Accurate determination of lithium isotope ratios by MC-ICP-MS without strict matrix-matching by using a novel washing method

Abstract

The accurate determination of Li isotopic ratios by MC-ICP-MS has traditionally been hampered by the high background and severe memory effect of Li. In this study, a novel method to efficiently reduce the high background and memory effect of Li was developed. It was found that the Li background can be significantly reduced by a factor of 15 to 70 by using a 5% NaCl rinse solution. In addition, the “mismatching effects” reported previously, which are caused by different acid and Li concentrations between the sample and standard, were eliminated once the Li background was efficiently reduced, suggesting that the crux of the two types of matrix effects is actually the high Li instrumental background and memory effect. Applying the background reduction technique, a method without strict matrix-matching was developed for the accurate and precise determination of Li isotopic ratios. The proposed method was validated by the analysis of eight reference materials with satisfactory results, even when the Li concentration in the samples was not matched with that of standards. The external precision of this method is better than ±0.25‰ (2SD) for δ⁷Li, suitable for the identification of small fractionation of Li isotopes occurring in geological processes. With the proposed method, there is no need to match the Li and acid concentration of the sample and standard, which significantly reduced the sample preparation time and increased the sample throughput.