Plutonium isotope ratio analysis at femtogram to nanogram levels by multicollector ICP-MS

Plutonium isotope ratios have been determined on solutions with concentrati ons covering four orders of magnitude, 100 fg ml(-1) to 600 pg ml(-1), usin g multicollector ICP-MS (Micromass IsoProbe). Discrimination between differ ent sources of anthropogenic plutonium requires both precise and accurate i sotope ratio determination at environmental concentration levels. To achiev e a precision of better than 1% at these concentrations, we have developed an analytical procedure in which an equal atom U-236-U-233 double spike was added to solutions to correct for drift in signal intensity between peak j ump sequences. This double spike is also used to correct for instrument mas s bias during each ratio determination. Analyses were made on about 1 ml of solution which, in the case of a 100 fg ml(-1) sample with a fallout Pu-24 0/Pu-239 of about 0.18, means a Pu-239 content of about 80 fg in the analyt e. Pu-240/Pu-239 can be reproducibly measured to within 1.4% (2s) at 100 fg ml(-1) and better than 0.3% at >3 pg ml(-1). Using this same technique we have also successfully measured Pu-242/Pu-239 with a precision of better th an 2% on solutions containing 30 fg of Pu-242 and better than 10% with 5 fg of Pu-242.