Determination of Uranium Isotopic Ratio by ICP-OES Using Optimal Sensitivity Position Analysis

Isotopic composition analysis of natural and depleted uranium by inductively coupled plasma optical emission spectrometry (ICP-OES) requires the use of a high-resolution instrument due to the very slight isotopic shifts between the atomic emission spectra of the 238U and 235U isotopes. In this work, we show that conventional ICP-OES (without high-resolution optics) can be used for highly accurate uranium isotopic analysis, on par with the results obtained by ICP-MS. Such accurate measurements are achieved by applying a preparatory mathematical procedure termed the optimal sensitivity position (OSP) procedure. In the OSP procedure, a theoretical spectrum for 238U is constructed and subtracted from the measured spectra of calibration solutions encompassing various isotopic ratios. The resultant spectra are used to locate the optimal measurement positions for both major uranium isotopes (238U and 235U) and the background measurement position. Herein, the optimal measurement positions are revealed to be located not at each isotope's peak maximum but at 424.427, 424.409, and 424.390 nm for 238U, 235U, and the background position, respectively. These locations remained stationary during the period of this work, implying that the use of a routine mathematical procedure is not required before every analysis. The OSP procedure offers a direct and highly accurate approach for determining the isotopic ratio of uranium in a relatively cheap and simple fashion. Furthermore, this work also demonstrates the possibility of greatly enhancing ICP-OES capabilities using mathematical analysis of spectra.