Low-Level Mercury Speciation in Freshwaters by Isotope Dilution GC-ICP-MS

Atmospheric deposition of anthropogenic Hg has led to increased Hg concentrations in many ecosystems. Modeling is an effective method for predicting the complex dynamics of Hg transport in watersheds. Such models require accurate concentrations of water column methylmercury, CH3Hg+, for input parameters, yet these concentrations are very difficult to measure precisely because they are so low. We developed a method for aqueous CH3Hg+ quantification in Lake Champlain, VT, where ambient CH3Hg+ concentrations are <0.04 ng L-1. The analysis utilized species-specific isotope dilution, purge and trap gas chromatography inductively coupled plasma mass spectrometry and provided instrument detection limits of about 0.3 fM (0.06 pg L-1) and method detection limits of 15 fM (0.003 ng L-1) for CH3Hg+, which are among the lowest reported. Artifactual methylation of inorganic Hg2+ was shown to be minor, and the precision of the isotope dilution method was generally <5% relative standard deviation, which is much lower than would have been the case for an external calibration approach. The method is accurate even at low concentrations of 0.025 ng L-1. This combination of precision, accuracy, and low detection allows for quantification of significant differences in CH3Hg+ concentration between bays and over time within bays of Lake Champlain, where mean CH3Hg+ concentrations differ by only 0.006 ng L-1 at concentrations as low as 0.014 ng L-1.