Measurement of lithium isotope ratios by inductively coupled plasma mass spectrometry: application to geological materials

Abstract

A method is described for the measurement of Li isotope ratios in minerals using ICP-MS. The technique is capable of achieving a relative precision of 0.8‰, and has successfully been applied to minerals containing less than 300 µg g^–1 of Li. Sample material is dissolved in a combination of HCI and HF in sealed PTFE vessels. Separation of Li from the matrix is accomplished by column chromatography using Bio-Rad AG50W X-8 100–200 mesh resin. The use of isotopic reference material IRM-016 Li₂ CO₃ is suggested as a reference standard for calculating and correcting instrumental mass discrimination. The presence of excess amounts of concomitant ions in solution is shown to lead to matrix-induced mass discrimination. Lithium isotope ratios (⁷Li: ⁶Li) for the minerals studied range from 12.698 (–33.2‰ relative to the IRM-016) for an amblygonite sample from Tanco Mine, Manitoba, Canada to 12.250 (+3.5‰ relative to the IRM-016) for a lithiophorite sample from North Mountain Mine, AR, USA.