Species-specific isotope dilution with permeation tubes for determination of gaseous mercury species

摘要

Instrumentation and methodology for determination of the gaseous mercury species Hg-0, (CH3)(2)Hg, and CH3Hg+ has been developed. The method is based on continuous addition of gaseous isotopically enriched Hg species (tracers) at the point of sample acquisition, in combination with reduced pressure sampling on Carbotrap adsorbent tubes. Permeation tubes are used for generation of the tracers. Collected species are thermally desorbed and purged through an aqueous sodium tetraethylborate solution for derivatization of CH3Hg+. The purged gas is dried with a Nafion membrane, and the Hg species are subsequently collected on a smaller Tenax TA adsorbent tube. Species are then thermally desorbed from the Tenax TA and introduced into a gas chromatograph connected to an inductively coupled plasma mass spectrometer for separation and detection. To be able to add tracers during field sampling, we developed a portable device, supplying the permeation tubes with a thermostated and mass flow-controlled air stream of 5.0 +/- 0.1 degreesC and 50.0 mL min(-1), respectively. Typical permeation rates obtained during a period of more than 6 weeks were 12.93 +/- 0.56, 0.42 +/- 0.01, and 0.49 +/- 0.03 (mean +/- standard deviation) pg of Hg min(-1) for a set of Hg-199(0), (CH3)(2)Hg-198, and (CH3Hg+)-Hg-200 tubes, respectively. Methodological detection limits (3sigma) were determined to 700 pg of Hg m(-3) for He and 50 pg of Hg m-3 for (CH3)2Hg and CH3Hg+. The collection efficiencies for sampled volumes of 400 L of synthetic air on the Carbotrap tubes used in this study were 13 +/- 2, 102 +/- 2, and 99 +/- 4% for Hg-0, (CH3)(2)Hg, and CH3Hg+, respectively. Desorption efficiencies for the above species and tubes were 98 2, 98 1, and 90 4%, respectively. Fractions (20-40%) of the added (CH3)(2)Hg-198 and (CH3Hg+)-Hg-200 tracers were found to be transformed during the analytical processing of collected air samples. Determined concentrations in the research laboratory air, corrected for species transformations, were 3-53, 8-11, and 1-2 ng of Hg m(-3) for He, (CH3)(2)Hg, and CH3Hg+, respectively. Concentrations in the ambient air were determined to be 2.1-2.6 ng m(-3) for Hg-0 and below the detection limit for (CH3)(2)Hg and CH3Hg+. [References: 26]