Precise tellurium isotope analysis by negative thermal ionization mass spectrometry (N-TIMS)

Abstract

We present a new method for precise isotope analysis of Te by thermal ionization mass spectrometry in negative ion mode (N-TIMS) along with an improved technique for chemical separation of Te from geochemical samples. In the chemical separation, Te was purified by a three-step column chemistry method employing an anion exchange resin and an extraction chromatographic resin, yielding 98% recovery. To achieve strong Te⁻ beam intensity in N-TIMS, we evaluated the optimum analytical conditions with respect to the filament material, ionization activator, and filament heating protocol. The reproducibilities of Te isotope ratios obtained by static multicollection using Faraday cups were 0.9%, 0.04%, 0.06%, 0.01%, 0.005%, and 0.008% for ¹²⁰Te/¹²⁸Te, ¹²²Te/¹²⁸Te, ¹²³Te/¹²⁸Te, ¹²⁵Te/¹²⁸Te, ¹²⁶Te/¹²⁸Te, and ¹³⁰Te/¹²⁸Te, respectively. This result is comparable to those of previous MC-ICP-MS studies. The reproducibilities of Te isotopes obtained by SEM jumping measurements were approximately ten times worse than those obtained by Faraday cup runs, excluding the ¹²⁰Te/¹²⁸Te ratio for which the reproducibility was comparable to that obtained by Faraday cup runs due to the extremely low abundance of ¹²⁰Te. To further improve the analytical precision, the ¹²⁶Te/¹²⁸Te ratio was determined by multidynamic collection for the study of ¹²⁶Sn–¹²⁶Te chronology. The 2s of the ¹²⁶Te/¹²⁸Te ratio of the multidynamic collection was 1.6 times better than that obtained by static measurements. Finally, the accuracy of our method was confirmed by the analysis of two carbonaceous chondrites and one terrestrial standard rock, which was consistent with previously reported values.