Quantitative Analysis of Manganese in Underwater Steel Samples Using Long–Short Double-Pulse Laser-Induced Breakdown Spectroscopy

The long–short double-pulse laser-induced breakdown spectroscopy (LS-DP-LIBS) method was applied to qualitative and quantitative analyses of underwater steel samples to improve the detection ability of underwater measurement. The stable plasma intensity and discrete emission lines were detected using LS-DP-LIBS when comparing the measured results of single-pulse LIBS (SP-LIBS) and LS-DP-LIBS. The long pulse produces a cavitation bubble without plasma, and the short pulse induces the plasma of steel samples within the bubble. The detection features of LS-DP-LIBS for underwater steel samples were discussed in different intra-pulse delay time, long-pulse width, and delay time conditions when analyzing the measured spectra, the signal intensity of Fe(I) at 400.524 nm and 402.187 nm, Mn(I) at 404.136 nm, and intensity ratio of Mn(I) 404.136 nm/Fe(I) 402.187 nm. The results indicated that the plasma stability and spectral signal intensity were improved significantly with a long-pulse width of 80 µs in the intra-pulse delay time of 70 µs, which were appropriate for bubble formation and plasma generation. According to the discussion of the delay time effect, the state of generated plasma was almost stable from 650 ns to 850 ns. Manganese (Mn) contents in steel samples were analyzed quantitatively when measuring five steel samples with different Mn contents using LS-DP-LIBS in optimal experimental conditions. A strong linear dependence was observed with R²=0.9842, which demonstrated the feasibility and appropriateness of quantitative analysis for underwater measurement using LS-DP-LIBS.