Collection of atmospheric gaseous mercury for stable isotope analysis using iodine- and chlorine-impregnated activated carbon traps
The trace levels of Hg in the atmosphere pose an analytical challenge for Hg stable isotope research. Here we propose a new method using iodine- and chlorine-impregnated activated carbon (IC and CLC) traps for collection of atmospheric total gaseous mercury (TGM). Decontamination of IC and CLC yielded low blank levels of 0.3 ng Hg per g of carbon, corresponding to <3% of collected Hg. IC and CLC traps with 250, 600 and 1000 mg of a sorbent collected TGM efficiently (>92%) at flow rates of 2, 10, and 20 LPM respectively. These flow rates enabled high-resolution (12–24 h) and low-resolution (bi-weekly) sampling of TGM. We also tested bi-weekly TGM sampling using commercial gold traps at a low flow rate of 0.3 LPM. TGM collected on IC and CLC traps at the Pic du Midi Observatory was desorbed and pre-concentrated into aqueous solution using an off-line combustion and trapping technique. Hg isotopic compositions were determined using cold vapor multicollector-inductively coupled plasma-mass spectrometry (CV-MC-ICPMS). The field measurements showed mean recoveries of 88 ± 5% (1σ, n = 11) for 12 h and 24 h and 90 ± 6% (1σ, n = 25) for the bi-weekly field samples. Laboratory additions of Hg vapor of known isotopic composition to CLC and gold traps indicated that recoveries >81% did not induce isotopic artifacts. Samples with recoveries <81% showed a positive mass dependent fractionation bias. Typical expanded uncertainties (2σ) on δ202Hg (mass dependent fractionation of 202Hg) and Δ199Hg (mass independent fractionation of 199Hg) for the bi-weekly sampling, processing and CV-MC-ICPMS analysis are 0.13‰ and 0.09‰, respectively. Our study suggests that the proposed method meets the criteria for accurate and precise measurements of atmospheric TGM isotopic compositions.