The goal of this research was to demonstrate and validate a capillary flow controller used for long-term air sampling to characterize chronic indoor air exposures from vapor intrusion (VI). This technology is a potential improvement over current technology which limits the duration of collection of a single SummaTM canister to 24 to 48 hours. The new canister method provides most of the advantages of both canisters and sorbent samplers and avoids many of their limitations by allowing for long-term (1-3 weeks) sample collection and characterization of volatile organic compounds (VOCs) in buildings at risk for VI. The costs of sampling and analysis are typically 30 to 70% of life-cycle costs (Environmental Protection Agency [EPA] 2004). The long-term capillary flow controller can substantially reduce these costs by decreasing the number of samples collected over the course of an investigation or long-term monitoring period as a result of minimizing variability. The approach is robust, comparable in cost or less expensive than current methods, allows for long-term sample collection, and requires one sample to capture a full range of analytes and concentrations.
Evacuated canisters are used to collect air samples in both indoor and outdoor air environments for the last three decades. Studies have shown good stability and recovery of many contaminants. The airflow into the canister is typically controlled using a diaphragm or critical orifice. These flow devices generally allow for sampling periods from a few minutes to 24 hours, depending upon the canister size and contaminant of interest (8-hours using 400 mL and up to 24-h using 6 L canisters). The current controllers in practice tend to be unreliable at durations greater than 24-hour (EPA TO-15).
A capillary flow controller (Figure 1) was developed to sample at lower flow rates of air into a sampling canister to allow for more reliable long-term sampling (Rossner 2002; 2004; 2005). With the development of this flow control device, the use of evacuated canisters for longer term sampling in typical or smaller canister volumes became possible. The capillary flow controller is a variation on a sharp edge orifice flow controller commonly used to control the flow of air into an air sampling canister. It essentially controls the velocity of the air as a function of the properties of the capillary diameter and length. The longer the capillary is, the slower the flow rate.
Figure 1. Capillary flow controller coupled with a standard air sampling canister.
Log-transformed concentration data are normal and are thus used as the outcome variable for the assessment of variable interactions. The results of the multivariate analysis suggests association between temperature, relative humidity, and sample date on trichloroethene concentration. A strong correlation was observed between the ratio of the 24 hour (diaphragm flow controller) and 6L and 15L (capillary flow controllers) respectively (r = 0.9655). In addition, a reasonable and expected correlation was observed between temperatures and humidity (r=0.6330). While the relationship between the average of the 24-hour canisters and 14-day canisters is strong, and the regression analysis displayed a strong correlation, further analysis was carried out to examine if an interaction effects between the parameters was impacting the outcome.
At the time the project team initiated the field portion of this study, they anticipated the need to track personnel time, sampling time, travel cost and shipping cost for the canisters. However, the diaphragm canister system and the capillary flow controller system were prepared, deployed, retrieved and analyzed in the same manner. The only difference was that the capillary flow controller sampling for 14-days required an additional trip to retrieve the canisters.
The long term flow controller performed well in the field under significant temporal and spatial variation while the low flow rates remained relatively consistent. Data from this project as well as from other VI investigations suggest indoor VOC concentrations can fluctuate by orders of magnitude from one day to another and from season to season. Long term sampling may be more representative of the long term average concentration within a building, thus, relying on one day averages may result in under or over estimation of VI. In addition, with shorter sampling times such as 24 hours, the timing of the sampling becomes much more critical and can be a limitation to interpreting the results. The ability to collect an air sample over a long period (weeks) of time can help enhance VI investigations by providing a larger fraction of days sampled vs the traditional 24 hour sample.
Current concerns for users should be limited in most areas because the technology is so similar to sampling with the current diaphragm flow controller. The benefit of gaining so much more data, 49 multiple days for the same cost, than 24 hours sample will be useful in the decision-making process. However, information from the traditional 24-hour sample have been used by regulators and organizations for many years, therefore it may be a concern how well these 14-day samples can be interpreted with respect to the 24-hour samples.
Nocetti, D., M. Crimi, and A. Rossner. 2019. Sampling Strategies in the Assessment of Long-Term Exposures to Toxic Substances in Air. Remediation.