Species specific isotope dilution with on line derivatisation for determination of gaseous mercury species

Abstract

A species specific isotope dilution (SSID) method for determination of the volatile mercury (Hg) species Hg⁰, (CH₃)₂Hg, CH₃HgX and HgX₂ (where X is a counter ion) in gaseous samples was developed. The procedure involved on line derivatisation of ionic Hg species with sodium tetraethylborate during sampling, before collection of species on a Tenax TA tube in series with an Au–Pt tube. Collected species were thermally desorbed and determined by GC-ICP-MS. The procedure provided absolute detection limits of 0.5 pg for CH₃HgX and (CH₃)₂Hg, 5 pg for HgX₂ and 25 pg for Hg⁰. Recoveries and transformations of the investigated species were studied for the different procedural steps and the method was tested for microcosm experiments on brackish water sediment samples, as well as for laboratory air samples. As a result of methylation and reduction processes within the sediment, gaseous emission rates could be determined for all investigated species after addition of an isotope enriched aqueous inorganic ¹⁹⁹Hg²⁺ standard to the sediment. Hg⁰ was the only specie detected in laboratory air samples (4.5 ng m⁻³). The procedure for derivatisation of ionic Hg species prior to collection on Tenax TA reduces memory effects and transformations of CH₃HgX and HgX₂, which is especially advantageous at low sample gas flow rates and for complex gaseous matrices, such as emissions from sediment samples. SSID provides simplified methodological development, as well as efficient correction for matrix induced losses and transformations of Hg species. The method could be useful for various biogeochemical studies of Hg, as well as for reliable risk assessments during, for example, waste management and soil remediation.