In situ high-precision lithium isotope analyses at low concentration levels with femtosecond-LA-MC-ICP-MS

Abstract

In this study we have established a new method for in situ measurements of stable lithium (Li) isotope ratios at low Li concentration levels using UV-femtosecond laser ablation coupled with multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Highly precise in situ determination of Li isotope ratios is challenging due to low Li concentrations in most natural minerals and matrix effects typically occurring during the ablation process and in the ion source. Here we demonstrate that matrix-dependent isotope effects in the plasma can be largely suppressed by operating the plasma under cool conditions (900 W) and a laser beam which is focused ∼130 μm below the sample surface. Under such conditions, precise and accurate measurements of δ⁷Li with ∼2‰ (2σ) analytical uncertainty have been performed for various glass reference materials and olivine. Depending on the Li concentration of the investigated samples, detector combinations have been optimized in order to achieve the best precision. At Li concentrations ranging from 2 to 10 μg g⁻¹ a combination of an ion counter for the determination of ⁶Li and a Faraday cup equipped with a 10¹³ Ω amplifier for ⁷Li has yielded the best precision. For Li concentrations > 10 μg g⁻¹, Faraday cups equipped with a 10¹³ Ω amplifier for ⁶Li and a 10¹¹ Ω amplifier for ⁷Li are recommended. Measurements applying a 10¹³ Ω amplifier require a tau correction due to the slower signal response of the amplifier. The accuracy of the here-established LA-MC-ICP-MS method was tested by comparing the results obtained for reference glasses that have been analyzed both in situ and with solution MC-ICP-MS (or other conventional methods, previously applied in other studies), and the results are overall in good agreement. The applicability to a zoned olivine phenocryst from the Massif Central volcanic region has been tested successfully.