Isotope ratio measurements of spent reactor uranium in environmental samples by using inductively coupled plasma mass spectrometry

As a result of the accident at the Chernobyl nuclear power plant (NPP) the environment was contaminated with spent nuclear fuel. The U-236 isotope was used in this study to monitor the spent uranium from nuclear fallout in soil samples collected in the vicinity of the Chernobyl NPP. A rapid and sensitive analytical procedure for uranium isotopic ratio measurement in environmental samples based on inductively coupled plasma quadrupole mass spectrometry with a hexapole collision cell (HEX-ICP-QMS) was developed. Figures of merit for the HEX-ICP-QMS were studied with a plasma-shielded torch using different nebulizers [such as an ultrasonic nebulizer (USN) and a Meinhard nebulizer] for the solution introduction. An U-238(+) ion intensity of up to 27 000 MHz/ppm in HEX-ICP-QMS with USN was observed by introducing helium into the hexapole collision cell as the collision gas at a how rate of 10 mL min(-1). The formation rate of uranium hydride ions UH+/U+ of 2 x 10(-6) was obtained by using USN with a membrane desolvator. The detection limit for U-236 in 10 mug L-1 uranium solution was 3 pg L-1 corresponding to the limit of U-236/U-238 ratio determination of 3 x 10(-7). in the case of low isotopic abundance of U-236 (<10(-5)). the measured isotopic ratio in the synthetic mixture was systematically lower than the calculated ratio. The precision of uranium isotopic ratio measurements in 10 <mu>g L-l laboratory uranium isotopic standard solution was 0.13% (U-235/U-238) and 0.33% (U-236/U-238) using a Meinhard nebulizer and 0.45% (U-235/U-238) and 0.88% (U-236/U-238) using an USN. The isotopic composition of all investigated Chernobyl soil samples differed from those of natural uranium: i.e. in these samples the U-236/U-238 ratio ranged from 10(-5) to 10(-3). Results of isotopic ratio measurement by HEX-ICP-QMS in soil samples are in agreement with those obtained by using a double-focusing sector field ICP-MS within experimental errors.(C) 2000 Elsevier Science B.V.