Online determination of mercury isotopic compositions at ultratrace levels by automated purge and trap coupled with multicollector inductively coupled plasma-mass spectrometry

Abstract

Characterizing natural mercury (Hg) isotopic compositions in geological and environmental matrices provides a better understanding of the biogeochemical cycles of Hg. However, preconcentration is indispensable for the determination of samples with low Hg contents (e.g., ng L⁻¹ levels), which is time-consuming and requires a lot of manpower and materials. Herein, a sensitive method for the online determination of Hg isotopic compositions by automated purge & trap (P&T) coupled with multicollector inductively coupled plasma-mass spectrometry (MC-ICPMS) was developed. Rapid Hg isotopic analysis was realized in 15 min (single sample-standard bracketing procedure) with a sample consumption of 25 mL per measurement. The optimization of the P&T module and MC-ICPMS was conducted to obtain a high sensitivity for Hg isotopic determination (²⁰²Hg max intensity over 10 V for 40 ng L⁻¹ Hg solution). Strategic signal acquisition and data treatment using the linear regression slope without time-lag correction were applied. Accurate isotopic compositions in a referenced standard solution at 8 ng L⁻¹ (NIST 3177, δ²⁰²Hg −0.55 ± 0.29‰, Δ¹⁹⁹Hg −0.02 ± 0.23‰, Δ²⁰⁰Hg 0.01 ± 0.19‰, Δ²⁰¹Hg −0.02 ± 0.16‰, 2SD, and N = 31) and certified reference materials (CRMs) at 40 ng L⁻¹ were obtained (NRC DORM-4, δ²⁰²Hg 0.45 ± 0.14‰, Δ¹⁹⁹Hg 1.73 ± 0.16‰, Δ²⁰⁰Hg 0.06 ± 0.08‰, Δ²⁰¹Hg 1.42 ± 0.13‰, 2SD, N = 13; NIST 1632e, δ²⁰²Hg −2.22 ± 0.19‰, Δ¹⁹⁹Hg −0.02 ± 0.07‰, Δ²⁰⁰Hg 0.00 ± 0.07‰, Δ²⁰¹Hg −0.01 ± 0.09‰, 2SD, and N = 20). The measuring concentration of this proposed method has decreased from μg L⁻¹ to ng L⁻¹ levels compared to CV introduction. The sample size and analytical time are 2–3 orders of magnitude lower compared to offline preconcentration methods (several hours to process several to dozens of liters). This proposed method can be used for the analysis of Hg isotopic compositions in solution samples with low Hg concentrations including natural aquatic samples.