Determination of total uranium and uranium isotope ratios in human urine by ICP-MS: results of an interlaboratory study

In biomonitoring studies, the very low levels of uranium [U] found in urine and the complexity of the urine matrix present challenges for accurate isotope ratio determination. The principal goals of this study were to evaluate current interlaboratory performance and measurement reproducibility for determining [U] and uranium isotope ratios in human urine, and to produce an archive of well-characterized urine reference materials for future method validation and quality control purposes. Six stabilized pools of urine were prepared following the same protocol that is used to prepare materials for the New York State Department of Health (NYSDOH) Proficiency Testing (PT) Program. The candidate reference materials were characterized by their [U] content, isotopic ratios, as well as for homogeneity, and stability. The interlaboratory study design was modeled on the PT program for trace elements in human urine operated by the NYSDOH's Wadsworth Center. Among the nine laboratories that returned results, sample preparation methodologies included simple dilution, acid digestion, column extraction, or a combination of these techniques. For [U] determination, four laboratories used SF-ICP-MS and five used Q-ICP-MS, whereas, for isotope ratios, five used SF-ICP-MS and four Q-ICP-MS. Sample introduction was achieved using either desolvation-nebulization systems or a nebulizer and spray chamber (ambient temperature or Peltier-cooled). Certified reference materials were used by six laboratories to validate [U], isotope ratios, or both. In general for values of [U] > 5 ng L-1, the assigned target values are in good agreement with predicted values and the standard uncertainties are relatively small. For U-235/U-238, there was generally good agreement between the target and predicted values, however the relative standard uncertainties for isotope measurements were higher than for [U]. For the minor isotope ratios U-234/U-238 and U-236/U-238, there was generally poor agreement between target and predicted values and the standard uncertainties were also relatively high. These results suggest that accurate and precise measurements of [U] in urine can be achieved using a wide range of ICP-MS instrument types and methodologies. In contrast, measurement of uranium isotope ratios in urine is more strongly dependent on instrumentation and analytical methodology.