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

U-234 belongs to the U-238 natural radioactive decay series, and at equilib rium, the abundance ratio, U-234/U-238, corresponds to the ratio of their h alf-lives, i.e. 54.8 x 10(-6). In natural waters, deviations from this rati o reflect radioactive and geochemical processes and can be correlated with the flow path of the water. Alpha spectrometry or thermal ionization mass s pectrometry require tedious procedures for separation and preconcentration of the uranium prior to analysis. Inductively coupled plasma quadrupole mas s spectrometry can yield reliable results for the U-234/U-238 ratio without any treatment of the sample (except acidification). Here, we show that com bining a flow-injection sample introduction system with an ultrasonic nebul izer, significantly increases the sensitivity and enhances the ability to c ope with a relatively high salt content in non-saline water samples. The re lative standard deviation attained is 5% at 50 pg l(-1) U-234. The method i s extremely fast (1 min per measurement); pre-treatment is not required and 1 ml of the sample is needed for triplicate measurements. Therefore, it ma y be used as a fast screening method. (C) 2000 Elsevier Science B.V. All ri ghts reserved.