Rapid dating of recent aquatic sediments using Pu activities and 240Pu/239Pu as determined by quadrupole inductively coupled plasma mass spectrometry

Quadrupole inductively coupled plasma mass spectrometry has been used to rapidly establish the chronology of recent aquatic sediments via measurements of the activities of (239)pu, (240)pu, and the atom ratio Pu-240/Pu-239. Following addition of 0.007 Bq of a Pu-242 spike isotope, Pu is leached from 3-20 g aliquots of dry-ashed sediments with HNO3. A selective anion exchanger is used to preconcentrate Pu into similar to2 mL aliquots, which are directly analyzed using a pneumatic nebulizer and double-pass spraychamber operating at 60 muL/min solution uptake rate, The ICPMS data collection is performed for 10 min per sample. The U concentrations were 0.01 -0.05mug/L in the analyzed solutions, and the interference of (UH+)-U-238-H-1 upon Pu-239(+) was negligible. The method has been applied to determining Pu activities, inventory, and (240)pu/Pu-239 in a complete sediment core from Old Woman Creek (Huron, OH). The Pu activity profiles, obtained in similar to6 h of instrumental measurement time, are in agreement with a gamma spectrometric Cs-137 profile. Peak Pu239+240 and Cs-137 activities in the core were 1.60 +/- 0.02 and 47.8 +/- 0.8 Bq/kg, respectively; inventories were 108 +/- 2 Bq/m(2) Pu239+240 and 2710 +/- 40 Bq/m(2 137)Cs. Detection limits, based upon the analysis of 20 g samples, were 0.004 Bq/kg Pu-239, 0.012 Bq/kg Pu-240, and 0.012 Bq/kg Pu239+240. Pu-240/Pu-239 atom ratios of 0.16-0.19 were obtained for all core intervals containing detectable Pu, which indicates that global fallout is the source of these radionuclides.