Validation of a simplified field-adapted procedure for routine determinations of methyl mercury at trace levels in natural water samples using species-specific isotope dilution mass spectrometry

A field-adapted procedure based on species-specific isotope dilution (SSID) methodology for trace-level determinations of methyl mercury (CH3Hg+) in mire, fresh and sea water samples was developed, validated and applied in a field study. In the field study, mire water samples were filtered, standardised volumetrically with isotopically enriched (CH3Hg+)-Hg-200, and frozen on dry ice. The samples were derivatised in the laboratory without further pre-treatment using sodium tetraethyl borate (NaB(C2H5)(4)) and the ethylated methyl mercury was purge-trapped on Tenax columns. The analyte was thermo-desorbed onto a GC-ICP-MS system for analysis. Investigations preceding field application of the method showed that when using SSID, for all tested matrices, identical results were obtained between samples that were freeze-preserved or analysed unpreserved. For DOC-rich samples (mire water) additional experiments showed no difference in CH3Hg+ concentration between samples that were derivatised without pretreatment or after liquid extraction. Extractions of samples for matrix-analyte separation prior to derivatisation are therefore not necessary. No formation of CH3Hg+ was observed during sample storage and treatment when spiking samples with Hg-198(2+). Total uncertainty budgets for the field application of the method showed that for analyte concentrations higher than 1.5 pg g(-1) (as Hg) the relative expanded uncertainty (REU) was approximately 5% and dominated by the uncertainty in the isotope standard concentration. Below 0.5 pg g(-1) (as Hg), the REU was > 10% and dominated by variations in the field blank. The uncertainty of the method is sufficiently low to accurately determine CH3Hg+ concentrations at trace levels. The detection limit was determined to be 4 fg g(-1)(as Hg) based on replicate analyses of laboratory blanks. The described procedure is reliable. considerably faster and simplified compared to non-SSID methods and thereby very suitable for routine applications of CH3Hdivided by speciation analysis in a wide range of water samples.