Determination of Concentration Profiles of Methyl Mercury Compounds in Surface Waters of Polar and other Remote Oceans by GC-AFD

Abstract

The concentration of monomethyl mercury (MeHg+) and dimethyl mercury (Me2Hg) was determined in surface sea-water samples of the Antarctic and Arctic Ocean as well as of other remote areas (South Atlantic and South Pacific) during expeditions of the German research vessel “Polarstern”. A purge and trap/gas chromatographic system, equipped with an atomic fluorescence detector (AFD), was used. For the analysis of MeHg+ conversion into the volatile methylethyl mercury by reaction with tetraethyloborate prior to the purging process was carried out. The detection limit for both methylated mercury compounds was 5 pg Hg/L, which allowed their determination in most ocean water samples even in those of the Antarctic and Arctic Ocean. A north-south concentration profile in the Atlantic Ocean, covering a distance from 51°N to 58°S, was also examined, which resulted in the most extended set of data in the environment for these important heavy metal species. In anthropogenically influenced areas of the North Atlantic from 51°N to about 40°N concentrations of methylated mercury in the range of 100–3000 pg/L were found. The contents of these species were significantly lower in remote areas, represented by a range of <5 pg/L to 150 pg/L. Concentrations of the methylated mercury species were compared with those of substances often used as biomass indicators, e.g. chlorophyll-a and adenosine triphosphate. A positive correlation was found, in general, in remote areas between the contents of methylated mercury and these parameters for bioactivity, demonstrating the biogenic origin of MeHg+ and Me2Hg, respectively. The concentration of MeHg+ normally exceeded that of Me2Hg, except at locations with especially high bioactivities. This result indicates that Me2Hg may be the main primary biogenic product. Oceanographic conditions are very well reflected by concentration profiles of methylated mercury. For example, in biologically very active upwelling regions peak concentrations of Me2Hg and MeHg+ were found, whereas in parts of the Antarctic Ocean, totally covered by ice, the concentration of methylated mercury was determined to be below the detection limit.