Uranium Isotope Ratio Measurements Research Articles

Investigation of potential polyatomic interferences on uranium isotope ratio measurements for the LS-APGD-Orbitrap MS system

The determination of actinide (e.g., U and Pu) content and isotopics is of importance to the nuclear forensics and safeguards communities. However, in the analysis of environmental samples, such as those collected by the International Atomic Energy Agency, uranium measurements can be complicated by isobaric interferences from polyatomic variants of heavy elements (e.g., Pb). This leads to complex, time-consuming sample manipulations (i.e., separations) before determining isotope ratios. An alternative strategy to sample pretreatment is to use high-resolution mass spectrometric platforms during the analysis to fully resolve the uranium isotopes from potential polyatomic interferences, negating the need for prior chemical separation. The liquid sampling - atmospheric pressure glow discharge (LS-APGD) coupled with an Orbitrap mass spectrometer provides a high-resolution (>70,000 m/Δm at m/z 200) inorganic mass spectrometry platform. As a demonstration of the power of this instrumental platform, and indeed, the high-resolution approach in general, uranium isotope ratios were determined in the presence of elemental impurities (e.g., Pb, Pt, Ta, W) commonly encountered with environmental sample swipe analysis, without any prior treatment. Even at elemental impurity concentrations of 1000–5000× relative to uranium, no interference was observed with the 235U or 238U signal. In addition, the 235U/238U isotope ratio for the samples with the concomitants present are within 2 standard deviations of the values obtained without their addition, indicating that these impurities do not impact the determined uranium isotope ratio. These findings represent a significant first step in leveraging the high resolution of the LS-APGD-Orbitrap-MS to overcome isobaric and molecular interferences instead of relying on chemical separations.

Direct Uranium Isotopic Analysis of Swipe Surfaces by Microextraction-ICP-MS.

The ability to directly measure uranium isotope ratios on environmental swipes has been achieved through a solution-based microextraction process and represents a significant advancement toward the development of a rapid method to analyze international nuclear safeguard samples. Here, a microextraction probe is lowered and sealed onto the swipe surface, and analytes within the sampling site (∼8 mm2) are dissolved and extracted into a flowing solvent of 2% nitric acid (HNO3). The mobilized species are subsequently directed into an inductively coupled plasma-mass spectrometer (ICP-MS) for accurate and precise isotope ratio determination. This work highlights the novelty of the sampling mechanism, particularly with the direct coupling of the microextraction probe to the ICP-MS and measurement of uranium isotope ratios. The preliminary method detection limit for the microextraction-ICP-MS method, utilizing a quadrupole-based MS, was determined to be ∼50 pg of 238U. Additionally, precise and accurate isotope ratio measurements were achieved on uranium reference materials for both the major (235U/238U) and minor (234U/238U and 236U/238U) ratios. While the present work is focused on directly measuring uranium isotopic systems on swipe surfaces for nuclear safeguards and verification applications, the benefits would extend across many applications in which direct solid sampling is sought for elemental and isotopic analysis.

Precise measurement of uranium isotope ratios in Fukushima soils using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS)

Precise measurement of 234U/238U and 235U/238U isotope ratios was performed using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) in this study. The 234U ions were measured using a Daly detector, the 235U ions were measured using a Faraday cup with a 1012 Ω resistor, and the 238U ions were measured using a Faraday cup with the usual 1011 Ω resistor. Chemical separation of uranium was carried out by three different methods using either combinations of anion exchange and UTEVA resins or UTEVA resin three times. Chromatography combining three UTEVA resin columns provided the highest recovery (%) of uranium and biggest improvement in precise measurements of uranium isotope ratios. The certificate reference material (CRM) JLk-1 was used as an in-house standard for the uranium isotope ratio measurements. The long-term reproducibility of the in-house standard was 0.60% for 234U/238U and 0.05% for 235U/238U. The measured ratios were compared with the natural ratios, and the relative deviation obtained for uranium isotope ratios was less than 0.005% for 234U/238U and 235U/238U. Finally, the present method was applied to measure uranium isotope ratios in Fukushima microwave-digested soil samples.

