In situ speciation of sulfur vapors by X-ray absorption near edge structure spectroscopy

• We developed a furnace system for XANES spectroscopy to measure vapors and gases.
• The optimized design reduces potential contamination from air, water, and steel.
• Sulfur gases created from heated solids (363 to 550 K) comprise different species.
• The findings resolve long-held interpretive discrepancies for S vapor speciation.
• Quantified S vapor species can solve kinetic problems in geologic systems.

Despite the tremendous importance for geologic systems, there is limited knowledge of sulfur-containing gas-phase geochemical reactions that take place at elevated temperatures, including up to 1000 K. This deficit is at least partly caused by a lack of suitable experimental techniques to monitor and quantify potential reactions. We developed a new furnace design that can heat solid samples to specific temperatures while in situ X-ray absorption spectra of the gas-phase species are collected at the sulfur K-edge at about 2400 eV for X-ray absorption near edge structure (XANES) spectroscopy. Our experimental design improves previous furnace systems developed to generate sulfur vapors for spectrometry measurements and earlier sulfur XANES spectroscopic analyses of sulfur-containing gases and vapors. The new design minimizes contamination by air, water vapor, and stainless steel. Our results demonstrate that vapors emitted from elemental sulfur heated to different temperatures are similar to previous findings, but we can now resolve long-held interpretative discrepancies for sulfur vapor speciation, some of which resulted from the misidentification of vapors due to reactions between the sulfur vapors and furnace materials. We also used the new design to quantify the sulfur vapor species produced from elemental sulfur reacted with water over a range of temperatures. At low temperatures, like 363 K (~ 90 °C), the gas consisted of different sulfur species, whereas at temperatures greater than 550 K (~ 280 °C), the gas present in the furnace was only SO2. These quantitative findings show promise that future reaction kinetics can be done by using our experimental spectrometry system.