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 homogeneous. 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 addition 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 (TRU.Spec by EiChrom Industries, Inc.) before a second pass through the ICP-MS