Mapping air contaminants indoors using a prototype computed tomography system.
Ann Occup Hyg 1996 Dec; 40(6):675-691
The use of a prototype open path Fourier transform infrared computed tomography (OP/FTIR/CT) system to map the locations and levels of a gas compound released in an exposure chamber was examined. A sulfur- hexafluoride (SF6) concentration of 5,000 parts per million was added to the exposure chamber. Point samples were analyzed with a gas chromatograph equipped with an electron capture detector (GC/ECD) and were reconstructed with the OP/FTIR/CT system, in which a single spectrometer was employed. Two different optical ray configurations, a 100 ray and a 136 ray configuration, were investigated. An iterative tomographic reconstruction algorithm, the maximum likelihood solution using expectation maximization, was used to generate a two dimensional concentration map. The OP/FTIR/CT system accurately located single peaks within 1 foot of the maximum peak concentration in 16 of the 20 experiments performed using the 100 ray configuration and in 17 of the 18 experiments performed with the 136 ray configuration. All of the double peaks were accurately positioned when the 136 ray configuration was employed. However, when the 100 ray configuration was used, many artifacts were observed. Differences in the shapes of the peaks and the presence of artifacts were observed between the OP/FTIR/CT maps and the GC/ECD maps. Despite these discrepancies, the OP/FTIR/CT maps accurately located the highest concentration peak, based on the GC/ECD maps. However, in 30 out of 34 OP/FTIR/CT maps, the peak concentrations were underestimated. Peaks were reconstructed within 4 to 50% of the GC/ECD estimations. The apparent discrepancies in peak concentration decreased when the differences in peak width were also considered. The authors conclude that the above method offers a promising and accurate approach to determining the distribution of gaseous molecules in an enclosed space.
NIOSH-Publication; NIOSH-Grant; Grants-other; Exposure-chambers; Spectrographic-analysis; Analytical-methods; Gas-sampling; Gas-chromatography; Gas-detectors; Analytical-instruments
Environmental Sciences & Engr University of North Carolina CB #7400 Chapel Hill, NC 27599-7400
Other Occupational Concerns; Grants-other
Annals of Occupational Hygiene
University of North Carolina Chapel Hill, Chapel Hill, North Carolina