Grantee Research Project Results

In Phase I, LGR has demonstrated technical feasibility by fabricating a MIR iCRDS system optimized for the detection of trichloroethylene (TCE) and tetrachloroethylene (PCE) that provides continuous unattended monitoring of these and several other compounds. LGR developed a custom suite of software to collect data automatically from multiple sample sources and two reference sources (zero air and known standard), process the data and fit the resulting spectrum to a basis set library to yield a quantitative measure of volatile organic compound concentrations in the ppb to ppt range. LGR performed advanced modeling on reinjection techniques to optimize the system performance. The instrument was shown to be precise: 5.6 ppb for benzene (1σ over 4 measurements), 1.57 ppb for TCE (1σ in 16 minutes), and 2.8 ppb for PCE (1σ in 16 minutes). When averaged, the precision increased to 500 ppt for TCE (1σ in 800 minutes) and 466 ppt for PCE (1σ in 258 minutes). The instrument was shown to be extremely linear, having a measured vs. actual slope of 0.993 ± 0.017 (R² = 0.99791) for TCE and 1.015 ± 0.010 (R² = 0.99791) for PCE. Once calibrated with a known standard, the instrument was found to be accurate to 0.52 ppb for TCE at 88.7 ppb and to 3.56 ppb for PCE at 97.5 ppb. The minimum detectable concentration was found to be 3.57 ppb for TCE and 7.68 ppb for PCE at a SNR ratio of 3σ. The instrument was found to have cross interferences for TCE with water but not PCE with water. It was shown that the TCE interference can be completely eliminated by drying the air with a Nafion drier. It was found that both TCE and PCE are measured accurately when a contaminated wet stream is dried using Nafion. By varying the concentration of a TCE + PCE stream, it was found that there was no cross interference between these species despite an overlap between their absorption lines.

 

After these laboratory validation studies the instrument was deployed to the Superfund site at Moffett airfield for one week where it measured absorption spectra unattended. Ground water at the site has been contaminated from engine cleaning shops, dry cleaners and semiconductor processing. Building 10 at Moffett airfield is a known vapor intrusion site for cis-1,2-dichloroethylene, TCE and PCE through steam tunnels with ground water intrusion. During the deployment, the instrument measured breathing zone air and steam tunnel air with automatic, periodic measurement of zero air and known concentration of PCE in zero air. The site recently was equipped with ventilation to draw air from the breathing zone, through the tunnel to an external exhaust, and LGR was permitted to turn this ventilation off for the weekend at the end of the deployment. During this period, LGR observed a significant rise in the TCE and PCE concentration in tunnel air that was consistent with previously values measured before ventilation by summa canister collected GC/MS (Figure 1).

 

 

With these laboratory data, LGR has shown that the instrument is precise and accurate to the ppb level and has an extremely linear response, with an MDC in the low ppb range for TCE and PCE. Additionally, LGR has shown that interferences with water can be eliminated using Nafion drying for TCE and PCE and that these two compounds do not interfere with each other when present in the same gas sample.

 

The successful deployment, with measurement and quantitative reporting of TCE and PCE concentrations in breathing zone and tunnel air demonstrate the feasibility of the iCRDS technique for autonomous, continuous, in-the-field monitoring of ppb levels of toxic VOCs from vapor intrusion.

 

Dr. Leen has been invited to present this instrument and the Phase I deployment results at several EPA events, and LGR will present literature for the instrument at the upcoming fall meeting of the American Geophysical Union in San Francisco, CA.