MeHg in solids by distillation, isotope dilution, Brooks-Rand ICPMS
Analysis of Methylmercury in Solids by Distillation, Gas Chromatography Separation, and Speciated Isotope Dilution Mass Spectrometry.
The following standard operating procedure (SOP) is used by the U.S. Geological Survey’s Mercury Research Laboratory (MRL), and describes the preparation of soil, sediment, and plant matter by distillation and subsequent analysis for methylmercury (MeHg). Quality assurance and control protocols are employed throughout sample distillation and analysis, including: laboratory practices to prevent sample contamination, method blanks, analysis of certified reference materials (CRM), replicate analyses, and isotopic MeHg additions for quantification by isotopic dilution (ID). Prior to analysis, distillation is required to disassociate MeHg from the sample matrix and reduce matrix interference during analysis. Solids are weighed into Teflon distillation vials, to which 40 ml of reagent water, an isotopically enriched MeHg spike, and 2 ml of acidified KCl/CuSO4 solution are added. The distillation vials are heated to 121°C, and purged with nitrogen gas. The resulting water vapor carries the disassociated MeHg from the sample and is condensed in chilled Teflon receiving vials (distillate). Distillation is stopped when approximately 25% of the water remains in the distillation vessels. Analysis is conducted via the Brooks-Rand “MERX” automated MeHg analytical system coupled to the Elan inductively coupled plasma-mass spectrometer (ICPMS). Distillate is added to 42 ml glass vials and buffered with Sodium Acetate/Acetic Acid to a pH of 4.5 – 5.0. Sodium Tetraethylborate (NaTEB) is added to the sample resulting in ethylation of the oxidized mercury species (Hg2+ and MeHg+ ). The volatile ethylated species, as well as elemental mercury, are stripped from the distillate with argon gas, retained on Tenex traps, thermally desorbed back into the sample stream, and separated by mass with a gas chromatography column. The elemental and ethylated mercury species are released from the column en masse into the sample stream, thermally oxidized to elemental mercury, and introduced to the ICPMS for detection. Quantification of MeHg concentrations in the samples are calculated using isotopic dilution.
Mercury Research Laboratory, 2016