Isotopic composition analysis of dissolved mercury in seawater with purge and trap preconcentration and a modified Hg introduction device for MC-ICP-MS

Abstract

This study aimed to solve the common problems in Hg isotope analysis of water samples at low concentration. The isotope composition of dissolved Hg in seawater is reported for the first time. A modified device for introducing Hg into a multi-collector inductively coupled plasma mass spectrometer and a preconcentration method for the preconcentration of dissolved Hg were developed to enhance the sensitivity of the isotopic composition analysis method. The modified cold-vapor generator was used to transfer dissolved Hg²⁺ from the matrix into gaseous Hg⁰. The purge and trap method was developed and employed to preconcentrate dissolved Hg in water samples. Keeping other parameters the same, the Hg signal generated with the modified Hg introduction device was twice as much as the commercial one (HGX 200). In the measurement of NIST SRM 3133, the external precision for δ²⁰²Hg was 0.06‰ (2SD, n = 310), and the δ²⁰²Hg value of the UM-Almadén in-house secondary standard was −0.57 ± 0.10‰ (2SD, n = 49), indicating that the modified device was stable and reliable. Factors influencing the efficiency of the purge and trap method, e.g., concentration of KMnO₄ in the trapping solution, flow rate of the purge gas and purge time, were optimized. With ultrapure water (blank) and seawater (matrix) spiked with NIST SRM 3133 at Hg concentrations of 5.00–35.50 ng L⁻¹ and 10.00–35.50 ng L⁻¹, the δ²⁰²Hg values of the blank spike and matrix spike were 0.00 ± 0.04‰ (2SD, n = 19) and −0.02 ± 0.04‰ (2SD, n = 12), respectively. The results indicated that the purge and trap method was free from matrix interference. The results of this practical application showed good stability and reproducibility of the proposed methods.