Determination of uranium and thorium isotopes in kaolinitic samples by ICP-MS/MS

The use of natural uranium and thorium long-lived isotopes was applied to study the evolution of kaolinitic materials. To do that, a radiochemical method for separation of Th and U in phyllosilicate samples was developed with the main aim of their analysis by ICP-MS/MS. Given the lack of certified phyllosilicate reference materials with certified isotope abundances, the method was tested using a stepwise approach. First, a preliminary setup was done using a sediment reference sample. Then, the extrapolation of this method to the samples of interest has been tested using a phyllosilicate sample certified for U and Th concentrations. The results showed that an extension of the plateau time of digestion in the used microwave system was peremptory as to get full digestion of the sample. The use of the isotope dilution technique allowed both the quantification of major isotopes (U-238 and Th-232) simultaneous to the determination of isotope ratios (U-234/U-238, U-235/U-238 for quality control and Th-230/Th-232). The established method was applied to six kaolinitic samples from the Iberian Range (NE Spain), showing the potential of this isotope approach for the characterization of their time evolution.