Uranium isotope ratios

Turner S, van Calsteren P, Vigier N, Thomas L (2001) Determination of thorium and uranium isotope ratios in low concentration geological materials using a fixed multi-collector-lCP-MS. J Anal At Spectrom 16 612-615... 

Instrumental layouts and developments in AMS are reviewed by Kutschera.195 Today AMS is the most powerful, sensitive and selective mass spectrometric technique for measuring long-lived radionuclides at the level of natural isotopic abundances (10-16 to 10-12). Accelerator mass spectrometry (AMS) allows uranium isotope ratio measurements with an abundance sensitivity for 236U in the range of l(rlo-10 l2.l98J"... 

An analytical procedure has been proposed for precise uranium isotope ratio measurements in a thin uranium layer on a biological surface by LA-ICP-MS using a cooled laser ablation chamber.125 One drop of uranium isotope standard reference materials NIST, 350, NIST 930, of our isotopic laboratory standard CCLU 500 (20p.l, U concentration 200 ng 1) and of uranium with natural isotopic pattern were deposited on the leaf surface and analyzed by LA-ICP-MS at well defined laser crater diameters of 10, 15, 25 and 50 p.m. A precision for measurements of isotope ratios in the range of 2.1-1.0% for 235U/238U in selected isotope standards was observed whereby the precision and the accuracy of isotope ratios compared to the non-cooled laser ablation chamber was improved.125... 

The results of uranium isotope ratio measurements by MC-TIMS (TRITON Thermo Fisher Scientific) on a single uranium oxide reference particle (10 pan) are illustrated in Figure 8.5.9,144 The high precision of multiple ion counters and high detection power in the MC-TIMS are... 

Figure 8.5 Precision of uranium isotope ratios measured by MC-TIMS (Triton, Thermo Fisher Scientific, Bremen). (]. S. Schwieters etai., Ceochim. Cosmochim. Acta, 68, A60 (2004). Reproduced by permission...

Table 8.7 Variation of uranium isotope ratios in contaminated soil samples in the vicinity of Chernobyl.

The ratio of to would be expected to be unity as long as the uranium stays locked inside undisturbed crustal rock in secular equilibrium with its progeny, but measurements show that the ratio is typically different than unity (EPA 1994). This disequilibrium occurs when the rock is disturbed by chemical or physical changes involving water. In the environment, a portion of the separates from the by what is theorized to be a physical process (alpha recoil ejection of the Th decay product from surfaces of soil particles) or a combination of physical and chemical processes (a transformation at the soil particle surface fractures the surface allowing access for water to dissolve the more soluble Th product) (NCRP 1984a). These processes can change the uranium isotope ratios in air, soil, and water. 

Ayalon, A., Bar-Matthews, M. and Kaufman, A., 1999, Petrography, strontium, barium and uranium concentrations and strontium and uranium isotope ratios in speleothems as palaeoclimatic proxies Soreq Cave, Israel, Holocene 9 715-722. 

Enrichment Uranium isotope ratios Vegetation, water, soil, sediments, swipes TIMS... 

When using two lasers and applying two-photon spectroscopy, only those atoms that do not have a velocity component in the observation direction will undergo LEI. Then the absorption signals become very narrow (Doppler-free spectroscopy). This enhances the selectivity and the power of detection, however, it also makes isotope detection possible. Uranium isotopic ratios can thus be detected, similarly to with atomic fluorescence [673] or diode laser AAS. Thus for dedicated applications a real alternative to isotope ratio measurements with mass spectrometry is available. 

Smith B. W., Quentmeier A., Bolshov M. and Niemax K. (1999) Measurement of uranium isotope ratios in solid samples using laser ablation and diode laser-excited atomic fluorescence spectrometry, Spectrochim Acta, Part B 54 943—958. 