Measurement of uranium isotope ratios in Fukushima-accident contaminated soil samples using multi collector inductively coupled plasma mass spectrometry

In the present study, 137Cs and 238U activity concentrations, 234U/238U activity ratio, and 235U/238U isotope ratio were measured in fifteen soil samples collected from the exclusion zone around the Fukushima Daiichi Nuclear Power Station (FDNPS). The 137Cs activity concentrations of Fukushima-accident contaminated soil samples ranged from 29.9 to 4780 kBq kg−1 with a mean of 2007 kBq kg−1. On the other hand, the 238U activity concentrations of these soil samples ranged from 5.2 to 22.4 Bq kg−1 with a mean of 13.2 Bq kg−1. The activity ratios of 234U/238U ranged from 0.973 to 1.023. The 235U/238U isotope ratios of these exclusion zone soil samples varied from 0.007246 to 0.007260, and they were similar to the natural terrestrial ratio confirming the natural origin. Using isotope dilution technique, the 235U/137Cs activity ratio was theoretically estimated for highly 137Cs contaminated soil samples from Fukushima exclusion zone ranged from 5.01 × 10−8 - 6.16 × 10−7 with a mean value of 2.51 × 10−7.

Considerations for uranium isotope ratio analysis by atmospheric pressure ionization mass spectrometry.

The accurate measurement of uranium isotope ratios from trace samples lies at the foundation of achieving nuclear nonproliferation. These challenging measurements necessitate both the continued characterization and evaluation of evolving mass spectrometric technologies as well as the propagation of sound measurement approaches. For the first time in this work, we present the analysis of uranium isotope ratio measurements from discrete liquid injections with an ultra-high-resolution hybrid quadrupole time-of-flight mass spectrometer. Also presented are important measurement considerations for evaluating the performance of this type and other atmospheric pressure and ambient ionization mass spectrometers for uranium isotope analysis. Specifically, as the goal of achieving isotope ratios from as little as a single picogram of solid material is approached, factors such as mass spectral sampling rate, collision induced dissociation (CID) potentials, and mass resolution can dramatically alter the measured isotope ratio as a function of mass loading. We present the ability to accurately measure 235UO2+/238UO2+ down to 10s of picograms of solubilized uranium oxide through a proper consideration of mass spectral parameters while identifying limitations and opportunities for pushing this limit further.

Accurate measurement of uranium isotope ratios in solid samples by laser ablation multi-collector inductively coupled plasma mass spectrometry

A multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) coupled to a 213 nm ns-laser was used to measure uranium isotope ratios (234U/238U, 235U/238U, and 236U/238U) in six solid nuclear certified reference materials (CRMs).

Optimized Chemical Separation and Measurement by TE TIMS Using Carburized Filaments for Uranium Isotope Ratio Measurements Applied to Plutonium Chronometry.

An optimized method is described for U/Pu separation and subsequent measurement of the amount contents of uranium isotopes by total evaporation (TE) TIMS with a double filament setup combined with filament carburization for age determination of plutonium samples. The use of carburized filaments improved the signal behavior for total evaporation TIMS measurements of uranium. Elevated uranium ion formation by passive heating during rhenium signal optimization at the start of the total evaporation measurement procedure was found to be a result from byproducts of the separation procedure deposited on the filament. This was avoided using carburized filaments. Hence, loss of sample before the actual TE data acquisition was prevented, and automated measurement sequences could be accomplished. Furthermore, separation of residual plutonium in the separated uranium fraction was achieved directly on the filament by use of the carburized filaments. Although the analytical approach was originally tailored to achieve reliable results only for the (238)Pu/(234)U, (239)Pu/(235)U, and (240)Pu/(236)U chronometers, the optimization of the procedure additionally allowed the use of the (242)Pu/(238)U isotope amount ratio as a highly sensitive indicator for residual uranium present in the sample, which is not of radiogenic origin. The sample preparation method described in this article has been successfully applied for the age determination of CRM NBS 947 and other sulfate and oxide plutonium samples.

Erratum to: Comparison of mass spectrometric methods (TE, MTE and conventional) for uranium isotope ratio measurements

The isotope ratio measurement techniques—total evaporation (TE), modified total evaporation (MTE), and conventional—used for characterization measurements of certified reference materials by thermal ionization mass spectrometer instruments are compared. The advantages of each method, the fractionation profiles resulting from the measurement techniques, and factors affecting systematic components of bias in the isotope ratio measurements are discussed. The TE and MTE techniques yield major ratios and the conventional and MTE techniques yield minor ratios of comparable quality (in terms of precision and accuracy).

