DETERMINATION OF MERCURY ISOTOPE RATIOS IN SAMPLES CONTAINING SUBNANOGRAM AMOUNTS OF MERCURY USING INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY

The determination of sub-nanogram amounts of mercury demands systems o f high sensitivity. To determine isotopic ratios a large signal Is req uired to obtain a high number of counted ions. It Was found that the p recision in the isotope ratio determination in samples containing less than 300 pg of mercury is limited by the counting statistics. With 30 0 pg of mercury the precision was 0.2% (relative standard deviation) i n the Hg-202:Hg-199 ratio. The detection limit was lowered by concentr ation of mercury vapour on gold traps, and the sensitivity of the inst rument was increased 4-6-fold by addition of nitrogen or hydrogen to t he central gas flow.