Measurement of ultra-trace level 237Np in soil and sediment by triple-quadrupole inductively coupled plasma-mass spectrometry with different mass modes combined with chemical separation

Abstract

Long-lived trans‑uranium nuclides such as ²³⁷Np, may reside in the environment for long period and result in ecological impact and human health threat. Accurate measurement of ²³⁷Np in the environment is important for the risk assessment and ecological evaluation. However, the spectral peak tailing and polyatomic ions interference greatly limit the measurement of ²³⁷Np with triple quadrupole ICP-MS (ICP-MS/MS), which have been successfully used for analysis of ultra-trace level ^239,240Pu and ²⁴¹Am. The novel methods for ²³⁷Np measurement with ICP-MS/MS in both on-mass and mass-shift modes were examined in this paper. When helium was used as the collision gas for the ICP-MS/MS measurement with on-mass mode, the interference contribution to the counts for ²³⁷Np (at m/z 237) as low as 4 × 10⁻⁸ was observed only from Hg, while no measurable interference from Tl and Pb was observed. Moreover, when the mass-shift mode is employed with O₂/He and He as the collision/reaction gas, the interference contributions from Hg, Tl and Pb were all negligible. The detection limit of ²³⁷Np was reduced to 0.014 fg/g and 0.019 fg/g for on-mass and mass-shift mode, respectively. The novel methods were demonstrated to enable accurate determination of 1.0 fg/g level ²³⁷Np in the emulated sample solutions with a ²³⁷Np/²³⁸U atomic ratio as low as 10⁻⁹. Combined with appropriate chemical separation procedure, the developed methods were verified with certified reference materials and soil samples.