A protocol for synthesizing reference materials for multi-isotope analysis via high-temperature and high-pressure sintering: A demonstration with Ti-Hf in rutile

Abstract

Multi-isotope tracers are vital in exploring complex systems and processes in environmental science, archaeology, and Earth sciences. Laser ablation multiple-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), as a robust technique for such isotope analyses, offers high spatial resolution to identify micro-regional isotopic heterogeneity. However, high-precision in-situ isotope measurements typically necessitate the use of matrix-matched reference materials to correct for matrix effects, instrumental bias, and fractionation induced by laser ablation. Natural minerals with homogeneous multi-isotopic compositions are extremely rare and inadequate to fully meet diverse experimental requirements. This study proposes a robust synthesis protocol to make matrix-matched reference materials tailored for in-situ multi-isotope analysis via LA-MC-ICP-MS. This protocol integrates elemental doping (ED) with high-temperature and high-pressure (HTHP) techniques, offering a versatile approach to prepare solid reference materials tailored for multiple isotopes of interest. To validate the reliability of this method, a HfO₂-doped rutile (NWU-RT) was synthesized for Ti-Hf isotope determination. Comprehensive in-situ analyses demonstrate that NWU-RT exhibits exceptional homogeneity in Ti and Hf isotopes. The mean δ⁴⁹Ti_OL-Ti value is 0.42 ± 0.12 ‰ (2SD, n = 666), with a ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratio of 0.282265 ± 0.000071 (2SD, n = 561), aligning with results obtained from SN-MC-ICP-MS. Reproducibility tests involving six batches of NWU-RT synthesized under identical experimental conditions over a year confirm the significant potential of the ED-HTHP method for synthesizing reliable solid reference materials for in-situ multi-isotope measurements using LA-MC-ICP-MS.