Validation of uranium determination by SF-ICP-MS from QC samples from the IAEA Safeguards Analytical Laboratory

Full text: A high-resolution sector field ICP-MS (SF-ICP-MS) (ThermoElectron ELEMENT2) with an ESI Apex high sensitivity inlet system and ESI autosampler was installed at the IAEA Safeguards Analytical Laboratory (SAL) in late 2005. The instrument has at least three intended uses: (1) quality control in the form of uranium determinations from room blank samples collected in the IAEA Clean Laboratory and process blank samples from U-Pu chemical separations of environmental safeguards samples, (2) forensics in the form of trace element determinations in nuclear samples collected by IAEA Safeguards inspectors, and (3) rapid determination of uranium isotopic composition from diluted samples collected from uranium enrichment facilities. The validation of the methods for (2) and (3) will be reported at a later date. The importance of (1) bears on the frequency of QC measurements and therefore the confidence of cleanliness of the critical working areas within the Clean Laboratory. Here we report the results of method validation quality control samples collected in the SAL Clean Laboratory. The ELEMENT2 is located in the nuclear chemistry area of the SAL, therefore the validation, which requires the analysis of uranium concentrations down to 10 ppq, was envisioned as a significant challenge. The need for careful chemical treatment and standard preparation techniques was recognized in order to mitigate potential external contamination effects. A series of uranium standards covering the concentration level from 1000 ppq down to 10 ppq was prepared gravimetrically in the SAL Clean Laboratory and transferred and measured on the ELEMENT2. Although the measurements were not made under clean room conditions, uranium down to 10 ppq was measured with good linearity and repeatability. The associated standard deviations were typically below 10%. Due to the uranium background in the lab, concentration measurements below 10 ppq were much more variable, and therefore the reliable detection limit for uranium is in the lower picogram range. The validation approach encompassed more than 300 measurements and tested instrument signal stability, selectivity, sensitivity, linearity and consistency within short and long time frames. The results presented here show that a SF-ICP-MS placed in a nuclear laboratory can make reliable determinations of uranium down to 10 ppq when preparation and handling techniques are carefully considered and carried out. The use of the SF-ICP-MS for quality control in the IAEA SAL Clean Laboratory will significantly improve confidence in all safeguards activities by allowing for greater frequency of sample collection and measurement due to the instrument's rapid analysis times and high throughput compared to TIMS. (author)