H2S interference on CO2 isotopic measurements using a Picarro G1101-i cavity ring-down spectrometer

K. Malowany, J. Stix, A. Van Pelt, G. Lucic
2015 Atmospheric Measurement Techniques Discussions  
Cavity ring-down spectrometers (CRDS) have the capacity to make isotopic measurements of CO<sub>2</sub> where concentrations range from atmospheric (~ 400 ppm) to 6000 ppm. Following field trials, it has come to light that the spectrographic lines used for CO<sub>2</sub> have an interference with elevated (higher than ambient) amounts of hydrogen sulfide (H<sub>2</sub>S), which causes significant depletions in the δ<sup>13</sup>C measurement by the CRDS. In order to deploy this instrument in
more » ... is instrument in environments with elevated H<sub>2</sub>S concentrations (i.e., active volcanoes), we require a robust method for eliminating this interference. Controlled experiments using a Picarro G1101-i optical spectrometer were done to characterize the H<sub>2</sub>S interference at varying CO<sub>2</sub> and H<sub>2</sub>S concentrations. The addition of H<sub>2</sub>S to a CO<sub>2</sub> standard gas reveals an increase in the <sub>12</sub>CO<sub>2</sub> concentration and a more significant decrease in the <sub>13</sub>CO<sub>2</sub> concentration, resulting in a depleted δ<sup>13</sup>C value. Reacting gas samples containing H<sub>2</sub>S with copper prior to analysis can eliminate this effect. However, experiments also revealed that the addition of H<sub>2</sub>S to CO<sub>2</sub> results in the formation of carbonyl sulfide (OCS) and carbon disulfide (CS<sub>2</sub>), causing a decrease in the overall CO<sub>2</sub> concentration without affecting the δ<sup>13</sup>C value. It is important for future work with CRDS, particularly in volcanic regions where H<sub>2</sub>S is abundant, to be aware of the H<sub>2</sub>S interference on the CO<sub>2</sub> spectroscopic lines and to remove all H<sub>2</sub>S prior to analysis. We suggest employing a scrub composed of copper to remove H<sub>2</sub>S from all gas samples that have concentrations in excess of 1 ppb.
doi:10.5194/amtd-8-5651-2015 fatcat:runuxesgczhyrdik4qgrcee2qy