Comparison of mass spectrometric methods (TE, MTE and conventional) for uranium isotope ratio measurements

Comparison of mass spectrometric methods (TE, MTE and conventional) for uranium isotope ratio measurements

Polyatomic interferences on high precision uranium isotope ratio measurements by MC-ICP-MS: applications to environmental sampling for nuclear safeguards

Modern mass spectrometry and separation techniques have made measurement of major uranium isotope ratios a routine task; however accurate and precise measurement of the minor uranium isotopes remains a challenge as sample size decreases. One particular challenge is the presence of isobaric interferences and their impact on the accuracy of minor isotope 234U and 236U measurements. We present techniques used for routine U isotopic analysis of environmental nuclear safeguards samples and evaluate polyatomic interferences that negatively impact accuracy as well as methods to mitigate their impacts.

Development of an analysis method of minor uranium isotope ratio measurements using electron multipliers in Thermal Ionization Mass Spectrometry

A simple analytical procedure was developed to measure with high accuracy the isotope ratio of minor isotope of natural uranium present in small quantities using a thermal ionization mass spectrometer (TIMS). The reduction of quantities used for analysis and the measurement of non-abundant isotopes are of prime interest in the nuclear industry. Indeed it is necessary to reduce the analyst received dose and the effluent released, as well as realizing measurement at trace level. The new generation of TIMS is equipped with a multicollection system of electron multipliers: discrete dynode electron multiplier (SEM) and continuous dynode electron multiplier (MIC), that improve the sensitivity compared to faraday cups. The procedure developed was verified using Certified Reference Material IRMM 052. Results were evaluated relying on NF T 90-210 norm regarding method validation. First, the isotope ratio 234U/238U was examined by total evaporation using the SEM and MIC to measure 234U and the faraday cup to measure 238U. In a second approach, the isotope ratio 235U/238U was studied by total evaporation using the SEM to measure 235U and the faraday cup to measure 238U. The classical method with peak-jumping SEM measurement was also used. Total evaporation method employing only the faraday cup was used to confront the results obtained. The analyzable quantity was reduced from 250ng to 50ng for the 235U/238U isotope ratio and from 1270ng to 50ng for the 234U/238U isotope ratio with acceptable uncertainties thanks to the use of electron multipliers. For all experiments were the accuracy was achieved, the calculated uncertainties were below to 0.28% for the 235U/238U isotope ratio and 5% for the 234U/238U isotope ratio.

Total evaporation method for uranium isotope-amount ratio measurements

Total evaporation (TE) is an analysis technique for the measurement of uranium isotopic abundance ratios using thermal ionization mass spectrometry (TIMS). A small mass dependent bias observed in this analytical technique is determined by an external correction factor using well characterized standards (most often certified reference materials, CRMs). The technique had been demonstrated to be highly precise and accurate for major isotope-amount ratio measurements of uranium and plutonium. We compare the performance of the TE analytical technique for uranium isotope ratio measurements on two TIMS instruments (TRITON and MAT261) using well characterized CRMs from NBL and investigate the dependence of the instrumental mass bias on the amount of sample analyzed. It is concluded that the mass bias during a TIMS uranium isotopic analysis by TE is independent of the amount of material analyzed. Unlike the major ratio, minor isotope ratio measurements by TE are biased high due to peak-tailing from the major isotopes. The biases in the minor isotope ratio data using TE are evaluated using well characterized NBL CRMs.

The jackknife as an approach for uncertainty assessment in gamma spectrometric measurements of uranium isotope ratios

The jackknife as an approach for uncertainty estimation in gamma spectrometric uranium isotope ratio measurements was evaluated. Five different materials ranging from depleted uranium (DU) to high enriched uranium (HEU) were measured using gamma spectrometry. High resolution inductively coupled plasma mass spectrometry (ICP-SFMS) was used as a reference method for comparing the results obtained with the gamma spectrometric method. The relative combined uncertainty in the gamma spectrometric measurements of the 238 U/ 235 U isotope ratio using the jackknife was about 10–20% ( k = 2), which proved to be fit-for-purpose in order to distinguish between different uranium categories. Moreover, the enrichment of 235 U in HEU could be measured with an uncertainty of 1–2%.

A novel inductively coupled plasma atomic emission spectrometry method for uranium isotope ratio measurements using chemometric techniques

Inductively coupled plasma atomic emission spectrometry was applied for uranium isotope ratio measurement. Even high resolution spectrometers cannot resolve small isotopic shift between 235U and 238U emission lines. Multivariate calibration techniques are useful tools for overcoming spectral overlap. The emission spectra of natural and depleted uranium samples were modelled by the laboratory written PLS-2 program (according to PLS algorithm). Four factors were selected as the optimum number of factors. The PLS model was applied on an independent test set. The results were validated by thermal ionization mass spectrometry (TIMS). A good agreement between the predicted results from the PLS-2 model and the TIMS results were obtained.

