Determination of thallium isotopic composition through MC-ICP-MS with mass bias corrected using admixed W

Abstract

Despite their widespread application in planetary and earth sciences in the past two decades, thallium isotopic compositions have been determined via MC-ICP-MS with instrumental mass bias corrected by employing a combination of lead doping and standard-sample bracketing. Residual Pb, either from the sample matrix or contamination, may significantly distort the isotope ratio of Pb for mass bias correction, thereby affecting the accuracy of Tl isotope measurement. In this study, we employed ¹⁸⁶W/¹⁸⁴W (NIST SRM 3163 W) instead of ²⁰⁸Pb/²⁰⁶Pb (NIST SRM 981 Pb) to correct instrumental mass bias during the measurement of Tl isotopic compositions through MC-ICP-MS. The plot of ln(¹⁸⁶W/¹⁸⁴W) vs. ln(²⁰⁵Tl/²⁰³Tl) shows a strong linear correlation (R² > 0.993) in the repeated analyses of NIST SRM 997 Tl using W. Two pure Tl standards, Fluka Tl and GSB Tl, produced ε²⁰⁵Tl_{NIST 997} values of −1.9 ± 0.7 (2SD, n = 97) and 0.8 ± 0.7 (2SD, n = 31), respectively. The ε²⁰⁵Tl_{NIST 997} of five geological reference materials (SCo-1, GSS-6, GSP-2, NOD-P-1, and NOD-A-1) are also reported here, and the external reproducibility of these results is better than 0.08‰ (2SD). ε²⁰⁵Tl_{NIST 997} values are in good agreement with previously published values determined using Pb doping to correct mass bias, demonstrating that W can serve as a calibrator and facilitate high-precision ε²⁰⁵Tl_{NIST 997} measurement of geological materials.