Direct detection and isotope analysis of individual particles in suspension by single particle mode MC-ICP-MS for nuclear safety

Abstract

Isotopic fingerprint information carried by an individual particle is of great value in nuclear safety, nuclear safeguards, and nuclear environment. Several techniques, such as single particle mode ICP-MS, laser ablation ICP-MS and LA-MC-ICP-MS, have been developed for direct detection or isotope analysis of individual particles in the solid sample. In this study, single particle mode MC-ICP-MS was proposed for the precise isotopic measurement of individual particles in suspension. Erbium oxide powder with natural isotopic abundance used as uranium particle surrogate was dispersed in ultrapure water to prepare a suspension sample. Uranium isotope at different enrichment levels could be covered by the erbium isotope range. Erbium solution was nebulized before and after particle analysis to determine the mass bias factors in the mass spectrometer. The submicron-sized erbium particle was nebulized and then introduced into the ICP torch for detection and isotopic measurement in single particle mode. From the histogram of pulse intensities, the spherical equivalent physical diameter was estimated to be around 226 nm with ¹⁶⁴Er amount of 7 × 10⁻¹⁶ g. Two data processing strategies, point by point (PBP) and linear regression slope (LRS), were used to determine the erbium isotope ratios. Results show that the precisions of ¹⁷⁰Er/¹⁶⁶Er, ¹⁶⁸Er/¹⁶⁶Er, and ¹⁶⁷Er/¹⁶⁶Er ratios determined by PBP method are 5.5%, 4.6% and 3.9%, respectively. The relative errors of these measured isotope ratios after mass bias correction lies between 0.2–4%. The precision and accuracy of ¹⁶⁴Er/¹⁶⁶Er ratio are worse due to the weak signal of ¹⁶⁴Er. By LRS method, the precisions of determined ¹⁷⁰Er/¹⁶⁶Er, ¹⁶⁸Er/¹⁶⁶Er, ¹⁶⁷Er/¹⁶⁶Er and ¹⁶⁴Er/¹⁶⁶Er ratios improved by one order of magnitude at least. The precisions of ¹⁷⁰Er/¹⁶⁶Er, ¹⁶⁸Er/¹⁶⁶Er and ¹⁶⁷Er/¹⁶⁶Er ratios are better than 0.3%. The precision and accuracy of the ¹⁶⁴Er/¹⁶⁶Er ratio are significantly improved. Moreover, the ¹⁶²Er/¹⁶⁶Er ratio at 10⁻³ level could also be determined. The proposed technique is suitable for the detection and isotope analysis of particles with size range of 130 nm–3 μm. The sample preparation process is simple, which eliminates possible contamination. The technique combines fast screening, sensitive detection, and isotopic identification of an individual particle.