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. 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.


Hydrobromic acid (48%) and cis-stilbene were purchased from Alfa Aesar (Karlsruhe, Germany). Trans-stilbene was obtained from TCI. All other chemicals were of analytical reagent grade. Deionized water (18 MΩ cm) was used for aqueous solutions.

3-Phenyloxirane-2-carboxylic acid
Following the synthesis of Corey and Ward (1986) and Shee et al. (2019) to 0.75 mmol trans-cinnamic acid in 500 μL acetone 3.3 mmol NaHCO3 in 500 μL in water was added dropwise. A solution of Oxone (1.4 mmol, 1.8 euiv. KHSO3) in 1.6 mL 0.4 mM ethylenediaminetetraacetic acid disodium salt was added dropwise over an hour while the temperature was kept approximately at. 20 25 °C and pH 7.5. After another hour of stirring the mixture was cooled to 0 °C and acidified with 12 M HCl to pH 2. After adding 5 mL ethyl acetate with rapid stirring the mixture was filtered and extracted with 3 times 50 mL ethyl acetate. The combined organic fractions were washed with NaCl and dried over MgSO4, filtered and dried under vacuum. 1 mL ethanol was added to the resulting oil and cooled in ice. After adding 3.6 mmol KOH in another 1 mL ethanol the mixture was filtered, washed with ethanol and dried under vacuum. Approximately 75 % of the total yield of the potassium salt was obtained. The crude product was used as a coating 25 material.

9,10-Epoxystearic acid
The synthesis of 9,10-epoxystearic acid followed Findley et al. (1945) with optimization of Milchert and Smagowicz (2009) and Milchert et al. (2010). 0.22 g acetic acid and 0.07 g sulfuric acid were added to 3.00 g oleic acid. While stirring for 15 min the solution was heated to 40 °C in a water bath. 1.08 g H2O2 were added with continued stirring to the black solution. The solution 30 turned colourless. After 4 hours stirring the phases were separated and 5 mL water was added to the organic phase and stored at 4 °C for 15 min. The colourless precipitation was filtered. And washed with water until the filtrate was pH 7. 6 mL hexane and 2 mL cyclohexane were added to 3.0 g raw product and heated to 50 °C. The product separated as a liquid phase underneath the solvents. After 16 h at 4 °C, colourless precipitation was obtained, filtered and dried. Pure products were isolated by SiO2 column chromatography with 1:1 of hexane and ethyl acetate as a mobile phase. 35 1.07-2.11 g product was obtained (37.5-73.9% yield).

5-Bromo-6-hydroxy-5,6-dihydro-[1,10]-phenanthroline (EPBr)
A solution of 1 mmol EP in 1 mL 48% aqueous HBr was stirred for 1 hour at room temperature. When neutralizing the solution with 8 mL aqueous saturated NaHCO3-solution the EPBr precipitated. The colourless solid has been extracted, washed twice with 2 mL saturated NaHCO3-solution and three times with 3 mL water. The product has been dried under vacuum at room temperature 45 resulting in 72% of the theoretical yield. This is a revised prescription of the principal reported in Chini et al. (1992) and Haufe et al. (1977). Synthesis of EPBr after Porter et al. (1995)

Denuder preparation
A system for the parallel coating of four denuders has been created (Fig. S3). The system can hold four denuders at an angle of 10°.
The denuders are not fixed tight so a system of a geared motor (Modelcraft, 12 V, 2.1 A) and toothed belts can rotate the denuders with approximately 87 rounds/min. Each denuder is connected to a N2-stream of approximately 0.5 L/min. The N2-Stream is drying the applied solution and prevents leaking of the solution on the lower end of the denuder. 60 Alternatively, denuder can be coated handheld. The denuder has to be connected to a N2-stream. After the application of solution, the denuder has to be rotated until the solution has dried. Particular attention has to be paid on the angle of the denuder to prevent leaking.

2.3
Gas Chromatography oven temperature programs  showing broad peaks between minute 7 and 8 presumably caused by 9,10-dihydroxystearic acid and 10-chloro-9-hydroxystearic acid. Note that the coating 9,10-epoxystearic acid disappeared, presumably has been used up by the reaction with water and HCl. (d) zoom of chromatogram (c) that shows 10-bromo-9-hydroxystearic acid the product of the coating with HBr and the high and variable background. We assume that the shift of retention times to an earlier position is caused by the overload of the analytical column from the coating.     115 Figure S9: Stability of extracted and concentrated samples. The samples were stored in the freezer at -4°C. Stability of field-like lab samples during the first 80 days. Assignment of the samples can be seen in Table S3. The regression model indicates that a collective significant effect was not found. The field samples described in Sect. 3.5 have been measured again after 2-3 years. All samples revealed a loss of -20 to -40 %. That refers to 0.03 ± 0.01 %/day. Note that the x-axis has a logarithmic scale.