A feasibility study on the use of isotope dilution as a tool for quantifying uranium isotopic concentrations by quadrupole ICP-MS

The determination of uranium elemental concentrations and isotope ratios has been a major task for the BGS inductively coupled plasma mass spectrometry (ICP-MS) laboratory over the last five years. UK government departments and the nuclear industry have ongoing environmental monitoring programmes which require high quality data. The need for a greater understanding of chemical pathways, sinks and sources has been a driver for BGS to continue to improve analytical performance but with a minimal increase in costs. This study seeks to identify whether isotope dilution could provide a significant increase in analytical performance with minimal cost implications. This report reviews the fundamental concepts of isotope dilution and its advantages over related techniques such as internal standardisation. It considers the potential benefits of quadrupole ICPMS (ICP-QMS) over other techniques where high productivity is an issue, such as for environmental monitoring. The optimisation of the ICP-MS acquisition parameters including peak dwell times, detector dead-times, spike concentrations and operating conditions are examined. Isotope dilution (ID) relies heavily on the accurate calibration of the 233U spike and the process of reverse isotope dilution is used and discussed as part of this study. A series of experiments was conducted to ascertain the degree of chemical preparation required for accurate isotope dilution determination. These included comparing the effectiveness of using the 233U spike with internal standardisation by a proxy element, to overcome ICP-MS matrix effects and changes in sensitivity. Finally, the proposed methodology was tested using a range of natural rock reference materials with known uranium concentrations covering various common igneous and metasedimentary types. This project established that the use of 233U, a by-product of nuclear fuel processing, provided superior precision when used as an internal standard for measurements of other uranium isotope concentrations, compared to indium, rhenium or bismuth. The data suggest that the within sample precision is better when full ID quantification is performed, regardless of the matrix, than when using the spike as an internal standard. The IDMS technique could potentially eliminate the current column separation procedure, which would achieve significant savings in staff time and consumables. A small systematic bias has been observed in the data. It is thought that one of the most likely factors is the assumption made about the density of the 233U spike solution, which would affect the IDMS calculations but have no influence when 233U is used as an internal standard. It will not always be appropriate to use this method for the determination of uranium isotope concentrations and a decision tree needs to be devised to determine the appropriate method to be used for each potential application. The investigation has proved the value and potential of this methodology for isotope concentration analysis by quadrupole ICP-MS. Once a robust, fit for purpose methodology can be applied routinely, the possibility of the use of IDMS for other stable isotopes should be considered.