Determination of U isotopic ratios in environmental samples by ICP-MS

Abstract

Inductively coupled plasma mass spectrometry (ICP-MS) has been used for the determination of trace elements in a variety of materials. For uranium, the concentration of 238U can be measured with a detection limit of less than 0.1 ppt in a few minutes. However, because of the extremely low 234U concentrations in environmental materials, it is necessary to separate U from the matrices and to remove interfering elements from the sample solution for measurement of the 238U/234U ratio by ICP-MS. In this study, a simple and rapid separation method for U with an extraction chromatographic resin (TEVA resin) was developed for ICP-MS and the U isotopic ratios in phosphogypsum samples, collected in Spain, were measured. The resin efficiently retained U in 6 M HCl medium and more than 98% of the U was easily eluted with the first 30 mL of 0.1 M HNO3. The separated solutions were free from most of the matrix elements and the concentrations of U in the solutions ranged from 35 to 70 ppb. The 238U/234U isotopic ratios in the solutions were measured by ICP-MS. Moreover, due to the presence of a higher number of 235U atoms in comparison with 234U, the 238U/235U isotopic ratios were also determined with high precision by ICP-MS. The results obtained agreed well with the ratios measured by alpha-spectrometry. The method for this determination is more rapid than alpha-spectrometry, and is considered to be more suitable for environmental monitoring.