High-precision stable zirconium isotope ratio measurements by double spike thermal ionization mass spectrometry

Abstract

A new analytical method for the accurate and precise determination of stable zirconium isotope ratios in geological materials by thermal ionization mass spectrometry (TIMS) is presented. Isobaric interference from Mo was largely eliminated during filament heating and further reduced by off-line Mo interference correction. This methodology allows the complete elimination of Mo interference even with a Mo/Zr ratio up to 1. Separation of Zr from natural sample matrices was achieved by using a single DGA resin column with a small usage of acids (15 mL of 7 mol L⁻¹ HNO₃ and 3 mL of 1 mol L⁻¹ HF). A ⁹¹Zr–⁹⁶Zr double spike was admixed with samples before digestion, which enabled the correction of isotope fractionation that could occur during column separation and mass spectrometry measurement. The long-term measurement reproducibility of δ⁹⁴Zr is generally better than ±0.06‰ (uncertainty represented by two standard deviations, hereinafter referred to as 2 SD), assessed by repeated analyses of Zr standard reference NIST SRM 3169 and eleven geological reference materials. The Zr isotope ratios of geological reference materials BHVO-2 and AGV-2 obtained in this study are consistent with the reported values measured by MC-ICP-MS, confirming the accuracy of the proposed method. This new method greatly simplifies the chemical separation process and is particularly suitable for Zr isotope analysis of complex matrix (especially for high Mo) samples.