Increased interest in measuring uranium isotope ratios in environmental samples (biological materials, soils, dust particles, water) has come from the necessity to assess the health impact of the use of depleted uranium (DU) based ammunitions during recent military conflicts (e.g., Gulf war, Kosovo) and from the need to identify nondeclared nuclear activities (nuclear safeguards). In this context, very important decisions can arise which have to be based on measurement data of nondisputable uncertainty. The present study describes the certification to 2.5% (k = 2) relative combined uncertainty of n(235U)/n(238U) at ultralow uranium levels (approximately 5-20 pg g(-1)) in human urine samples. After sample decomposition and matrix separation, the isotope ratios were measured by means of a single-detector magnetic sector-field inductively coupled plasma mass spectrometry instrument fitted with an ultrasonic nebulizer. Correction for mass discrimination effects was obtained by means of the certified isotopic reference material IRMM-184. The analytical procedure developed was validated in three complementary ways. First, all major sources of uncertainty were identified and propagated together following the ISO/GUM guidelines. Second, this quality was controlled with a matrix matching NUSIMEP-3 sample (approximately 0.06-0.7% difference from certified). Third, the instrumental part of the procedure was proven to be reproducible from the confirmation of the results obtained for three samples remeasured 7 months later (approximately 1.5% difference). The results obtained for 33 individuals indicated that none seemed to have been exposed to contamination by DU.

An analytical method is presented based on reverse isotope dilution single detector inductively coupled plasma magnetic sector mass spectrometry (ID-ICP-SMS) and applied to the specific case of the certification of a 111 Cd enriched candidate Cd spike calibration material (nominal mass fraction 10 mg kg Ϫ1 in 5% HNO 3 solution). Uncertainty propagation was used as a tool for both determining the analytical approach and validating it. The robustness of close to "exact matching" reverse IDMS to correction of measured isotope intensities for multiplicative (mass discrimination) and (semi)additive effects (dead time, instrumental background, and isobaric interference) is discussed. The very low experimental relative standard deviation of the mean (0.08%) of eight replicate determinations indicated that all significant sources of uncertainty had probably been taken into account for the estimation of the final combined uncertainty statement (U c ϭ 0.17%, k ϭ 1). IRMM-621 was used as comparator. Uncertainties on IUPAC isotopic abundances of 111 Cd and 112 Cd, for the natural Cd solution involved between the two enriched materials, formed nearly 60% of U c . The repeatability of the isotope ratio measurements contributed less than 10%. Correction for procedural blank necessitated somewhat unusual calculations (potential contamination of an enriched material with natural Cd). The procedure also involved a quadrupole based ICP-MS judged to be appropriate for the characterization of the isotopic composition. For comparison purposes, direct IDMS results are simulated using identical experimental input data. Finally, a significant background signal in the 106 -116 mass region, observed only with the magnetic sector instrument, was attributed to argon based isobaric interferences. I sotope dilution mass spectrometry (IDMS) was recognized as having the potential to be a primary ratio method of measurement [1]. When applied correctly, it can be of "practical use in establishing traceability to the SI unit system" [2] and accurate results with sufficiently small uncertainties can be achieved. The advantages and disadvantages of the IDMS method have been discussed [3,4] since its invention nearly 50 years ago. One of the significant advantages over other approaches is that the analyte recovery does not have to be quantitative, providing that isotopic equilibration has been achieved between all of the analyte and added spike material. Commonly there is the reverse IDMS (or two-way IDMS) whereby the spike material is calibrated against a well-characterized assay material. The alternative is direct IDMS (or one-way IDMS) which is faster but requires a spike material already reliably certified "once for all" for the element content and isotopic composition. Direct IDMS has the potential to lead to relative combined uncertainties on the amount content determination of ϳ1% (k ϭ 2) even with quadrupole ICP-MS (inductively coupled plasma mass spectrometry) instrumentation [5]. In many cases this is entirely fit for the purpose and a bett...

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