Betti (1996) and co-workers used GD-MS for sample screening in isotopic measurements of zirconium, silicon, lithium, boron, uranium, and plutonium in nuclear samples. The results obtained from the GD-MS were compared with results from thermal ionization mass spectrometry (TIMS). For boron and lithium concentrations from //g/g to ng/g levels, isotopic ratios determined by GD-MS were comparable to TIMS in terms of accuracy and precision. Uranium isotopic ratios determined by GD-MS were also in good agreement with values measured by TIMS with regards to accuracy. Chartier et al. (1999) used GD-MS to analyze erbium and uranium in molybdenum-uranium fuel samples. The ratio of 166Er to 238U was then compared to numbers determined by thermal ionization mass spectrometry. The ratio of erbium to uranium was accurate to within 3% of the number determined by TIMS. 

Manninen, P. K. G. 1995. A rapid method for uranium isotope ratio measurement by inductively-coupled plasma-mass spectrometry. J Radioan Nuc Ch Le 201(2), 71-80. March, R. E. and Hughes, R. J. 1989. Quadrupole storage mass spectrometry in Chemical... 

Pappas RS, Ung BG, Paschal DC (2003) A practical approach to determination of low concentration uranium isotope ratios in small volumes of urine. J Anal At Spectrom 18 1289-1292... 

Determination of uranium isotope ratios in urine has proven important in the last two decades for purposes related to assessing potential direct human exposures to depleted uranium from hardened munitions, from environmental exposures in areas where heavy use of such munitions took place during conflicts, and to consideration of whether there was significant likelihood of long-term health threats as a result of such exposures. In the nuclear industry, or as a possible outcome of a nuclear facility accident, it would be important to determine the possibility and extent of enriched uranium exposures for the same reasons. Since uranium is excreted in urine, analysis of urine samples has often been chosen as a noninvasive way to determine exposures. 

For individuals with high urine uranium concentrations (1.0 pg/L uranium or more in urine) such as from uranium milling workers, wounded combatants whose tissues were embedded with pieces of steel hardened with depleted uranium, or those with similar circumstances, a simple analysis of a 1/10 dilution of urine with a standard introduction system on most ICP-MS instruments may be used to derive an acceptably accurate uranium isotope ratio. As an example. Figure 23.1 illustrates the results obtained from a urine sample spiked with 1.0 pg/L uranium, diluted 1/10 with 5% nitric acid, and analyzed with a well-optimized single collector magnetic sector ICP-MS instrument. However, in the vast majority of urine samples, the uranium concentrations are far below 1.0 pg/L so that accurate determinations are a challenge in most cases [2]. 

Despite instrumental improvements, problems remain for urine uranium isotope ratio analyses at low concentrations by ICP-MS. The most accurate and precise instruments determine only the ratios of detected ions. If ions are produced as a result of introduction of a sample with a matrix that inherently causes formation of isobaric mass interferences within the mass window of one or more of the isotopes to be determined, then the resulting inaccurate elemental isotope ratios will accurately and precisely reflect a determination of the combined elemental isotope and interference ions that were counted indiscriminately by the detector. 

Besides suppression of sensitivity caused by solutions with high ionic strength [11,12], high salt concentrations also affected the accuracy of isotope ratio determinations, possibly due to greater losses of the lower-mass uranium isotopes as a result of ion-ion repulsions in the ion beam known as the space-charge effect. While this may not be much of an issue for water analyses, the application of these principles to the accurate determination of uranium isotope ratios in urine imphes that either one needs to attempt a close matrix match for every sample or sample preparation should include the elimination of as many neutral and ionic compounds as possible within the constraints of minimal sample preparation complexity. 

The sample preparation techniques discussed here, which utilize chelation resin SPE columns, were developed in order to maximize urine uranium isotope ratio accuracy, while consuming as little preparation time as possible. Removal of hydrophobic organic substances... 

Low U/ U isotope ratio QC samples are prepared by quantitatively pipetting an aliquot of a depleted U standard such as NBL CRM 115, U005, and an enriched standard such as U020 or equivalent (if licensing is available for enriched uranium standards) into the characterized base urine described above. Equivalent standards may be obtained from IRMM. The spiked-urine uranium isotope ratio is characterized to verify the expected versus measured ratio. The isotope ratio, if different than expected, is verified by calculation as the mass-weighted average of the natural uranium isotope ratio and the spiked depleted uranium isotope ratio. 

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