Impact of matrix effects on the accurate measurement of Li isotope ratios by inductively coupled plasma mass spectrometry (MC-ICP-MS) under “cold” plasma conditions

Abstract

Lithium isotopic measurements by MC-ICP-MS using “cold” plasma conditions are characterised by fewer baseline interferences and improved reproducibility as compared with conventional hot plasma techniques. The 2σ precisions for 1200 W, 800 W and 680 W are conservatively estimated as 1.1‰, 0.7‰ and 0.5‰. However, matrix effects are most significant at the coldest plasma temperatures, with deviations of >0.5‰ δ⁷Li being observed at 680 W for 100 ng g⁻¹ Li solutions containing ≥500 ng g⁻¹ Na. However, no matrix effects were detected at 800 W for solutions containing up to 5000 ng g⁻¹ Na. Lithium isotopic measurements under all conditions are strongly sensitive to variations in acid concentration, requiring precise matrix matching between samples and standards. Self-induced Li matrix effects are not evident for Li concentrations below 120 ng g⁻¹, however blank corrections are necessary. The isotopic ratios of Li separated from seawater at 680 W (δ⁷Li = +30.7 ± 0.4‰) and 800 W (+30.4 ± 0.9) are subtly lower than those obtained for 1200 W (δ⁷Li = +32.0 ± 0.2), although all lie within error given the reported reproducibility and all are within the range of previously published analyses. With minimisation of matrix induced fractionation, the precision and accuracy achievable at 680 W and 800 W equals or surpasses that previously reported for TIMS and conventional hot plasma ICP-MS.