An improved method for recovering and preconcentrating mercury in natural water samples for stable isotope analysis

Abstract

The composition of stable mercury (Hg) isotopes provides important chemical signatures for tracing the sources and transformation pathways of Hg in the environment. However, such measurements are challenging for natural water samples due to the ultratrace levels of Hg and therefore preconcentration of Hg in water is needed prior to isotope measurements. In this study, we developed a preconcentration method using a modulated apparatus that can be deployed for field measurements. The system includes a 3 L bubbler, a chlorine-impregnated activated carbon (ClC) trap, a zero-air filter connected to the inlet, and a vacuum pump. Hg in aqueous phase samples is first oxidized with BrCl and then reduced with SnCl₂. The produced Hg⁰ is purged from the aqueous phase at a flow rate of 2.5 L min⁻¹ for 1 hour and collected by a trap containing 550 mg of ClC. Hg collected in the ClC trap is then thermally desorbed in argon carrier gas and preconcentrated into a 40% mixed acid solution (4 M HNO₃ and 1.3 M HCl). This method was evaluated using solutions spiked with NIST SRM 3133, UM-Almadén and BCR 482 standards at Hg concentrations of 0.5 to 40 ng L⁻¹. The results showed an analytical recovery of 98.8 ± 3.7% (1SD, n = 37) with enrichment factors of up to 1000 times and no significant difference in isotope compositions between the recovered Hg and original standards (t-test, p-δ²⁰²Hg = 0.24, p-Δ¹⁹⁹Hg = 0.58, and p-Δ²⁰⁰Hg = 0.87). Duplicate experiments on field aqueous samples showed that the analytical precision was 0.13‰, 0.06‰ and 0.07‰ (n = 16) for δ²⁰²Hg, Δ¹⁹⁹Hg and Δ²⁰⁰Hg, respectively. The developed method is reliable and efficient for concentrating Hg in natural water samples for determining the Hg isotopic compositions.