Elemental fractionation effect in fiber laser induced breakdown spectroscopy

Elemental fractionation (EF) is a non-stoichiometric effect in laser induced breakdown spectroscopy (LIBS), yet the relevant study is scarce due to the lack of proper way to indicate it and the adjustability of laser parameters. In this study, we developed a new method to calculate relative sensitivity coefficient (RSC), which is an estimation of EF, by applying multiple emission lines of each element to calculate plasma parameters so as to figure out actual concentration ratio values instead of spectral emission values for RSC calculation. Compared with traditional single-line method, this method obtained RSC values in reasonable range (0.1‒10) and circumvented the significant variation induced by different line-selection criteria in single-line method. The influence of laser pulse width (150‒350 ns), pulse energy (1.25‒4.17 mJ) and repetition rate (1/24000 Hz) with a fiber laser on EF was studied. Pulse width showed little impact in all the experiments, whereas pulse energy obviously affected the behavior of elements, which can be divided into three groups, namely increasing (Cu), declining (Fe, Mg, Si) and showing local maxima (Cr, Mn) when pulse energy increased. The difference of heat conductivity was deemed to be the potential reason. Lower laser repetition rate helped to reduce EF, which was ascribed to less heat accumulation. After evaluation with chemometrics model of Partial Least Squares, boiling point, melting point and vaporization heat were found to be highly correlated with RSC values, which supports the idea that thermal processes play the vital role in the origination of EF. The findings in this research help to further optimize and get better understanding of underlying mechanism of LIBS.