Development of an Isotope Dilution Mass Spectrometric Method for Dimethylthallium Speciation and First Evidence of Its Existence in the Ocean

Abstract

A sensitive analytical method for the determination of dimethyl thallium (Me2Tl+) in environmental samples by positive thermal ionization isotope dilution mass spectrometry (PTI-IDMS) was developed. For the necessary low detection limit and for species selective determination, PTI-IDMS was connected with species-unspecific enrichment of thallium from ocean water samples by a strongly basic anion exchanger and a species-specific extraction step, respectively. For isotope dilution, a 203Tl-enriched Me2Tl+ spike solution was synthesized and characterized with respect to its isotopic composition and concentration. The spike solution was stable for the 30-month period of investigations under dark storage conditions at pH = 2 and 4 °C. The detection limit of the developed method is 0.4 ng L-1 for 500 mL ocean water samples and the species selectivity of Me2Tl+, compared with that of inorganic thallium, is >500. In different surface water samples of the Atlantic Ocean from 50°N to 64°S and in one depth profile down to 4000 m, Me2Tl+ concentrations in the range of <0.4 to 3.2 ng L-1 were determined. This is the first time that dimethyl thallium could be detected in environmental samples. Positive detection of the organothallium compound was found in about 20% of all samples analyzed. By also analyzing the total thallium content with PTI-IDMS it was found that in these cases 3−48% of thallium exists in the methylated form. From its occurrence in the remote areas of the South Atlantic, from positive correlation with the bioactivity in the corresponding ocean water, and from the fact that Me2Tl+ is not known as an anthropogenic substance, it follows that a biogenic origin must be assumed.