| Environmental Hg pollution is an issue of global concern due to the high toxicity of its many forms, the most environmentally-relevant of which is Monomethylmercury (MeHg), CH3Hg+, a bioaccumulative neurotoxic organometallic compound formed naturally by anaerobic bacteria. Understanding the cycling of this element requires measurement of both MeHg and its precursor, mercuric (HgII) mercury. Currently state-of-the-art analytical methods for mercury speciation in solid and aqueous-phase environmental samples are very accurate, but also very time consuming, employing two separate, expensive measurements for MeHg and HgII mercury, arguably limiting the pace of discovery in this important area. The development of a single automated system for measurement of both forms in one analysis could significantly research in this important field. To this end, a new system of sample preparation and analysis of MeHg and mercuric HgII mercury has been developed. The system couples a series of ligand-exchange reactions for analyte isolation and concentration to liquid chromatographic separation and continuous-flow post-column chemistry; specifically, cationic thiourea complexes of monomethyl and mercuric ion are separated using high-pressure ion chromatography and quantified by subsequent on-line cold-vapor generation and gas-phase detection of Hg0 by atomic fluorescence spectrometry (CVAFS). The system has a high throughput (one sample every ∼10 minutes), uses external calibration, and has an average precision of ∼2.5% and absolute detection limit of <1 pg. Very low relative detection limits are possible by coupling the system to on-line thiol-resin preconcentration.; Two specialized sample preparation chemistries were developed to be used with the sample introduction methods of the HgTu/IC-CVAFS system: one for biotic tissues, and one for sediments. A digestion system for the simultaneous measurement of MeHg and HgII in biota samples using sequential HNO3 digestion and thiourea leaching coupled to on-line thiolic preconcentration has been designed for use with the system and shown to give accurate results with biotic reference materials and have limits of detection of ∼10 pg g-1 with 100 mg of dried sample. A specific solvent extraction for quantitative recovery of MeHg in high organic-carbon sediments, using HNO3, HCl and CuSO4 in the aqueous phase leaching solution and toluene for an organic solvent phase, has been developed for coupling to on-line thiolic preconcentration. The application have been proven accurate with reference materials and shows ∼100% recovery from polluted wetland sediments, a test which variably yielded between ∼70--85% recovery from a commonly used solvent extraction procedure employing H 2SO4, KBr, and CuSO4. |
