Simultaneous determination of femtogram level of ²³⁷Np, ²³⁹Pu and ²⁴⁰Pu in environmental solid samples using extraction chromatography and ICP-MS/MS

An analytical method based on triple quadrupole inductively coupled plasma mass spectrometry (ICP-MS/MS) was developed for simultaneous determination of femtogram levels of ²³⁷Np, ²³⁹Pu, and ²⁴⁰Pu in environmental samples. By carefully controlling the valence states of Np and Pu in the entire separation procedure using a simple single extraction chromatographic column (TK200), the consistent behavior of Np and Pu was achieved, allowing for the reliable application of ²⁴²Pu as the chemical yield tracer for ²³⁷Np, ²³⁹Pu, and ²⁴⁰Pu. A high decontamination factor of 3.2 × 10⁷ for the most critical interfering element, uranium, was achieved in the chemical separation step. The interferences of ²³⁸U¹H⁺ and peak tailing of ²³⁸U⁺ during the measurement of plutonium isotopes were effectively eliminated by utilizing 7.5 mL/min He–1.1 mL/min CO₂ as reaction gases in the octupole collision/reaction cell and employing sequential quadrupole mode for mass separation in ICP-MS/MS. Specifically, the interference of ²³⁸U¹H⁺ was reduced to 10⁻⁶ and the peak tailing of ²³⁸U⁺ to 10⁻¹⁰, surpassing the performance of measurement method without reaction gases by 3 orders of magnitude. The developed method enables the accurate determination of femtogram levels of ²³⁷Np, ²³⁹Pu, and ²⁴⁰Pu in the samples with U/Np and U/Pu atom ratios of up to 10¹⁷ and 10¹², respectively. The developed method was validated by analyzing standard reference materials and spiked soil samples.