PNNL

Abstract

The detection and quantification of Li and its isotopes is performed in a variety of geochemical, nuclear, and energy storage applications. Therefore, the accurate elemental and isotopic analysis of Li in various substrates is needed. Laser induced breakdown spectroscopy is a promising method for the rapid analysis of Li isotopes in real time with no sample preparation requirement. The resonance transition at $\approx$670.8 nm is well suited for Li isotopic analysis via laser induced breakdown spectroscopy considering its relatively large isotopic shift of $\approx$15.8 pm. However, spectral broadening in laser produced plasmas due to the Doppler effect, presence of fine and hyperfine structures, self-absorption and self-reversal effects on this Li I transition make accurate analysis of Li isotopic ratios challenging. Such line broadening and distortion mechanisms are impacted by the nature and pressure of the ambient gas. Here, we perform spatially and temporally resolved optical emission spectroscopy of plasmas produced via laser ablation of LiAlO$_2$ substrates. The present study explores the influence of Ar, N$_2$, and He ambient gases over the pressure range of 0.1 - 100 Torr on line broadening and self-absorption/self-reversal of the Li I ($\approx$670.8 nm) transition. Our results show that the self-reversal of Li I ($\approx$670.8 nm) is most prominent at higher pressures (10 - 100 Torr), and at a reduced pressure of $\approx$ 100 mTorr, regardless of the ambient gas, Li I shows the narrowest linewidth. Spatial and temporal analyses reveal that a position of 5 mm from the target surface and a gate delay time of 20 $\mu$s minimizes Li I linewidth while achieving reasonable emission intensity. Using optimized plasma conditions that minimize line broadening and distortions, we report predicted versus known $^6$Li/$^7$Li ratios for LiAlO$_2$ substrates in the form of a calibration curve. We demonstrate that with the use of chemometric methods, specifically principal component analysis and regression, we can predict $^6$Li/$^7$Li ratios with $\sim$ 96 \% accuracy.