Direct determination of 237Np in nuclear effluent by ICP-MS/MS

Abstract

A new method for the rapid quantification of neptunium 237 in nuclear effluent was developed. This method is based on the direct measurement of neptunium 237 by ICP-MS/MS in mass-shift mode as NpO and using 20% CO₂ as a reaction gas. These measurement conditions allowed the best signal-to-noise ratio at m/z 253 (²³⁷Np¹⁶O) to be obtained by the efficient elimination of the interferences due to the abundance sensitivity of uranium 238 and hydrides of uranium 235 and uranium 236 while minimizing the elemental fractionation between neptunium and plutonium. Moreover, plutonium 242 can be used as a tracer for neptunium 237 isotopic dilution-based quantification. After an optimization of the measurement parameters, excellent figures of merit were obtained. Indeed, this protocol allows an accurate direct measurement of neptunium 237 in various nuclear effluent samples containing up to 2 μg L⁻¹ (2.4 Bq L⁻¹) of uranium 236 and up to 20 mg L⁻¹ (250 Bq L⁻¹) of uranium 238 with excellent accuracy and uncertainties lower than 15%, for samples containing 10⁻⁵ to 10⁻¹ μg L⁻¹ (10⁻³ to 13 Bq L⁻¹), and a limit of detection for ²³⁷Np as low as 4 pg L⁻¹ (10⁻⁴ Bq L⁻¹).