Determination of the 234U/ 238U ratio in water samples by inductively coupled plasma mass spectrometry

Abstract

234U belongs to the 238U natural radioactive decay series, and at equilibrium, the abundance ratio, 234U/ 238U, corresponds to the ratio of their half-lives, i.e. 54.8×10 −6. In natural waters, deviations from this ratio reflect radioactive and geochemical processes and can be correlated with the flow path of the water. Alpha spectrometry or thermal ionization mass spectrometry require tedious procedures for separation and preconcentration of the uranium prior to analysis. Inductively coupled plasma quadrupole mass spectrometry can yield reliable results for the 234U/ 238U ratio without any treatment of the sample (except acidification). Here, we show that combining a flow-injection sample introduction system with an ultrasonic nebulizer, significantly increases the sensitivity and enhances the ability to cope with a relatively high salt content in non-saline water samples. The relative standard deviation attained is 5% at 50 pg l −1 234U. The method is extremely fast (1 min per measurement), pre-treatment is not required and 1 ml of the sample is needed for triplicate measurements. Therefore, it may be used as a fast screening method.