Evaluation of techniques for sampling volatile arsenic on volcanoes

Abstract

Volatile arsenic (As) species, like arsine, mono-, di-, and trimethylarsine (AsH3, MeAsH2, Me2AsH, Me3As) are reported to be released from volcanoes but their determination is difficult because of low concentrations, low boiling points, and high reactivity, especially in the presence of volcanic gases like H2S and SO2. We tested needle trap devices (NTDs), cryotrapping, and Tedlar® bags for quantitative and species-preserving sampling. NTDs did not trap AsH3, MeAsH2, Me2AsH, did not release sorbed Me3As quantitatively, and lead to artifact formation of dimethylchloroarsine, which also questions the reliability of previous reports from solid phase micro extraction fibers using the same sorption materials. Cryotrapping in dry ice was insufficient to trap AsH3 and MeAsH2; Me2AsH and Me3As were only partially retained. Sampling in Tedlar® bags remained the best alternative. Stability of all four arsines was confirmed for dark storage at 5°C for 19days in a matrix of dry N2, 11days in 20% O2, and 19days in 3800ppmv CO2 (>80% recovery for all species), while in the presence of H2S, Me3As recovery was only 67% and in the presence of SO2, Me2AsH and Me3As recovery was 40 and 11%, respectively. Removing interfering reactive gases by a NaOH trap, we sampled natural volcanic emissions at fumaroles of Vulcano and Solfatara (Italy). Detected total arsine concentrations of 0.5–77ng·m−3 were 1–2 orders of magnitude higher than the calculated background. Inorganic arsine was the dominant species suggesting that secondary microbially catalyzed methylation is a process of minor importance in the fumarolic gases.