Measurement of uranium isotope ratios in solid samples using laser ablation and diode laser-atomic absorption spectrometry

A diode laser was used for the selective detection of 235U and 238U in a laser-induced plasma ignited by a Nd:YAG laser beam focused onto uranium oxide samples. The diode laser was sequentially tuned to the absorption lines of both isotopes (682.6736 nm for 235U, and 682.6913 nm for 238U). The absorption was measured on a pulse-to-pulse basis; the transient absorption peak was used as an analytical signal. Three samples were used with the relative abundance of the minor isotope 235U of 0.204%, 0.407% and 0.714%. Optimal conditions for the detection of the minor isotope were obtained at a distance of ∼3 mm from the sample surface, an argon pressure of ∼3 kPa and for 7.5 mJ pulse energy of the Nd:YAG laser. Absorption in the wing of the broadened line of the 238U isotope was found to be the main source of background for the measurement of the absorption of the minor isotope. The limit of detection of the minor isotope, evaluated on the basis of the 3σ criteria was estimated to be 100 μg g−1. At the optimal conditions for the detection of the minor isotope optical thick conditions in the line centre of the main isotope were observed. Therefore, the isotope ratio measurements were performed by rationing the intensity of the net absorption signal measured in the line centre of the minor isotope and the absorption signal measured in the wing of the main isotope. This strategy was checked by determination of the isotope ratios for the two samples with depleted 235U concentration using the sample with the natural isotope composition (0.714%) as a standard. The accuracy and precision for this measurement strategy was evaluated to approximately 10%.