Determination of Uranium Isotopes using ICP-QQQ-MS

Naturally-occurring uranium has three isotopes: ²³⁴U, ²³⁵U, and ²³⁸U. ²³⁸U has the highest abundance at 99.3%. On the other hand, due to the uranium enrichment process, enriched uranium contains more ²³⁴U than natural uranium. Uranium enrichment is aimed at obtaining ²³⁵U; however, the process results in the concentration of the lighter isotopes. Uranium activity in waters is often determined by alpha spectrometry measuring ²³⁴U and ²³⁸U. Alpha spectrometry requires chemical separation before analysis, making sample preparation difficult and, therefore, increasing the total time per analysis.

Inductively coupled plasma mass spectrometry with triple quadrupole technology (ICP-QQQ-MS) is a very powerful analytical technique for the determination of uranium concentrations. It is not uncommon to achieve sub-parts-per-trillion detection limits for total uranium analysis, but more importantly, it is also an alternative to alpha spectrometry for the analysis of uranium isotopes.

Brooks Applied Labs has developed a new method for the determination of very low ²³⁴U, ²³⁵U, and ²³⁸U concentrations in most water samples. The sensitivity of the Agilent 8800 ICP-QQQ-MS, coupled with a front-end preconcentration system, allows us to determine even the lowest abundance isotope (²³⁴U at 0.005%) at sub-parts-per-quadrillion levels, corresponding to an activity level far below 1 mBq (0.027 pCi).

Chromatogram for the analysis of a 4 parts-per-quadrillion 234U standard (equivalent to 27 fCi)

Chromatogram for the analysis of 234U in an open-ocean seawater certified reference material