Optimization of operating conditions for improved precision of zirconium and hafnium isotope ratio measurement by inductively coupled plasma mass spectrometry (ICP-MS)

Abstract

The optimal instrument operating parameters (IOPs) for improving the precision of Zr and Hf isotope ratio measurements by inductively coupled plasma mass spectrometry (ICP-MS) were investigated, as an initial step towards improving precision in the determination of Zr and Hf abundances in samples with low concentrations of these elements by isotope dilution ICP-MS. An Elan Model 5000 ICP-MS was employed. Isotopes investigated were ⁹⁰Zr, ⁹¹Zr, ⁹²Zr, ⁹⁴Zr, ¹⁷⁷Hf, ¹⁷⁸Hf, ¹⁷⁹Hf and ¹⁸⁰Hf. Varying the r.f. power from 1000 to 1300 W did not significantly affect the measured isotope ratios. Mass bias, always showing preferential reduction in response of light isotopes, occurred for both Zr and Hf isotope ratio measurements. Significant variations of measured isotope ratios were found as a function of ion lens setting and dwell time. Precision was optimized and mass bias minimized at a B lens setting of 60 digipots and a dwell time of 30 ms. The ratios ⁹¹Zr : ⁹⁰Zr and ¹⁷⁹Hf : ¹⁷⁸Hf are the best for natural samples. Argid and oxide interferences on these masses are minimal; for example ¹⁶²Dy¹⁶O and ¹⁶³Dy¹⁶O translate into uncertainties of 0.02% in measured ¹⁷⁹Hf : ¹⁷⁸Hf ratios. By optimizing IOPs, it is not necessary to correct for potential interferences from rare earth element oxides, without sacrificing precision and accuracy of isotope ratio measurement. Precision and mass bias are insensitive to the concentration of Zr and Hf over the range of sub-ppb to ppm. An overall precision of 0.2–0.6% relative standard deviation can be achieved for Zr and Hf isotope ratio measurement, and can be maintained over 7 h periods.