title: 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.
abstract: A field-adapted procedure based on species-specific isotope dilution (SSID) methodology for trace-level determinations of methyl mercury (CH(3)Hg(+)) 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 CH(3) (200)Hg(+), and frozen on dry ice. The samples were derivatised in the laboratory without further pre-treatment using sodium tetraethyl borate (NaB(C(2)H(5))(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 CH(3)Hg(+) concentration between samples that were derivatised without pre-treatment or after liquid extraction. Extractions of samples for matrix-analyte separation prior to derivatisation are therefore not necessary. No formation of CH(3)Hg(+) was observed during sample storage and treatment when spiking samples with (198)Hg(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 CH(3)Hg(+) 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 CH(3)Hg(+) speciation analysis in a wide range of water samples.
authors.affiliation: Umeå Marine Sciences Centre, Norrbyn, 910 20, Hörnefors, Sweden. lars.lambertsson@umf.umu.se
authors.forename: Lars
authors.initials: L
authors.lastname: Lambertsson
authors.forename: Erik
authors.initials: E
authors.lastname: Björn
mesh_terms: D004784:Environmental Monitoring / Q000379:methods; D005615:Freezing; D008401:Gas Chromatography-Mass Spectrometry / Q000379:methods*; D008629:Mercury Isotopes; D008767:Methylmercury Compounds / Q000032:analysis* / Q000737:chemistry; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014867:Water / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D008629:Mercury Isotopes; D008767:Methylmercury Compounds; D014874:Water Pollutants, Chemical; D014867:Water