Open-path laser-based hydrogen sulfide detection at single-ppm level

Hydrogen sulfide (H
₂
S) is a highly toxic gas that can be frequently found in the vicinity of industrial sites such as petroleum refineries, natural gas plants, or waste water treatment facilities. Its open-path detection with sensitivities at single ppm (parts-per-million) levels is critical for safety reasons. Laser-based H
₂
S sensing typically relies on a quartz-enhanced photo-acoustic spectroscopy [1, 2] or methods that use high fineness cavities [3] providing minimum detection limits from few tens of ppm down to below 1 ppm. Unfortunately, both techniques are (in general) not suitable for open-path sensing, they require additional pumps, pressure controllers and sampling lines. Moreover, in both cases operation in a close-cell arrangement with reduced pressure helps to avoid interference from other atmospheric species (this can significantly alter the measurement results in open-path arrangements). In this work we will present an open-path system for H
₂
S detection that relies on a wavelength modulation spectroscopy (WMS) and a Herriott-type multipass cell. We will demonstrate that using this non-complex sensing approach the detection at single ppm level with negligible interference from other gases is possible.