Advancing the analysis of low level uranium and plutonium in bioassay samples using SF-ICP-MS

Abstract

The routine monitoring of workers exposed to uranium and plutonium traditionally uses urine samples. Conventionally this analysis, typically using ?1 L urine samples, is carried out by alpha spectrometry. A method for the rapid, simultaneous determination of plutonium and uranium in urine has been developed. It significantly improves on traditional methods by exploiting the increased sensitivity achievable using SF-ICP-MS, and uses much smaller (40 mL) sample volumes. The ultimate aim for detection limits is 1 ?Bq/L (0.4 fg/L) for Pu, and <0.2 mBq/L (8 ng/L) for U, measuring both the concentration and isotope ratios.

Isolation of the uranium and plutonium is achieved through a calcium phosphate precipitation followed by separation using a single UTEVA column. A rapid clean up of the plutonium fraction, using TEVA resin is required to ensure high uranium decontamination. The samples are measured using an Element XR ICP-MS and the Aridus sample introduction system. The procedural blank contribution is paramount to the success of the method because of the combined demands of low detection limits, and the need to control the uranium hydride interference (238U+ on 239Pu). Sub boiled acids are used throughout ultra-pure reagents and MilliQ water, and all work is carried out in a class 100 clean laboratory. Instrumental detection limits (calculated as 3 ? SD of blank) achieved are ?1 ppq for uranium, and ?0.1 ppq for plutonium.

As part of this work, the retention of material by the ICP-MS has been explored. As samples are ‘consumed’ by the instrument, any activity contained within the sample will be retained by the components of the instrument. Retention of material by the ICPMS raises questions of a potential radiation dose to instrument operators. The cones and sample introduction system retain the most material. The potential radiation dose for an instrument used for low level radioactive material has been calculated and demonstrated to be less than the public radiation dose limit in the UK.

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