Vapor Intrusion Assessment: From Passive Sampling to Real-Time Monitoring

Vapor monitoring is necessary for protecting the public from exposure to potentially toxic pollutants from industrial and/or naturally occurring processes. Low level vapor and aerosol emissions whether from open or leaky chemical process activities, can cause environmental and health concerns, even when transient. Vapor intrusion (VI) is the migration of volatile chemicals from soil and/or groundwater into buildings, which may expose individuals to harmful levels of  volatile organic compounds (VOCs).¹ Advancements in our understanding of VI pathways, along with increasingly stringent regulatory guidance, requires accurate and precise analytical detection methods that are flexible enough to meet the desired risk based screening levels.

Fundamental advancements in understanding vapor intrusion pathways require advances in analytical techniques to better determine risk.

Conventional Active Air Sampling

Conventional VI testing involves the collection of multiple air samples over time at strategic points within an indoor (or outdoor) area of interest, such as a below a floor slab in a building. Samples are analyzed by standardized analytical methods such as EPA Method TO-15² or TO-17.³ Air sample collection typically requires  the use of specially prepared canisters (e.g., Summa canisters), cylinders, or sorbent tubes connected to a pump that actively draws air. These approaches require advance preparation of the sampling apparatus and are limited to collection of short duration “grab samples”. Due to the short sampling duration of these methods, VI events that may be transient or intermittent are often missed. Sample collection over a longer duration or in real-time is considered advantageous providing both discrete (real-time) or time averaged (passive sampling) data. In the case of real-time monitoring, evaluating potential acute exposure impacts on local workers/inhabitants and industrial processes becomes a possibility.

Quantitative Passive Sampling for Vapor Intrusion

Passive sampling has been growing in popularity for the measurement of VOCs due to lower costs, simpler sampling protocols and ease of deployment, with applications ranging from indoor/outdoor air quality to soil VI assessments. Quantitative passive sampling is the collection of vapors by diffusion or permeation in response to concentration gradients at known and controlled uptake rates, such that the time-weighted average concentration can be calculated from the mass of each analyte collected over a given period of time.⁴ The passive sampler acts as a sink for the analytes, which establishes the concentration gradients. The sampler is exposed to the air for a measured amount of time (t), during which VOCs diffuse or permeate into the device and a certain mass (M) of VOCs will be trapped on the adsorptive medium within the device. Once the adsorbed mass has been quantified, the time weighted average concentration of a particular analyte in the atmosphere being sampled can be calculated using the simple formula:

Passive sampling is less susceptible to extreme variations in concentration than active sampling techniques.

Compound specific uptake rates are typically measured in controlled laboratory tests, where VOCs at known concentrations are exposed to samplers over time. The ability to quantify concentrations is an important improvement over semi-quantitative passive sampling, because knowing indoor air concentrations allows comparison to risk-based targets, when assessing human health risks due to VI.

Waterloo Membrane Sampler™

The Waterloo Membrane Sampler™ (WMS™) is a passive permeation sampler patented for quantitative soil gas analysis and VI studies (https://www.siremlab.com/waterloo-membrane-sampler-wms/). The WMS™ incorporates a polydimethylsiloxane membrane across the face of a vial filled that is filled with a sorbent medium and has several advantages over alternate sampling methods including resistance to water vapor, low cost, ease of use, small size and predictable uptake rates for virtually any VOC. Tuning the sampler for lower uptake rates reduces the risk of the “starvation effect”, which occurs when the uptake rate of the sampler exceeds the rate of transport of chemicals to the face of the sampler.⁴ This situation results in a reduction in vapor concentrations near the sampler, and can lead to a low (or negative) bias in the calculated concentrations. Preventing starvation by controlling uptake rates allows the WMS™ to provide quantitative VI assessment for long-term monitoring of VOCs that correlates well with active sampling by TO-15.⁴

Real-Time Fourier Transform Infrared (FTIR) Continuous Monitoring for Vapor Intrusion

Real-time VI monitoring uses instruments that quantify vapors and provide data that allows changes over time to be quantified by real-time monitoring techniques have the advantage of providing a continuous data-feed. Previous challenges in achieving adequate sensitivity have limited these applications. Fortunately, new solutions and technologies have been developed that make this application a reality. A specialized extractive Fourier Transform Infrared (FTIR) spectrometer has been developed and calibrated to sensitivities that allow for real time VI monitoring as most gas molecules absorb light proportional to their concentration and the presence of specific compounds  can be determined by their spectral signatures. The hardware and software enhancements incorporated into the FTIR spectrometer allow for sub-parts per billion by volume (ppbv) detection limits for various gases, is EPA approved,⁵ and able to provide detection and profiling for dozens of different target analytes, simultaneously, to enable on-site characterization of emissions with ample time resolution. The sampling and measurement procedure for FTIR spectroscopy consists of a central analyzer system, which runs in a continuous and automated fashion. Care is taken to maintain sample integrity so that quantitative measurements are conducted of species in their natural state.  Spectral calibrations for several molecules of interest may require special laboratory setups before field application, but once they become part of the quantitative analysis library, they are easily transferable to subsequent real-time field VI measurement projects. The traditional limitations of extractive FTIR have often pointed to its relatively higher detection limits (tens to hundreds of ppbv) when providing data on minute-by-minute time scales but the recent developments focused on “enhanced” FTIR instrumentation and quantitative analysis software to drive down real-time detection limits to levels well below 1 ppbv, as shown in the Table 1, make real-time VI monitoring a reality.

Table 1: Detection limits of FTIR Spectroscopy with Software Enhancements

The advancement in FTIR techniques allows for quantification of VOCs in real-time for vapor Intrusion assessments.