Cavity ring-down spectrometers (CRDS) have the capacity to make isotopic measurements of CO₂ where concentrations range from atmospheric (~ 400 ppm) to 6000 ppm. Following field trials, it has come to light that the spectrographic lines used for CO₂ have an interference with elevated (higher than ambient) amounts of hydrogen sulfide (H₂S), which causes significant depletions in the δ¹³C measurement by the CRDS. In order to deploy this instrument in

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... ironments with elevated H₂S 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 H₂S interference at varying CO₂ and H₂S concentrations. The addition of H₂S to a CO₂ standard gas reveals an increase in the ₁₂CO₂ concentration and a more significant decrease in the ₁₃CO₂ concentration, resulting in a depleted δ¹³C value. Reacting gas samples containing H₂S with copper prior to analysis can eliminate this effect. However, experiments also revealed that the addition of H₂S to CO₂ results in the formation of carbonyl sulfide (OCS) and carbon disulfide (CS₂), causing a decrease in the overall CO₂ concentration without affecting the δ¹³C value. It is important for future work with CRDS, particularly in volcanic regions where H₂S is abundant, to be aware of the H₂S interference on the CO₂ spectroscopic lines and to remove all H₂S prior to analysis. We suggest employing a scrub composed of copper to remove H₂S from all gas samples that have concentrations in excess of 1 ppb.