Extending the application range of Hg isotopic analysis to sub-μg L−1 levels using cold vapor generation multi-collector inductively coupled plasma-mass spectrometry with 1013 ohm Faraday cup amplifiers

Abstract

High-precision determination of the isotopic composition of mercury (Hg) is of paramount importance for unraveling its biogeochemical cycle and for identifying the origin of Hg in environmental compartments. Cold vapor generation multi-collector inductively coupled plasma-mass spectrometry (CVG-MC-ICP-MS) is the standard approach for such application. Cold vapor generation provides a high Hg introduction efficiency into the ICP, while chromatographic Hg isolation is not required as a result of the selective reaction between Hg²⁺ and SnCl₂. For environmental or biota samples with low Hg concentrations, however, this approach still presents challenges and reliable measurements typically require a Hg concentration ≥1 μg L⁻¹ in the solution analyzed. Recent improvements of MC-ICP-MS instrumentation, including the introduction of the so-called Jet interface and 10¹³ Ω Faraday cup amplifiers, enhance the signal-to-noise ratio. In this study, it was investigated to what extent this allows Hg isotopic analysis at lower concentration. Performance in Hg isotopic analysis was compared using two different sets of cones (standard vs. Jet), two plasma conditions (wet vs. dry) and two amplifier types (10¹¹ Ω vs. 10¹³ Ω). Satisfactory accuracy and precision were achieved at a Hg concentration down to 0.1 μg L⁻¹ in the solution measured when using Jet cones, dry plasma conditions, and the four available 10¹³ Ω amplifiers. The uncertainty expressed as 2SD for the δ²⁰²Hg values measured for the in-house standard solution was ±0.2‰ at 0.25 μg Hg L⁻¹ and ± 0.3‰ at 0.1 μg Hg L⁻¹. The method was subsequently applied to the analysis of real surface water samples contaminated with toxic metals.