A study of laser-induced breakdown spectroscopy for analysis of the composition of solids submerged at oceanic pressures

The application of laser-induced breakdown spectroscopy for analysis of the chemical composition of solids immersed in water at oceanic pressures has been investigated. Well defined emission spectra were observed from plumes generated from underwater solids after excitation using a single laser pulse of duration less than 10 ns. It is demonstrated that an increase in water pressure from 0.1 to 30MPa (300 atm) does not have a significant effect on the intensity and broadness of the observed spectral lines. Shadowgraph images demonstrate that even at pressures of 30MPa, beyond the critical pressure of water, cavitation occurs around the ablated region. Furthermore, it is demonstrated that during the early stages, less than 1 μs, after irradiation the size of the cavity is largely independent of the external fluid pressure for pressures up to 30MPa. It is suggested that the high pressure shock wave induced by the focused laser dominates the local pressure regime for close to 1 μs after irradiation and generates a transient low pressure region in which a cavity can form for the plume to expand into. Measurements of craters formed in the solids after ablation at different pressures demonstrate that the amount of material ablated by the laser stays within the same order for all hydrostatic pressures tested. The results of this study suggest that laser-induced breakdown spectroscopy is, in principle, a technique suitable for in situ elemental analysis of both shallow water sediments and deep sea minerals.