DEMETHYLATION OF METHYLMERCURY DURING INORGANIC MERCURY DETERMINATION BY THE MAGOS COLD VAPOR ATOMIC-ABSORPTION TECHNIQUE

Previous animal experiments suggested that the Magos cold vapor atomic absorption spectroscopic (CVAAS) method might overestimate the concentrations of inorganic mercury (I-Hg) in the presence of methylmercury (MeHg). In the present study it is shown that this error is due to a fast degradation of MeHg during the formation of the analytical signal. For brain samples, about 5% of the total amount of MeHg in the reaction vessel is degraded to I-Hg. Speciation of Hg in aqueous solution of MeHg chloride, after purification with ion-exchange chromatography using the Magos method, showed that about 9% was I-Hg. Analysis by NMR of MeHg chloride and MeHg hydroxide showed that less than 1% was in the form of I-Hg. The absolute magnitude of the error in the CVAAS method is dependent on the amounts of SnCl2 and MeHg in the reaction vessel; however, the ratio of I-Hg to total (T-Hg) is shown to be independent of the amount of MeHg (25.5 - 255 ng as Hg) in the reaction vessel. A procedure for corrections is proposed, based on the results from these studies and empirical data from speciation analyses of brain tissue from MeHg-exposed rats and rabbits.