Real-Time FTIR Monitoring of Parts Per Trillion Level Vapor Intrusions and/or Airborne Molecular Contaminations
(Geosyntec Consultants, 8217 Shoal Creek Blvd., Austin, TX.)
Low level vapor and aerosol emissions, even those of a transient nature, from various open or leaky chemical process activities can cause environmental and health concerns, as well as detrimental impacts on product yields (as with semiconductor wafer manufacturing, for instance). Characterization of these intrusions may call for real-time monitoring techniques, but achieving adequate pptv-level sensitivities with temporal measurement resolutions is challenging. Presented here is an air monitoring method based on specialized signal-to-noise enhancements of an extractive FTIR spectrometer. The hardware and software techniques incorporated in this system have been proven in the Geosyntec laboratories to enable sub-ppbv detection limits for various gaseous acids and amines, some gaseous and aerosolized arsenic compounds, and various VOC’s. Laboratory calibration and EPA Method 301 field validation procedures were carried over to additional field measurement campaigns at some semiconductor fabs, outdoor agricultural sites and indoor public facilities. The nature of vapor intrusion or airborne molecular contamination (AMC) testing often involves the collection of multiple time integrated air samples at strategic points within an area of interest, followed by laboratory analysis of these samples via some precision analytical method(s), ala EPA Method TO-15. The air sampling containers usually are specially prepared canisters, tedlar/Teflon bags, sorbent tubes, filter media (for particulate/aerosol matter), or even wet impingers. All of these require some preparation prior to field used and are not capable of providing “real-time” measurement results on-site. Also, any information associated with vapor intrusion or AMC events that may be considered transient or intermittent is lost as these sampling schemes combine volumes of air over a period of time before any analysis takes place. Such information would be critical in understanding the chemistry and by-product formation associated with certain vapors, as well as their acute exposure impact on local workers/inhabitants and manufacturing processes. The measurement method presented here is an EPA recognized one (see EPA Method 320) that can provide detection and profiling for multiple target analytes, simultaneously, that enables on-site characterization of emissions with ample time resolution. The 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. Recent work at Geosyntec has focused on driving down real-time detection limits to levels well below 1 ppbv. Enhanced iterative software algorithms are then invoked to achieve real-time detection limits (DLs) in the hundreds of pptv, with sample integrated (8-hour spectral coadded) DLs approaching the tens of pptv.