Determination of methyl mercury in humic-rich natural water samples using N2-distillation with isotope dilution and on-line purge and trap GC-ICP-MS

In this study a method for the determination of low methyl mercury (CH3Hg+) concentrations in non-filtered humic-rich natural water samples using an ethylation-purge and trap with isotope dilution GC-ICP-MS was developed. Purge and trap system was connected on-line to GC which was then coupled to ICP-MS with a simple non-heated in-house made interface. Samples analyzed in this study contained high amounts of dissolved organic matter (16-83 mg DOC L− 1) which is known to interact strongly with mercury affecting the ethylation efficiency. Thus, prior to instrumental determination CH3Hg+ was separated from the interfering matrix using a nitrogen-assisted distillation. Isotopically enriched spike was added to samples prior to distillation to correct the incomplete analyte recovery during the whole analysis procedure. The method detection limit for a 20 mL sample volume was 0.05 ng L− 1 (3σ). The method precision obtained by replicate analyses of natural water samples was found to be better than 10% (RSD). Since there is no certified reference material available for low-level CH3Hg+ in water, the accuracy of the method was verified by comparing the results with the results obtained with gas chromatography - cold vapor atomic fluorescence spectrometry (GC-CV-AFS) based on the EPA method 1630. The results obtained with the two different methods were in good agreement with each other (r2 = 0.87, n = 25) and the method was successfully applied to non-filtered humic-rich natural waters.