Ultrahigh energy resolution gamma-ray spectrometers for precision measurements of uranium enrichment

Superconducting gamma-ray detectors offer an order of magnitude higher energy resolution than conventional high-purity germanium detectors. This can significantly increase the precision of non-destructive isotope analysis for nuclear samples where line overlap affects the errors of the measurement. We have developed gamma-detectors based on superconducting molybdenum-copper sensors and bulk tin absorbers for nuclear science and national security applications. They have, depending on design, an energy resolution between ∼50 and ∼150 eV FWHM at ∼100 keV. Here, we apply this detector technology to the measurement of uranium isotope ratios, and discuss the trade-offs between energy resolution and quantum efficiency involved in detector design.

Evaluating the status of uranium isotope ratio measurements using an inter-laboratory comparison campaign

The REIMEP 18 (Regular European Inter-laboratory Measurement Evaluation Programme) campaign for the measurement isotopic ratios of uranium in nitric acid solution was completed in December 2006. The task for all participating laboratories was to measure the uranium isotopic composition of four uranium samples ranging from depleted to slightly enriched uranium. With 71 participating laboratories REIMEP 18 has become the largest nuclear isotopic measurement campaign organized by IRMM so far. Participation in this kind of measurement campaign is an integral part of the external quality control required for nuclear safeguards laboratories worldwide. For the first time also a significant number of academic laboratories, mainly from the geochemistry area was included. Certification measurements were carried out at IRMM using state-of-the-art mass spectrometric methodology. A MAT511 UF 6-gas source mass spectrometer (GSMS) was used to determine the n( 235U)/ n( 238U) ratios and a TRITON thermal-ionization mass-spectrometer (TIMS) for the minor isotope ratios n( 234U)/ n( 238U) and n( 236U)/ n( 238U). Verification measurements on ampouled samples were performed successfully prior to sample shipping and showed good agreement with the certified ratios. The results of the REIMEP 18 campaign confirm in general the excellent capability of nuclear safeguards and scientific laboratories in measuring isotopic abundances of uranium, although some problems were discovered for the measurements of the minor isotope ratios n( 234U)/ n( 238U) and n( 236U)/ n( 238U) and the calculation of measurement uncertainties for isotope ratios in general. This paper describes the outcome of the REIMEP 18 campaign. It includes a graphical evaluation and discussion of the results, an evaluation of the applied measurement and calibration techniques and a discussion of conclusions and actions to be taken.

Final report on CCQM-P48: Uranium isotope ratio measurements in simulated biological/environmental materials

CCQM-P48 was a Pilot Study of the Inorganic Analysis Working Group of the ComitéConsultatif pour la Quantité de Matière (CCQM) and was organized by the Institute forReference Materials and Measurements (IRMM) of the European Commission. It was the firstCCQM comparison dedicated to isotope ratio measurements.It involved a rather rare combination of 5 national metrology institutes (NMIs), representing at theComité International des Poids et Mesures (CIPM) 3 Member States of the Metre Conventionand 2 international organisations, and 10 invited laboratories (5 'nuclear experts', 5'geochemistry/mass spectrometry experts') selected outside CIPM. The aim was to comparecapabilities to measure the n(234U)/n(238U), n(235U)/n(238U) and n(236U)/n(238U) ratios in 4simulated biological/environmental materials prepared at a nominal uranium mass fraction of5 µg g-1 (series A) and, optionally, in 4 simulated biological/environmental materials preparedat a nominal uranium mass fraction of 5 ng g-1 (series B). In addition, a diluted solution of IRMM-184, a natural uranium isotopic certified reference material (ICRM), had also been distributedfor rehearsal as well as for calibration purposes. Participants, however, were free to apply themeasurement strategy of their choice.The test material can be assimilated to a biological/environmental type of sample as thematrix simulated approximately the composition of urine, from the combination in ~ 3%HNO3 solution of purified inorganic salt (aquarium seawater salts to 18 ± 2 g kg-1) and urea(to 17 ± 2 g kg-1). Four uranium isotopic mixtures were produced by mixing uranium ICRMsin the gas phase as UF6. Fractions of these mixtures, following their transformation under theform of solid UO3, were dissolved individually into ~ 3% HNO3 to get separate ~ 1000 mguranium kg-1 mother solutions. Both series of test materials derived from these solutions(simple dilution into batches of simulated urine), but participants were not informed about thissimilitude.The absolute value of the n(235U)/n(238U) ratio in each isotopic mixture was established at theearliest stage (i.e. in the gas phase) by calibrated gas source mass spectrometry measurements,and thus could serve as independent reference value (expanded, k = 2, uncertainty U = 0.05%,relative) for results from series A. 'Mixture mode' (MM) median data based on participants'results were proposed as consensus reference values for all the other sets of results produced.For samples from series A, 6 laboratories implemented a combination of sample digestion andmatrix separation steps, whereas 3 laboratories did not report any particular sample treatment(apart possibly from dilution). Only mass spectrometry was employed for isotope ratiomeasurements, including 9 MC-ICPMS, 3 HR-ICPMS, 2 TIMS and 1 ICP-QMS. For n(235U)/n(238U) ratios, results from 11 laboratories were in agreement within stated U withreference values for at least 3 mixtures; stated relative U were always > 0.0120% (except for1 participant) and always ⩽ 0.7% (except for 1 participant with U > 1% for 3 samples).Results on n(234U)/n(238U) ratios were almost always within ± 3% around the calculated MMmedian value. For the n(236U)/n(238U) smallest ratio, ~ 5 × 10-7, results relative to thecalculated MM median value were within ± 20% for 8 participants, between 20% and 50%above for 3 other participants, over 50% above for 1 participant, and 3 participants did notreport results. The perception of which factor caused the largest contribution to theseuncertainty estimations differed among participants because of differences in the analyticalmethodologies deployed but mostly because of wide differences in concepts of uncertaintyestimation.Main text.To reach the main text of this paper, click on Final Report.The final report has been peer-reviewed and approved for publication by the CCQM IAWG.

