Precise determination of the lithium isotope ratio in geological samples using MC-ICP-MS with cool plasma

Abstract

Lithium has two naturally occurring isotopes, ⁶Li and ⁷Li, with approximate relative abundances of 7.5% and 92.5%, respectively. Due to large Li isotope variations in nature, lithium isotopes have the potential to reveal important information relevant to nuclear technology, biomedicine, astrophysics, and geochemistry. With the advent of multi-collector inductively coupled mass spectrometry (MC-ICP-MS), studies of Li isotopes have largely focused on the analysis of geological materials, with varying degrees of accuracy. However, this technique has often been affected by either baseline interferences or isobaric interferences on mass 6 and 7 during ionization in Ar plasma, which is mainly due to the Li compound with hydrogen gas, and double-charged nitrogen and carbon ions at higher levels of RF power. In this study, we reduced baseline interferences in Ar plasma using a cool plasma (∼800 W) technique with a X-type cone. Lithium was separated using a cation exchange column (BioRad AG50W-X8, 200–400 mesh) with a mixture of 6 N HNO₃ and 80% methanol at <0.2 mL min⁻¹ elution speed. The short-term reproducibility of δ⁷Li values of the NASS-5 seawater standard was 30.55 ± 0.45‰ (2σ, n = 15). Measured δ⁷Li values of rock and seawater standards ranged from 2.48 to 30.55‰, in good agreements with reported values.