Calibration-Free Analysis of Li Isotope Ratios Using Laser Ablation and Laser Absorption Spectroscopy

We introduce a rapid, calibration-free, all-optical method for high-precision lithium isotope ratio measurements in solid materials using laser ablation combined with tunable laser absorption spectroscopy. A new asynchronous method is used to acquire time-resolved, high-resolution absorption spectra of the ⁶Li and ⁷Li D1 and D2 transitions near 671 nm, probing atoms generated from ablation of LiAlO₂. Isotope ratios and atomic column densities are determined from measured spectra via a physics-based fitting model including hyperfine structure. Under 1 Torr air, spectra recorded ≥0.75 ms after plasma onset exhibit narrow line widths corresponding to Doppler temperatures ≤400 K, enabling resolution of the isotope peaks with high signal-to-noise ratios. Analysis of LiAlO₂ samples with varying ⁶Li/⁷Li ratios demonstrates relative isotopic precisions of 0.6-1.8% for spectra acquired in 30 s. Isotope ratios determined from the spectral fits show relative accuracy within -0.3% to -5% of ICP-MS measurements, without requiring calibration to external standards, and offering a rapid analysis approach to lithium isotope determination in solid materials relevant to nuclear energy, safeguards, and geochemistry.