Determination of plutonium 240/239 ratios in Lake Ontario sediments.

A thermal ionization mass spectrometric procedure was developed for determination of 240/239 ratios in subpicogram quantities of plutonium (Pu), and was applied to Pu extracted from Lake Ontario sediment. The detection limit was ∼ 4 fg of Pu and the precision of 240/239 ratio measurement was ∼ 7% in the 0.07-0.42 pg range. Results of sediment analyses at various depths showed in all cases 240/239 ratios near the mean global fallout value of 0.176 ± 0.014. The lack of variability of the ratios with depth negated the use of Pu, present in excess in the near-surface samples due to leakage from a nearby reprocessing plant, as a geochronological meter. The 240/239 ratio of the leakage Pu appeared to be very similar to that of fallout Pu.