High resolution inductively coupled plasma mass spectrometry for the tracedetermination of plutonium isotopes and isotope ratios in environmental samples

A high resolution inductively coupled plasma mass spectrometric (HR-ICP-MS) method for the determination of plutonium isotopes and the Pu-240/Pu-239 i sotope ratio utilising ultrasonic nebulisation was developed. Total plutoni um concentrations (Pu239+240) measured in environmental samples by this HR- ICP-MS method were in good agreement with data obtained from alpha-spectrom etry. Quantification was performed by both external calibration and isotope dilution and the best agreement was found by applying isotope dilution. De tection limits of 5, 1 and 1 fg ml(-1) were found for Pu-239, Pu-240 and Pu -242, respectively. These represent total amounts of 50, 10 and 10 fg or 0. 1, 0.08 and 0.002 mBq of the three isotopes in a 10 mi sample volume. The p recision (RSD) on the measurement of the Pu-240/Pu-239 ratio in environment al samples was approximately 2%, which is close to the theoretical precisio n (Poisson statistics). The influence of dwell time, number of sweeps and s ample uptake rate on the measurement precision of the Pu-240/Pu-239 isotope ratio was investigated and the parameters were optimised using a 23 experi mental design (a central composite design followed by a ridge;analysis). Th e accuracy of the isotope ratio measurement was evaluated by comparing resu lts from this HR-ICP-MS method with results from high resolution a-spectrom etry and spectral deconvolution. Good agreement was found between the two t echniques.