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Assessment of Hydrogen Sulfide Minimum Detection Limits of an Open Path Tunable Diode Laser
HASHMONAY, R., M. MODRAK, AND E. D. THOMA. Assessment of Hydrogen Sulfide Minimum Detection Limits of an Open Path Tunable Diode Laser. In Proceedings, AWMA's 101st Annual Conference and Exhibition, Portland, OR, June 24 - 27, 2008. AWMA, Pittsburgh, PA, NA, (2008).
During June 2007, U.S. EPA conducted a feasibility study to determine whether the EPA OTM 10 measurement approach, also known as radial plume mapping (RPM), was feasible. A Boreal open-path tunable diode laser (OP-TDL) to collect path-integrated hydrogen sulfide measurements along single optical paths downwind of suspected H2S source areas. The data collected were used to assess the minimum detection limit (MDL) of the OP-TDL instrumentation. The results of the analysis of the field data suggests that the instrument MDL increases considerably when using longer path lengths. This is due to the fact that the instrument light intensity is lower along longer beam paths, and optical noise will increase as the beam diverges with increasing distance, leading to a lower signal-to-noise ratio. For future studies, it is possible to improve the instrument MDL by using larger retro-reflecting mirrors in the field and measuring over shorter path lengths when possible. Qualitatively, it was observed that the OP-TDL instrument was overestimating the actual hydrogen sulfide concentrations, as the levels approached the instrument MDL. Based on an analysis of the path-integrated concentration (PIC) values collected with a corresponding R2 value <0.20, a correction factor was developed. This factor was applied to all PIC values from the study with a corresponding R2 value less than 0.20 to retrieve a more representative PIC value.