In many cases, vapor intrusion (VI) investigations must rely on Volatile Organic Compound (VOC) measurements in soil gas samples, in order to find potential inhalation risks to residents and occupants close to the site. Although the EPA Method TO-15 can be used to configure the required Sub-Part per billion by volume (ppbv) risk-based screening levels, it does not record the intrusive matrix compounds. Laboratories typically must use different methods to increase detector sensitivity, such as dilution. Soil Vapor’s matrix compounds must be diluted in order to minimize the gross contamination. Although dilution is known as the “one practical option,” we still get high dilution factors, elevated reporting limits, and concentrations marked as not being detected. Therefore, although dilutions can help break down the matrix compounds, they are not ideal. A modified TO-15 method was introduced with the goal of achieving the required sensitivity limits, and resolve selected risk drivers found in a high concentration matrix. TO-15 High Selectivity and Sensitivity (TO-15 HSS) is a known as an enhanced TO-15 method and multidimensional gas chromatographic system. This system incorporates a custom Gas Chromatograph (GC), which has three columns where compounds are alluded. This allows for the risk drivers to be resolved from any remaining interference compound before detection into the mass spectrometer. This process is used in order to remove the matrix from the sample and isolate the target risk drivers, yet it also allows a reduction in the sample load volume.
In a study, the TO-15 HSS method was applied to two compounds of concern which are commonly found in soil gas at petroleum-impacted sites: Ethylene Dichloride (EDC) and Ethylene Dibromide (EDB). The goal of this study was to consistently achieve laboratory reporting limits below Environmental Protection Agency Resident Vapor Intrusion Screening Levels (VISL) in sub-slab and near source soil gas. All the requirements to positively identify EDC and EDB in the highly contaminated test matrix were met, including retention time, ratio of extracted ions and comparison of the sample spectrum to the NIST reference spectrum. In a field sample performance, the TO-15 HSS method was applied. There was a range of real-world petroleum impacted vapor samples evaluated. Results showed Total Petroleum Hydrocarbons (TPH) as high as 5% and whereas naphthalene was not detected at 25,000 ppbv in the conventional TO-15 GC/MS, however it was detected with the TO-15 HSS with a reporting limit of .25 ppbv.
In conclusion, the TO-15 HSS application demonstrates a significant improvement in sensitivity and selectivity for the target EOC’s (risk drivers) in high matrix vapor samples. The custom Gas Chromatograph configures to isolate EDB and EDC, as well as other compounds. Time of flight provides reporting limits low enough to effectively construct the risk and support decisions for sight closure. The TO-15 HSS is a specialized version of the TO-15, which incorporates time of flight and allows us to analyze samples with high matrix interference without dilution and effectively assess the risk of sites.
TestAmerica. (Producer). (2019). Enhancing EPA Method TO-15 Using TOF-MS to Address High Matrix Effects in Soil Gas [Video Webinar]. Retrieved 7/17/2019
TO-15 High Sensitivity/Selectivity. TestAmerica. (Oct. 2, 2018). Measuring Trace Level VOCs in High Concentration Soil Gas Matrices – A New Tool to Meet Risk-Based Screening Levels.