Low-memory, small sample size, accurate and high-precision determinations of lithium isotopic ratios in natural materials by MC-ICP-MS

Abstract

Here we report a comparatively simple, rapid, high-precision and accurate technique for the Li isotope ratio determination by MC-ICP-MS. In comparison with published methods, our MC-ICP-MS technique uses a low-memory APEX-IR desolvator and a high-sensitivity X-skimmer cone, together with a simple one-step column separation, to achieve an optimal condition for Li isotope determination in natural materials with different matrices at low levels of Li. This new method was validated by analyzing a series of certified reference materials, which span a wide range of δ⁷Li and cover different sample matrices in order to provide a useful benchmark for future works. On the basis of this approach, the long-term precision of δ⁷Li (±0.12‰, 2SD, n = 46) was significantly improved by a factor of 2 compared to other methods, with the additional advantage of requiring a rather small size of ca. 1.2 ng Li. The δ⁷Li for several reference standards were determined, including Li₂CO₃ (IRMM-016: 0.05‰), seawater (IAPSO: 30.84‰; NASS-5: 30.72‰), coastal waters (CASS-4: 30.69‰; SLEW-3: 30.45‰), river water (NIST SRM 1640: 9.36‰), coral standard (JCp-1: 20.16‰) and two silicate rocks (AGV-2: 7.98‰ and BHVO-2: 4.63‰). Applying the improved technique, we are able to produce accurate and high-precision δ⁷Li data for low [Li] natural materials, such as continental runoffs and marine biogenic carbonates, which can provide valuable information on continental weathering processes and paleoceanographic implications.