Real-time analysis of the fine particles in volcanic plumes: A pilot study of Laser Induced Breakdown Spectroscopy with Calibration-Free approach (CF-LIBS)

Volcanic plumes are rich in fine juvenile ash, representing magma fragments expelled from the crater during explosive eruptions. The composition of fine ash is of primary importance for volcano monitoring and hazard assessment, but not much effort has been put until now to build portable instrumentation allowing real-time measurements. In this paper, a laboratory experiment is carried out to build and test an apparatus based on Calibration-Free Laser Induced Breakdown Spectroscopy (CF-LIBS). This methodology is extremely promising because it does not require any standard calibration strategies and the elemental composition is directly obtained by the laser-induced breakdown optical emission. In the experiment, volcanic ash samples are suspended in the air by a laser-induced shockwave in a dedicated chamber, as to mimic the conditions of a volcanic plume, and contemporaneously the laser-induced plasma (LIP) spectrum is analyzed. After spectrum acquisition, the quantitative analysis is performed using the so-called Calibration-Free (CF) analytical method, which is based on the assumption of plasma local thermodynamic equilibrium (LTE). Results show that by such methodology it is possible to measure ash composition in real-time, during its dispersion, with good precision. Based on the result of this experiment, the construction of a portable instrument to be used in the field for the monitoring of active volcanoes could be possible in the future.