Advances in high-precision lithium isotopic measurements with the NeomaTM MC-ICP-MS

Abstract

Lithium (Li) isotopes ratios (expressed as δ⁷Li) are increasingly utilized as tracers for environmental and biological processes, including recent studies on Li uptake by living organisms and its emerging role as a contaminant. However, the typically low Li concentrations in natural samples and sentinel species used for monitoring present significant analytical challenges, particularly in generating efficiently high-precision and accurate isotopic data. In this study, we present the results of multiple tests and an optimized protocol for Li isotopic analysis at ultra–trace levels (< 3 ng Li) using the Neoma MC-ICP-MS. We also provide long–term, high-precision isotopic data for marine and biological reference materials. First, we demonstrate that memory effects remain significant when analyzing low–concentration Li solutions. However, reducing the sample volume to 550 µL effectively minimizes these effects to just 3% of the ⁷Li signal. Our findings confirm that the Standard Sample Bracketing (SSB) method is effective for low–level Li isotopic measurements, though several precautions are necessary. Specifically, the molarity of nitric acid used for sample and LSVEC (bracketing standard) dilution must be carefully matched, with deviations of less than 0.3%. Additionally, the relative difference in ⁷Li voltages between standards and samples needs to be within ± 20% to avoid significant isotopic bias. Furthermore, we directly compared two desolvating systems (Apex Ω and Aridus III) for Li isotopic analysis under dry plasma conditions. This comparison enabled us to propose an optimized introduction system for nanogram–level analyses with minor memory effects. We then applied our protocol to multiple analyses of four reference materials (Li7–N, AEL, EDMM–1–H, Seawater, NIST SRM–1400, and PLK–VLFR), demonstrating efficient data acquisition with excellent long–term accuracy and precision for both marine and biological matrices. Future efforts should focus on reducing the time required for Li dissolution and purification from samples used in high-frequency environmental and bio–monitoring applications.