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 ²³⁹Pu, ²⁴⁰Pu. and the atom ratio ²⁴⁰Pu/²³⁹Pu. Following addition of 0.007 Bq of a ²⁴²Pu 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 ∼2 mL aliquots, which are directly analyzed using a pneumatic nebulizer and double-pass spraychamber operating at 60 μL/min solution uptake rate. The ICPMS data collection is performed for 10 min per sample. The U concentrations were 0.01-0.05 μg/L in the analyzed solutions, and the interference of ²³⁸U¹H⁺ upon ²³⁹Pu⁺ was negligible. The method has been applied to determining Pu activities, inventory, and ²⁴⁰Pu/²³⁹Pu in a complete sediment core from Old Woman Creek (Huron, OH). The Pu activity profiles, obtained in ∼6 h of instrumental measurement time, are in agreement with a γ spectrometric ¹³⁷Cs profile. Peak ^239+240Pu and ¹³⁷Cs activities in the core were 1.60 ± 0.02 and 47.8 ± 0.8 Bq/kg, respectively; inventories were 108 ± 2 Bq/m^{2 239+240}Pu and 2710 ± 40 Bq/m^{2 137}Cs. Detection limits, based upon the analysis of 20 g samples, were 0.004 Bq/kg ²³⁹Pu, 0.012 Bq/kg ²⁴⁰Pu, and 0.012 Bq/kg ^239+240Pu. ²⁴⁰Pu/²³⁹Pu 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.