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

Plutonium isotope ratios have been determined on solutions with concentrations covering four orders of magnitude, 100 fg ml–1 to 600 pg ml–1, using multicollector ICP-MS (Micromass IsoProbe). Discrimination between different sources of anthropogenic plutonium requires both precise and accurate isotope ratio determination at environmental concentration levels. To achieve a precision of better than 1% at these concentrations, we have developed an analytical procedure in which an equal atom 236U–233U double spike was added to solutions to correct for drift in signal intensity between peak jump sequences. This double spike is also used to correct for instrument mass 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 240Pu/239Pu of about 0.18, means a 239Pu content of about 80 fg in the analyte. 240Pu/239Pu 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 242Pu/239Pu with a precision of better than 2% on solutions containing 30 fg of 242Pu and better than 10% with 5 fg of 242Pu.