Effect of repeated irradiation on laser-induced breakdown spectroscopy of copper immersed in a sodium chloride aqueous solution and normalization with bubble collapse time

Abstract

Underwater laser-induced breakdown spectroscopy (underwater LIBS) has attracted attention as an on-site analytical technique for deep-sea investigation, but it suffers from serious signal fluctuation. In this study, we analyzed a copper (Cu) plate immersed in a sodium chloride (NaCl) aqueous solution by underwater LIBS and investigated the signal fluctuation caused by the repeated irradiation at a fixed position. The intensity of Cu and Na emission lines increased with increasing the number of laser shots. We found that there were strong correlations between the spectral line intensity and the collapse time of the laser-induced bubble. By normalizing the spectral line intensity with the bubble collapse time after removing the offset, the relative standard deviations of Cu and Na intensity obtained for the repeated irradiation were reduced from 41.4% to 27.0% and 28.8% to 17.7%, respectively. We also confirmed that elimination of the data obtained with the first irradiation of a fresh surface and the use of the spectral line intensity of dissolved species as a normalization parameter were valid for improving the precision of analysis. The results suggest simple approaches to improve the quantitative performance of underwater LIBS, which are easily applicable to the existing system.