Isotopic Measurements of Uranium Using Inductively Coupled Plasma Cavity Ringdown Spectroscopy

Inductively coupled plasma cavity ringdown spectroscopy (ICP-CRDS) is applied to isotopic measurements of uranium. We have successfully obtained the isotopic-resolved spectra of uranium at three different atomic/ionic transition lines, 286.57, 358.49, and 409.01 nm. Of the three lines, the largest isotope shift of approximately 9 pm was measured at the 286.57 ionic line. Isotopic-resolved spectra were recorded in ratio of 1:1 (²³⁵U/²³⁸U, 2.5 μg/mL) and at the natural abundance ratio of 0.714% (²³⁵U/²³⁸U, 1.25 μg/mL ²³⁵U). The smallest measurable isotope shift of approximately 3 pm was determined for the 409.01 nm ion spectral line. Detection limits (DL) were obtained under optimized ICP operating conditions to be in the range of 70∼150 ng/mL, except for the ²³⁸U component of the 286.57 nm line (300 ng/mL). This latter result was determined to be due to a strong, previously unreported, absorption interference from the argon plasma. The ²³⁵U isotope component (DL 70 ng/mL) was found to be unaffected. This work demonstrates the applicability of ICP-CRDS for uranium isotopic measurements. The potential of development of a field-deployable, on-line uranium isotope monitor using plasma-CRDS is discussed.