**4. Conclusions**

In this paper, artificial contamination was used to study the e ffect of the salt mixture in LIBS signals, especially the calibration curves and relative spectral intensity of di fferent Na and Ca samples. The results showed that the R<sup>2</sup> of the calibration curve between the element content and characteristic wavelength line intensity was a ffected minimally by the mixing ratio of di fferent compounds with the same cations. Thus, it is beneficial in the LIBS test on-site, that we can ge<sup>t</sup> the content of cations regardless of their compounds while the cations would contribute most in the soluble contamination. The laser energy per pulse in the LIBS test would a ffect the spectral significantly both in line intensity and shape. By reducing the laser energy intensity and selecting the proper emission wavelength of the analytes, the e ffect of the self-absorption can be weakened, and the detection accuracy can be improved. The PLSR model was used in the data process, which could improve the accuracy of Na and Ca linear analysis.

**Author Contributions:** Conceptualization and formal analysis, S.L. and X.W.; investigation, X.Q.; resources, S.L.; data curation, S.L. and T.W.; writing—original draft preparation, X.Q.; writing—review and editing, X.W.; supervision, T.W. and Z.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (51607101), Science and technology projects of Shanxi Electric Power Research Institute (SGSXDK00SPJS1900162).

**Conflicts of Interest:** The authors declare no conflict of interest.
