A simplified instrumental protocol for trace Nd and Hf isotope measurements (<10 ng) using a MC-ICP-MS and an Apex Omega de-solvating system†

Abstract

We reported a simple technique allowing accurate and precise Nd and Hf isotope measurements of samples with low contents using a Neptune Plus MC-ICP-MS and an Apex Omega membrane de-solvating system. By employing a highly sensitive jet sample cone and X skimmer cone, our protocol achieved a low oxide level (<0.05%) at high ion transmission (∼880 V ppm⁻¹ for Nd and 850 V ppm⁻¹ for Hf) and was found to have little effect on the mass fractionation of Nd and Hf. This was accomplished by simple adjustments of the sweep gas of the membrane de-solvating system and the sample gas of the nebulizer. Instrumental sensitivities were enhanced by a factor exceeding tenfold, in comparison to those of standard measurements with wet plasma. Long term measurements of 1 ng Nd solution standards yielded ¹⁴³Nd/¹⁴⁴Nd ratios of 0.512117 ± 0.000032 (2SD, n = 24) and 0.512227 ± 0.000036 (2SD, n = 42) for Jndi-1 and JMC-Nd solutions, respectively, while analysis of 1 ng JMC475 Hf gave ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.282163 ± 0.000027 (2SD, n = 21). These results are in agreements with their reference values. Potential interfering elements Ce and Sm are found to have an insignificant influence on ¹⁴³Nd/¹⁴⁴Nd ratios. Lu and Yb have a remarkable effect on ¹⁷⁶Hf/¹⁷⁷Hf ratios. Improved correction performance can be achieved by using Lu and Yb ratios obtained by analyzing pure Lu and Yb solutions. Five chemically purified rock reference materials are measured with the presented method and the results are consistent with their recommended values. Relative to other approaches, this protocol is rapid and accurate and can be widely applied in Nd and Hf isotope analysis of samples with trace amounts of Nd and Hf (<10 ng).