Study on laser-induced breakdown spectroscopy in high-pressure helium gas environment for deep ocean applications

Abstract

Laser-induced breakdown spectroscopy (LIBS) has gained wide acceptance as an in situ detection technique for elements. However, in deep-sea applications, the sensitivity of LIBS detection will be reduced due to the high-pressure environment and the nature of water. To address the negative effects of high-pressure water, this study used the method of draining the water from the sample surface by passing high-pressure helium gas, which allowed the plasma excitation environment to be converted from high-pressure water to high-pressure gas. The available spectral signals of solid samples at 60 MPa gas pressure were obtained for the first time, and the peak intensity and spectral broadening of the spectral lines at different pressures were analyzed for comparison. We found a nonlinear decrease in the spectral intensity and a gradual increase in the spectral broadening during the pressure increase. We also investigated the effect of laser energy on the intensity and width of the spectral lines in a high-pressure helium environment and found that increasing the laser energy in a high-pressure environment enhanced the spectral intensity and that the change in laser energy almost did not affect the line width. Finally, by observing the plasma images at different pressures with different energies, this study found that the laser penetrated the high-pressure helium gas in advance and leaved a column of light in the gas, and the plasma was slowly made smaller as the ambient pressure increases. This finding explained the cause of laser energy loss and demonstrated that the LIBS signal intensity can be improved by increasing the laser energy and shortening the laser transmission distance in a high-pressure gas environment.