COMPARISON OF DISTILLATION WITH OTHER CURRENT ISOLATION METHODS FOR THE DETERMINATION OF METHYL MERCURY-COMPOUNDS IN LOW-LEVEL ENVIRONMENTAL-SAMPLES .2. WATER

In the present paper two isolation procedures for the separation of methyl mercury compounds (MeHg) from natural water samples, followed by aqueous phase ethylation, precollection on the Carbotrap, isothermal gas chromatography and cold vapour atomic fluorescence (CV-AFS) detection were compared and evaluated. The first isolation technique is based on extraction of MeHg into methylene chloride and back extraction into water by solvent evaporation. The second is based on the distillation of MeHg compounds. A comparison of these two isolation procedures was performed on 110 water samples of various origin (oxic, anoxic fresh water and sea water) and containing a wide MeHg concentration range (from 0.01 to 25 ng l-1). A relatively good agreement of the results was obtained in the concentration range below 1 ng l - 1 as Hg (r2 = 0.919, n = 61). In anoxic, sulphide rich and organic matter rich water samples that usually contain higher MeHg values, significantly higher values (approx. 30%) were obtained by distillation. This is partly due to sulphide interference during the ethylation step, as well as incomplete release of MeHg from bound sites. Distillation was found advantageous to solvent extraction. It gives consistent and high recoveries (80-95%) for various water samples, and achieves lower detection limits (0.006 ng l-1 for 50 ml water sample). It is also less laborious and without the use of organic solvent. Additionally, distillation provides the specific separation of MeHg and dimethylmercury, if present.