Diode laser absorption measurement of uranium isotope ratios in solid samples using laser ablation

• Published in: Spectrochimica acta. Part B: Atomic spectroscopy Vol. 57; no. 10; pp. 1611 - 1623
• Main Authors: Liu, H, Quentmeier, A, Niemax, K
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.10.2002
• Subjects: Atomic absorption; Diode laser; Isotope ratio; Laser ablation; Uranium; Atomic absorption; Uranium; Laser ablation; Diode laser; Isotope ratio
• ISSN: 0584-8547; 1873-3565
• DOI: 10.1016/S0584-8547(02)00105-2

The isotopes 235U and 238U were simultaneously measured by absorption in a laser-induced plasma ignited by a Nd:YAG laser. Two separate diode lasers were used which probed almost the same plasma volume. The diode lasers were tuned to the absorption lines 682.6736 nm for 235U and 682.0768 nm for 238U. The 235U / 238U isotope ratio was measured on a pulse-to-pulse basis evaluating the transient absorption peaks as analytical signals. Three uranium oxide pellets were used for calibration with natural (0.714 wt.%) and depleted 235U concentration (0.407 and 0.204 wt.%). The measured 235U/ 238U mass ratio in an uranium containing mineral sample was (0.686±0.119)×10 −2 and found to be in good agreement with the mass ratio of naturally occurring isotopes (0.719×10 −2). The observed accuracy (<5%) and precision (∼17% R.S.D.) are encouraging if it is taken into account that the mineral sample was probed directly without any preceding preparation step. Spatially and temporally resolved emission spectroscopic measurements were performed to investigate the kinetics of the laser plasma and to optimize the operating conditions of the laser ablation with regard to sensitivity and selectivity of the 235U isotope detection. Optimal conditions were obtained at a distance of ∼0.3 cm from the sample surface, an argon pressure of ∼30 mbar and for 7 mJ pulse energy of the Nd:YAG laser. The limit of detection of the 235U isotope, evaluated on the basis of the 3σ criteria, was estimated to be 47 μg g −1 and improved by a factor of two compared with the sequential measurement with a single laser diode used in a previous work.