Abstract

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.