A laboratory comparison of chemical volcanic gas sampling techniques using an artificial fumarole

Volcanic gases play a critical role in nearly all volcanic processes. A number of chemical and spectroscopic techniques have been developed in recent decades to measure volcanic gas output. Giggenbach bottles, alkaline traps, filter packs, and venturi samplers are examples of chemical gas sampling techniques involving the reaction of acidic gas with an alkaline solution or a solid hydroxide salt on a filter. Aside from Giggenbach bottles all of these methods are disequilibrium techniques relying on the assumption that the kinetics of gas reaction are identical for all species. Given that equilibrium species solubilities differ, there is reason to suspect this assumption. While these chemical sampling methods have been used extensively in the field, we are aware of no direct calibration in a lab setting. To address this problem we have built an artificial fumarole in the laboratory for quantitative comparison and evaluation of gas sampling techniques. The fumarole consists of a metered mixture of H2 CO2 SO2, CO gas combined in a high-temperature mixing chamber with an aqueous solution of HCl and HF. The resulting equilibrium high-temperature gas is sampled at the furnace exit. Several gas mixtures were considered representing a range of compositions encountered at natural fumaroles. Results of analysis are compared with equilibrium gas concentrations calculated from the input gas composition and furnace temperature. We compared Giggenbach bottles, a variety of filter-pack chemistries, and passive alkaline trap samplers. In addition, we tested a venturi sampler of our own design; optimized for ruggedness, ease of field use, reliability and sampling efficiency. Each sample was analyzed by ion chromatography both before and after oxidation with H2O2 for fluoride, chloride, and several sulfur species. While reduced sulfur is not easily analyzed by IC, it can be determined from the difference in concentrations between total S in the oxidized samples and the other sulfur species in the non-oxidized samples. Preliminary results show that the alkaline trap is the least accurate method, while the remaining techniques show varying degrees of success. Our redesigned venturi sampler works particularly well and promises to be a very successful field instrument.