was used, a sample period of 5 minutes gave a 500mL sample.
The Summa canister with the headspace sample was then filled with Zero grade nitrogen. The flow control system is on the right side of the cabinet. The injection port used for the liquid samples is immediately to the right of the top of the Summa canister. The box to the left is a heater/thermal desorber used to clean sample transfer tubes.
The samples were analyzed for VOCs following United States Environmental Protection Agency (EPA) Method TO-15. The canister was connected to the inlet fitting of a Tekmar AutoCan concentrator and Agilent 6890/5973 GC/MSD analytical system. A known volume of sample was withdrawn from the canister by the inlet system, cryogenically focused and injected into the GC/MSD.
The samples were analyzed for VOCs following United States Environmental Protection Agency (EPA) Method TO-15. The canister was connected to the inlet fitting of a Tekmar AutoCan concentrator and Agilent 6890/5973 GC/MSD analytical system. A known volume of sample was withdrawn from the canister by the inlet system, cryogenically focused and injected into the GC/MSD.
Compounds in the sample were identified by comparing the retention time data from the total ion chromatogram (TIC) and mass spectral data of the sample peaks to known standards for 69 target volatile organic compounds and to a mass spectral database for non-target analytes. The largest peaks in the TIC were identified or tentatively identified, and the percent contribution of each peak to the total area in the TIC was calculated.
The VO Cs in the headspace of the MGP samples were not distinctive - no particular VOC or set of VOCs was found which would reliably indicate the presence of MGP waste. This chart shows the relative distributions of the most commonly identified VOCs in some of the MGP waste samples. The four samples on the left came from the same site, while the three on the right came from other sites. Other versions of this chart - normalizing the proportions, and looking at other indicators did not show any correlation or patterns which might reliably indicated the presence or absence of MGP waste.
In summary, the headspace analysis for VOCs found mostly aromatic hydrocarbons, which are also significant components of automotive gasoline. The relative proportions of those VOCs was not distinctive for MGP waste either.
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