Effects of mercury and thallium concentrations on high precision determination of mercury isotopic composition by Neptune Plus multiple collector inductively coupled plasma mass spectrometry

Abstract

Thallium (Tl) has been widely used as an internal standard for mass bias correction during high precision mercury (Hg) isotope ratio measurements using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). However, a recent study by Georg and Newman indicated the potential for Hg hydride formation (HgH_x, x = 1, 2) during Hg isotope measurements using an X skimmer cone with a Neptune Plus MC-ICP-MS. Mercury hydride formation could result in an artificial change in ²⁰⁵Tl/²⁰³Tl. Due to this observation, the applicability of using Tl as an internal standard for instrumental mass bias correction during high precision Hg isotope measurements has been questioned. In this study, using an adapted gas/liquid phase separator for Hg introduction and the NIST SRM 997 Tl standard for mass bias correction, mercury isotope measurements were performed using a Neptune Plus MC-ICP-MS. While we confirm Georg and Newman's observations, we show that Hg hydride formation is less important when Hg isotope measurements are conducted with high Tl and low Hg concentrations. With careful sample-standard bracketing (with Hg concentration matching within 10%), we demonstrate that measuring 20 to 50 ng mL⁻¹ of Tl and 0.5 to 3.0 ng mL⁻¹ of Hg, high precision Hg isotope ratio measurements are achievable. We caution researchers using other Hg inlet systems to recognize the importance of Hg and Tl concentrations and encourage the optimization of these values during their Hg isotope measurements.