COMPARISON OF DISTILLATION WITH OTHER CURRENT ISOLATION METHODS FOR THE DETERMINATION OF METHYL MERCURY-COMPOUNDS IN LOW-LEVEL ENVIRONMENTAL-SAMPLES .1. SEDIMENTS

In the Present study isolation of methyl mercury compounds (MeHg) from sediments by distillation, followed by aqueous phase ethylation, precollection on a Carbotrap, isothermal gas chromatography and cold vapour atomic fluorescence (CVAF) detection, was investigated. Results obtained by distillation were compared with those obtained by alkaline digestion and HCl leaching methods. A good agreement of the results for MeHg in sediments of various origins and wide concentration range (0.04-60.0 ng g-1, dry weight) obtained by distillation and KOH-methanol digestion was found. The main advantage of the distillation method over alkaline digestion is avoidance of matrix effects during the ethylation process. Distillation gives consistent and high recoveries (95 +/- 4%) and therefore provides more accurate results especially, at low MeHg concentrations. Detection limits as low as 0.001 ng MeHg g-1 as Hg for 100 mg sample weight were obtained. The HCl leaching method was found to be ineffective in releasing MeHg from sediment quantitatively. Even 8 M Ha could not release MeHg quantitatively; on the contrary, if the concentration of HCl was more than 4 M, decomposition of released MeHg occurred. Addition of Cu(II) ions increased the release of MeHg into solution, but not quantitatively. The difference in the results obtained by distillation and HCl leaching was in positive correlation with the concentration of MeHg (r2 = 0.92) and the concentration of total organic carbon (TOC) (r2 = 0.88). This suggests that the fraction of MeHg released from the sediment by HCl might be of certain interest in studies of the biogeochemical cycle of Hg. The results obtained in the present study clearly demonstrate the necessity for additional quality control exercises, particularly the establishment of certified reference materials such as sediment samples of various origin. The sediments currently certified for total Hg from NRCC (MESS-1, PACS-1), and NIES (Pond Sediment) were also analyzed for MeHg. Results obtained by three laboratories using different separation and detection systems showed good agreement. Additionally, the behaviour of MeHg under different storage conditions (deep frozen, refrigerated, dried) was also studied. No changes of MeHg concentration were found during a period of 8 months, if samples were deep frozen or refrigerated, while drying of sediments in an oven at 85-degrees-C resulted in a 15% decrease of total and 45% of MeHg.