Results for the “REIMEP 18” inter-laboratory comparison campaign for the measurement of uranium isotope ratios

Results for the “REIMEP 18” inter-laboratory comparison campaign for the measurement of uranium isotope ratios

Accurate measurement of uranium isotope ratios in soil samples using thermal ionization mass spectrometry equipped with a warp energy filter

A chemical and mass-spectrometric procedure for uranium isotopic analysis using a thermal ionisation mass spectrometer equipped with a Wide Aperture Retardation Potential energy filter has been developed and applied to uranium isotopic measurements for various soil samples. Soil samples were digested using a microwave digestor. Uranium was isolated from soil samples by the chemical separation procedure based on the use of anion-exchange resin and UTEVA extraction chromatography column. The isotope ratios were measured for two certified reference materials by using a VG Sector 54-30 thermal ionisation mass spectrometer in dynamic mode with Faraday cup and Daly ion counting system. Replicates of standard reference materials showed excellent analytical agreement with established values supporting the reliability and accuracy of the method. Precision of the 235U/238U ratio was achieved by a correction factor of 0.22% amu as a function of ion-beam intensity with sample loads of around 250 ng of U. The resulting reproducibility for standards and soil samples was better than 0.2% at two standard deviations (SD). Uranium isotopic compositions have been determined in several reference soil samples such as Buffalo river sediment, NIST 2704, river sediment SRM 4350b and ocean sediment NIST-4357 and a Chernobyl soil sample. There was a significant deviation from the natural uranium in comparison with Chernobyl soil samples.

Reduction of UH+ formation for 236U/238U isotope ratio measurements at ultratrace level in double focusing sector field ICP-MS using D2O as solvent

The main factors affecting the accurate and precise determination of 236U using ICP-MS are instrumental background, the isobaric interference of 235UH+ molecular ion on 236U+ analyte ions, and the presence of 238U+ and 235U+ peak tails. An optimized analytical method for attenuating the influence of these factors on uranium isotope ratio measurements at ultratrace level of environmental samples has been developed. In order to reduce 235UH+ formation, D2O (heavy water) is used as a solvent for the dissolution and dilution of uranium samples. Abundance sensitivity was improved by use of medium mass resolution (m/Δm = 4450) in comparison with low mass resolution in double-focusing sector field ICP-MS (ICP-SFMS). For solution introduction the performances of several different sample introduction systems (Meinhard, Aridus and ultrasonic nebulizer) were studied. It has been shown, that for all nebulization systems, a diminution in UH+/U+ is observed in D2O as compared with H2O as solvent. Optimum results were obtained in ICP-SFMS for a desolvating microconcentric nebulizer system (Aridus) with a minimum hydride formation rate of 9 × 10−7 and a limit for 236U/238U isotopic ratio measurements of 3 – 5 × 10−7. A comparison was performed of three commercially available sector field ICP-MS devices, with good agreement found between single collector and multiple collector ICP-MS (MC-ICP-MS).