Lithium Isotope Determination in Spodumene by fs-LA-MC-ICP-MS with Non-Matrix-Matched Calibration: Insights into ICP Operating Conditions and Data Evaluation

Abstract

Lithium isotope measurement in spodumene by femtosecond LA-MC-ICP-MS was investigated and the influence of plasma operating conditions and data reduction strategy on accuracy and precision was studied. It was found that "hot" plasma conditions led to an unstable baseline signal and substantial variations in the Li isotope ratios. By adding a constant amount of water to the carrier gas, a stable baseline was achieved and isotope ratios became reproducible and were consistent with data from solution-based MC-ICP-MS. The resulting biases were within +/- 0.51 parts per thousand and the reproducibility was better than 0.09 parts per thousand. Comparison of Li isotope ratios resulting from different data evaluation schemes showed that the mean of the transient intensity ratios, integration of the entire or a user-defined period of the ion signals resulted in good agreement with solution-based data, while linear regression underestimated the Li isotope ratios. It was also found that "cool" plasma operation produced a stable baseline signal, but the Li isotope results were biased by up to -4.3%, irrespective of water introduction and the data evaluation scheme. With the optimised "hot-wet" conditions, the Li isotope ratios in eleven spodumene materials were determined which successfully allowed distinguishing regional deposits and partially veins of the available samples.