Determination of manganese in submerged steel using Fraunhofer-type line generated by long-short double-pulse laser-induced breakdown spectroscopy

The emission spectral lines of underwater laser-induced plasma (LIP) are often influenced by a strong self-absorption effect. It is found that the long-short double-pulse laser-induced breakdown spectroscopy (LS-DP-LIBS) is able to generate the Fraunhofer-type signal from the submerged solid samples. In this work, the manganese concentrations in submerged steel samples are quantitatively analyzed using the Fraunhofer-type signal. We firstly show the Fraunhofer-type signal generated by LS-DP-LIBS. The parameters of the Fraunhofer-type absorption lines are briefly presented and discussed. Then, the Fraunhofer-type line at Mn I 403.307 nm is used to calculate the optical density (OD), which is a parameter for evaluating plasma absorbance. A calibration curve is established using OD values and certified manganese concentrations of ten samples. The results suggest that there is a linear relationship between OD values and Mn concentrations. Therefore, we have further conducted two prediction tests based on the measurement results of ten samples. The results show that the relative error of prediction is less than 10%, which demonstrates that the Fraunhofer-type absorption lines are worthy of study in the quantitative measurement of submerged solid samples.