Investigation of intrinsic origins of the signal uncertainty for laser-induced breakdown spectroscopy

The relative higher uncertainty of laser-induced breakdown spectroscopy (LIBS) is considered as one of the major problems and prevent it from large scale industrial application, and there was still very little profound investigation on the origins of LIBS signal uncertainties. In this paper, the temporal stability behavior of LIBS signal was investigated using time-resolved spectroscopy and fast photography technique. It was clarified that the plasma morphology was more repeatable at earlier stage of the plasma evolution, while being much less repeatable at later stage. With 100 ns gate-time, for a titanium alloy, the LIBS spectral signal firstly was suffered from high shot-to-shot fluctuation at very early stage of plasma evolution, then became most repeatable around the delay time of 800 ns, and thereafter experienced a deteriorating signal repeatability as plasma continued to evolve, which was consistent with the plasma morphology fluctuation pattern, implying that the observed total number density fluctuation by the optical system caused by plasma morphology variation could be the main factor that affects the LIBS signal repeatability. By evaluating the effects of the plasma parameters both theoretically and experimentally, the contributions of the fluctuation of plasma temperature, electron density, and total number density of the measured species, to spectral signal uncertainty, were quantified for the first time and it was found that the fluctuation of total number density was the controlling factor of the spectral signal fluctuation during most part of the plasma lifetime, especially at later stage of the plasma evolution due to the severe morphological fluctuation. At early stages of plasma evolution, the plasma temperature decreased more drastically, small jitters of laser energy and spectral acquisition time window would have more significant effect on temperature fluctuation, thus the effect of plasma temperature fluctuation on spectral signal fluctuation was particularly significant at early stage of the plasma (similar to first hundreds of nanoseconds). There also existed a "stable plateau" where the line intensity was not sensitive to the plasma temperature fluctuation. The fluctuation of electron density had little effect on the line intensity fluctuation. This study offers a fundamental understanding of the sources of LIBS signal fluctuation and a basic principle of stabilizing the LIBS signal, which is to reduce the fluctuation of the observed laser-induced plasma.