Determination of the Isotopic Composition of an Enriched Hafnium Spike by MC-ICP-MS Using a Regression Model

The isotope dilution (ID) method, owing to its high precision, is extensively used for the determination of element mass fractions in a wide range of natural samples. One of the prerequisites of the ID method is knowledge of the isotopic composition of an enriched spike, which is often challenging to accurately determine. In this study, we employ a regression mass bias correction model to accurately and precisely measure the isotopic composition of an enriched Hf spike (Lot No. 159293, Oak Ridge National Laboratory). A NIST SRM 3134 Re solution, whose isotopic composition was calibrated by an NRC IRIS-1 Ir isotope standard, was used to calibrate the isotopic composition of the Hf spike. The obtained ratios of(176)Hf/Hf-177,Hf-179/Hf-177 and(180)Hf/Hf-177 were 0.2406 +/- 0.0005 (u, k = 1), 2.8620 +/- 0.0005 (u, k = 1) and 384.65 +/- 0.05 (u, k = 1), respectively, which meet the required precision levels for Hf isotopes in the application of the ID method. The combined uncertainties of the calibrated Re and Hf isotope ratios were evaluated using a Monte Carlo method, in which the uncertainty of the primary calibrator (IRIS-1,Ir-193/Ir-191 = 1.6866 +/- 0.0005,u,k = 1) was also taken into consideration. The precision of Hf spike was improved significantly compared with the SSB and C-SSBIN methods. To the best of our knowledge, this is the first report of isotope ratio calibration in an enriched spike using the regression model. The precisely calibrated spike can lower the uncertainty of the Hf mass fraction significantly, thereby increasing the accuracy of the Lu-Hf chronometer.