Chemical Separation of Uranium and Precise Measurement of 234U/238U and 235U/238U Ratios in Soil Samples Using Multi Collector Inductively Coupled Plasma Mass Spectrometry

Nimelan Veerasamy,Tatsuo Aono,Rajamanickam Murugan,Sharayu Kasar,Masahiro Fukushi,Asako Takamasa,Sarata Kumar Sahoo,Kazumasa Inoue

doi:10.3390/molecules25092138

Abstract

A new chemical separation has been developed to isolate uranium (U) using two UTEVA columns to minimize iron and thorium interferences from high background area soil samples containing minerals like monazites and ilmenite. The separation method was successfully verified in some certified reference materials (CRMs), for example, JSd-2, JLk-1, JB-1 and JB-3. The same method was applied for purification of U in Fukushima soil samples affected by the Fukushima dai-ichi nuclear power station (FDNPS) accident. Precise and accurate measurement of 234U/238U and 235U/238U isotope ratios in chemically separated U were carried out using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). In this mass spectrometric method, an array of two Faraday cups (1011 Ω, 1012 Ω resistor) and a Daly detector were simultaneously employed. The precision of U isotope ratios in an in-house standard was evaluated by replicate measurement. Relative standard deviation (RSD) of 234U/238U and 235U/238U were found to be 0.094% (2σ) and 0.590% (2σ), respectively. This method has been validated using a standard reference material SRM 4350B, sediment sample. The replicate measurements of 234U/238U in SRM shows 0.7% (RSD). This developed method is suitable for separation of U and its isotope ratio measurement in environmental samples.

Highlights

Uranium is the heaviest naturally occurring element on earth and it has three naturally occurring radioisotopes, 234 U, 235 U and 238 U with relative isotopic abundances of 0.0055%, 0.72% and 99.27%, respectively [1]
The aim of our study is to develop a chemical separation method for U and its isotope ratios measurement using MC-inductively coupled plasma mass spectrometry (ICP-MS) from soil samples in high background radiation area (HBRA), Odisha, India, as well as Fukushima soil samples affected by the Fukushima dai-ichi nuclear power station (FDNPS), Japan, accident
A quadrupole ICP-MS was used for the measurement of stable isotopes of U, Th, La, Ce and Nd in reference materials and soil samples, which yielded detection limits of 0.01 μg L−1

Summary

Introduction

Uranium is the heaviest naturally occurring element on earth and it has three naturally occurring radioisotopes, 234 U, 235 U and 238 U with relative isotopic abundances of 0.0055%, 0.72% and 99.27%, respectively [1]. A trace amount of 236 U can occur naturally from neutron capture processes in uranium ores [2]. Uranium isotope ratios vary in natural samples due to various physical, chemical, or even biological processes, including mass fractionation, redox transitions, radioactive decay, radioactive disequilibrium, alpha recoil, and neutron capture [5]. Molecules 2020, 25, 2138 natural variability of more than 0.03% in the 235 U/238 U ratio for a range of natural materials has been reported [6,7]. This variation is due to the different redox states and nuclear field shift effects. It is very essential to measure uranium isotopic composition in environmental samples, which requires high sensitivity to detect the smallest amount of minor isotopes (236 U) and high accuracy to differentiate between small artificial components from natural uranium samples [8,9]

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