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