Determination of Total and Isotopic Uranium and Total Thorium in Soils by Inductively Coupled Plasma-Mass Spectrometry

Inductively coupled plasma-mass spectrometry (ICP-MS), using standard sample introduction by peristaltic pumping, is presented as a method to determine total and isotopic uranium (²³⁴U, ²³⁵U, ²³⁶U, and ²³⁸U) and thorium (²³²Th) in soil samples. Initial sample preparation consists of oven drying to determine moisture content, and grinding and mixing the soil to make it homogenous. This is followed by a nitric/hydrofluoric acid digestion to bring the uranium into solution. Bismuth (²⁰⁹Bi) is added prior to digestion to monitor for losses due to sample preparation and analysis. An additional digestion, using nitric/perchloric acid is performed if the total thorium concentration is required on the sample. The uranium and thorium content of this solution and the ²³⁵U/²³⁸U ratio are measured on an initial pass through the ICP-MS. The total uranium measurement is based on the ²³⁸U isotope measurement with correction for the presence of the U isotopes. To determine the concentration of the less abundant ²³⁴U and ²³⁶U isotopes, the digestate is further concentrated by using a solid phase extraction column (TRUSpec by EiChrom Industries, Inc.) before a second pass through the ICP-MS.