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

Abstract

A high resolution inductively coupled plasma mass spectrometric (HR-ICP-MS) method for the determination of plutonium isotopes and the 240 Pu/ 239 Pu isotope ratio utilising ultrasonic nebulisation was developed. Total plutonium concentrations ( 239+240 Pu) measured in environmental samples by this HR-ICP-MS method were in good agreement with data obtained from α-spectrometry. Quantification was performed by both external calibration and isotope dilution and the best agreement was found by applying isotope dilution. Detection limits of 5, 1 and 1 fg ml –1 were found for 239 Pu, 240 Pu and 242 Pu, 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 ml sample volume. The precision (RSD) on the measurement of the 240 Pu/ 239 Pu ratio in environmental samples was approximately 2%, which is close to the theoretical precision (Poisson statistics). The influence of dwell time, number of sweeps and sample uptake rate on the measurement precision of the 240 Pu/ 239 Pu isotope ratio was investigated and the parameters were optimised using a 2 3 experimental design (a central composite design followed by a ridge analysis). The accuracy of the isotope ratio measurement was evaluated by comparing results from this HR-ICP-MS method with results from high resolution α-spectrometry and spectral deconvolution. Good agreement was found between the two techniques.