Quantifying H2S with a Picarro CRDS G2201-i and the effect of H2S on carbon isotopes
Abstract. Cavity Ring-Down Spectroscopy (CRDS) is a popular analytical method with important applications in earth sciences including volcanology. A main disadvantage of using CRDS in volcanology is that the presence of H2S distorts some spectral lines causing errors in the measurements. In this study, we investigated the effects of H2S on measurements using a Picarro G2201-i instrument. We defined the interferences caused by H2S on CO2, CH4, and their carbon isotopic compositions. We found that 30 ppb H2S in 1000 ppm CO2 causes a difference of ~1.0 ± 0.2 ‰ on the δ13C-CO2 measurement, while 1 ppm H2S in 1 ppm CH4 per causes a difference of < 0.2 ‰ on the δ13C-CH4 measurement; this agrees with the results from previous studies using other models of Picarro instruments. Characterizing how H2S produces these interferences as a function of concentration, we further developed a series of equations to quantify H2S in gas mixtures in a concentration range of 1 to 270 ppm. We validated our method by analyzing a natural dry gas sample and comparing our results with those of two other independent analytical techniques, namely a CH4-MultiGAS and a “Giggenbach bottle”. When comparing the results between the CH4-MultiGAS and the Picarro G2201-i, we measured differences of ~ 4 %, while when comparing the results between the Giggenbach bottle and the Picarro G2201-i, we measured differences of ~ 9 %. The results of these three techniques show excellent agreement within error of each other. Our study demonstrates that the Picarro G2201-i instrument can accurately and precisely measure CO2, CH4, and H2S concentrations in the gas phase.
Status: open (until 08 Mar 2024)
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