Investigation of errors introduced by the species distribution of mercury in organic solutions on total mercury determination by electrothermal vaporisation–inductively coupled plasma mass spectrometry

This paper presents an investigation into the potential and limitations of an ETV system for the determination of total mercury, present as different species, in organic solution. The thermal and chemical stability of four mercury species in an electrothermal vaporiser, ETV, coupled to an inductively coupled plasma (ICP-MS) mass spectrometer was investigated when samples of hydrocarbon products were introduced. Calibration by the method of isotope dilution was tested with an isotopically enriched standard of mercury diethyldithiocarbamate, 201HgDDTC, in toluene. Sulfur-containing compounds, different palladium solutions and gold metals were investigated for use as matrix modifiers. A combination of Pd(NO3)2 in propanol with stabilising additives applied as a permanent modifier (A. Asheim, poster presentation, 3rd European Furnace Symposium, Prague, 1998) and thionyl chloride injected with each sample gave the most effective stabilisation of mercury species and reproducible signals with a relative standard deviation of 3% for 5 µg l−1 HgCl2. The recoveries, by isotope dilution, of HgCl2 and CH3HgCl in natural gas condensates ranged between 100 and 115%, while for Hg0 and (CH3)2Hg only 48–65% was recovered with optimised conditions. Losses of the latter two species occurred on drying of the sample in the furnace prior to heating to the pyrolysis temperature. Systematic errors in total mercury determination were, therefore, found to occur for samples containing Hg0 or (CH3)2Hg, even with isotope dilution calibration. The detection limit, based on 3s of 10 replicate measurements of the 202Hg+ signal for toluene spiked with 6 µg l−1 201HgDDTC, was 0.19 µg l−1.