Rapid selection of analytical lines for SAF-LIBS based on the doublet intensity ratios at the initial and final stages of plasma.

Self-absorption-free laser-induced breakdown spectroscopy (SAF-LIBS) can directly obtain the applicable quasi-optically thin lines by determining the optimal acquisition delay time according to the intensity ratio of doublet lines at specific transition wavelength of the analyzed elements, thus eliminating the influence of self-absorption on quantitative results. In quantitative analysis of samples with a certain content range, the key to the convenient application of this technique is to rapidly select the suitable doublet lines for the element to be analyzed. The theoretical analysis shows that the evolution trend of doublet intensity ratio is monotonous under the assumptions that the plasma is uniform and in local thermal equilibrium (LTE) and the area density (Nl) is a constant, which is also confirmed by the experimental results of Cu and Al. Thus, a rapid spectral line selection criterion for SAF-LIBS applications is derived: only when the doublet intensity ratios measured at the initial and final stages of plasma induced by the boundary sample with the highest element content lie on both sides of the theoretical ratio, the doublet lines can reach quasi-optically thin during plasma evolution and are suitable for SAF-LIBS measurements. This new criterion is helpful to promote the practicality and industrial application of SAF-LIBS technology.