Bromine speciation in volcanic plumes: new in situ derivatization LC-MS method for the determination of gaseous hydrogen bromide by gas diffusion denuder sampling

Abstract

Abstract. The chemical characterization of volcanic gas emissions gives insights into the interior of volcanoes. Bromine species have been correlated with changes in the activity of a volcano. In order to exploit the volcanic bromine gases, we need to understand what happens to them after they are outgassed into the atmosphere. This study aims to shed light on the conversion of bromospecies after degassing. The method presented here allows for the specific analysis of gaseous hydrogen bromide (HBr) in volcanic environments. HBr is immobilized by reaction with 5,6-epoxy-5,6-dihydro-[1,10]-phenanthroline (EP), which acts as an inner coating inside of diffusion denuder tubes (in situ derivatization). The derivative is analyzed by high-performance liquid chromatography coupled to electrospray ionization mass spectrometry (HPLC-ESI-MS). The collection efficiency for HBr (99.5 %), the collection efficiency for HBr alongside HCl (98.1 %), and the relative standard deviation of comparable samples (8 %) have been investigated. The comparison of the new denuder-based method and Raschig tubes as alkaline traps resulted on average in a relative bias between both methods of 10 ± 6 %. The denuder sampling setup was applied in the plume of Masaya (Nicaragua) in 2016. HBr concentrations in the range between 0.44 and 1.97 ppb were measured with limits of detection and quantification below 0.1 and 0.3 ppb respectively. The relative contribution of HBr as a fraction of total bromine decreased from 75 ± 11 % at the Santiago crater (214 m distance to the volcanic emission source) to 36 ± 8 % on the Nindiri rim (740 m distance). A comparison between our data and the previously calculated HBr, based on the CAABA/MECCA box model, showed a slightly higher trend for the HBr fraction on average than expected from the model. Data gained from this new method can further refine model runs in the future.