General: We now use the automated methylHg system MERX-M (Brooks Rand, Seattle, WA) interfaced with our ICP-MS instruments. The MERX consists of an autosampler, a purge and trap unit and a packed gas chromatography unit, and uses EPA method 1630 where ethylation with NaBH4 converts methylHg and inorganic Hg to the volatile ethylated derivatives. These volatile species are then purged with an inert gas (ultra pure N2) and trapped on Tenax traps. The Tenax traps are then thermally desorbed and the ethylated Hg species are carried in a Ar/Xe gas stream onto a packed gas chromatography column heated isothermally at 36 C. The effluent flow from the column is directed to an ICP-MS, which serves as an element specific detector to generate chromatograms for 199Hg, 201Hg, and202Hg. We use isotope dilution for quantification so all samples are spiked prior to analysis with an appropriate amount of enriched 199Hg2+ and CH3201Hg+.
Water samples are poured into pre-weighed 40 ml glass autosampler tubes and the weight of sample recorded. An appropriate amount of enriched isotope spike for each Hg species is then added to the sample and the weight of each spike is recorded. The sample is buffered with a citrate buffer (300 μl of 2M buffer) and the ethylating reagent (40 μl of 1% NaBH4) is added. The samples are then run directly on the MERX-ICP-MS system. Low level inorganic Hg determination ( < 1 ppt) is presently not reliably measured by this technique in our laboratory because of analytical blank issues. No such issues are present for methylHg which can be measured accurately to low pg/L concentrations.
Biological tissues are extracted in a 25% KOH/methanol solution. Approximately 50 mg sample is weighted into an amber glass vial and the enriched spikes are added and their weights recorded. Three ml of 25% KOH/methanol extractant is added and the sample is shaken overnight. A 50 μl aliquot of the extracted sample solution is then added to the 40 ml MERX autosampler vial and 40 ml DI is added to fill the vial. The vial is then buffered and ethylated and analysed as described above for water analysis.
Soil and Sediment samples are analyzed separately for methylHg and total Hg. Due to the very large ratio of inorganic Hg to methyl Hg in soils and sediments, the species have to be separated and analyzed individually. For methylHg determination, 0.5 g of sediment is weighted into a PP tube, along with the single isotope methylHg spike. Five ml of 18%KBr and 5% H2SO4 and 1 ml of 1M CuSO4 are added to each sample, then vials are capped and shaken for 1 hr. A 5 mL aliquot of CH2Cl2 is then added to each sample, and the samples are shaken for another hour. Samples are centrifuged at 3000 rpm for 15 min. then the CH2Cl2 fraction (containing methylHg) extracted with a pipette and transferred to another PP vial. Ten ml of DI water is added to each vial and the vial is placed in a warm water bath and bubbled with Ar until all of the CH2Cl2 has evaporated. Samples are then transferred to 40 ml amber glass vials buffered, ethylated and filled with DI water prior to analysis.
More in depth descriptions of our Hg speciation methods are published in:
Jackson, B., V Taylor, R.A. Baker, E. Miller. 2009. Low level mercury speciation in freshwaters by isotope dilution GC-ICP-MS. Environ. Sci. Technol. 43, 2463–2469
Taylor, V.F., B.P. Jackson, C.Y. Chen, 2008. Mercury speciation and total trace element determination of low-biomass biological samples. Anal. Bioanal. Chem. 392:1283–1290.
Taylor, V.F., Carter, A., Davies, C., Jackson, B.P. 2011.Trace-level automated mercury speciation analysis. Analytical Methods. 3, 1143-1148.