Direct determination of mercury at the sub-picogram per gram level in polar snow and ice by ICP-SFMS
An analytical method for the direct determination of mercury (Hg) in polar snow and ice cores and surface snow based on inductively coupled plasma sector field mass spectrometry (ICP-SFMS) has been developed. Various Hg isotopes, such as Hg-199, Hg-200, Hg-201 and Hg-202, appear to be free of polyatomic interferences in such matrices and allow the measurements to be made in low resolution mode, leading to high sensitivity. Ultra-low concentration Hg standards (from 1.5 to 20 pg g(-1)) were used for the calibration of the Thermo Finnigan MAT Element2, and a detection limit as low as 0.18 pg g(-1) was achieved using Hg-202. Ultra-clean procedures used from field sampling to final laboratory analysis show no significant blank contributions and appear suitable for the reliable determination of Hg at ultra-low concentrations. Precision of the Hg measurements was estimated to be 15% in terms of relative standard deviation on five replicates and accuracy was checked with an analytical reference material (102% recovery). Hg concentrations in surface snow samples from the Northern Hemisphere collected in the Canadian Arctic and in Svalbard (Norway) show high variability (1.2-32.0 pg g-(1)). In Antarctica, Hg was determined in different ice core sections from Dome C, spanning the last 18000 years BP (range from 0.7 to 3.2 pg g(-1)), and in snow samples from Coats Land covering the last 150 years (range from 0.2 to 16.1 pg g(-1)).