Abstract
Uranium isotope ( 235 U, 236 U, 238 U) ratios were determined for microscopic uranium-oxide grains using laser-ablation multi-collector inductively-coupled-plasma mass-spectrometry (LA-MC-ICP-MS). The grains were retrieved from contaminated soil and dust samples. The analytical technique utilised is rapid, requires minimal sample preparation, and is well suited for nuclear forensic 10 applications. Precision and accuracy were assessed by replicate analyses of natural uraninite grains: relative uncertainty for 235 U/ 238 U is 0.2 % (2σ), and the mean is in agreement with the natural ratio. A total of 115 uranium-oxide grains were analysed from environmental samples (soils and dusts); all of these were depleted uranium (DU) from a factory that produced uranium articles. Knowledge of the range of isotope ratios from particles of this controversial contaminant 15 has proven useful when interpreting isotope ratios from bulk samples. Variation of the measured isotope signatures reveals details of the history of uranium processing and emissions.
Introduction
Case-study National Lead Industries (NLI) operated a plant in Colonie (NY, USA), from 1958 -1984 . The plant processed uranium metals (depleted uranium and some enriched uranium); manufacturing kinetic energy penetrators (munitions), counterweights and radiation shielding from depleted uranium (DU). Scrap metal was combusted in a furnace prior to disposal as uranium-oxide, and this resulted in emissions of uranium-oxide particulate to the environment (at times via an unfiltered chimney). 1 Contamination of the suburban environment surrounding NLI by depleted uranium is evident in air filters, surface soils, reservoir sediments, and the urine of former employees and some residents [2] [3] [4] [5] [6] [7] . It is desirable to know the isotope ratios of the contaminant uranium when assessing bulk samples that comprise both background natural uranium and anthropogenic uranium. Furthermore it is possible that the uranium feedstock(s) used at NLI varied in isotopic composition. DU particulates from air filters collected 15.6 km NNW of NLI in 1979 had variable isotopic composition 2 . We hypothesise that bulk soil and dust samples aggregate particulates over several decades, and that individual primary uranium-oxide grains from these samples each record the isotopic composition emitted from NLI during a short interval of time. The bulk environmental samples of interest are contaminated soils (silica-rich medium to fine grained mineral sands with organic matter) and wind-blown dusts, typically comprising up to 500 mg kg -1 anthropogenic uranium, as microscopic uranium-oxide particulate in a matrix of natural mineral grains (that comprise trace natural uranium, less than 2.2 mg kg -1 ). The aim of this study is to analyse the isotopic composition(s) of a population of anthropogenic uranium-oxide grains on an individual particle basis.
Depleted Uranium (DU)
DU is the by-product of nuclear enrichment, and is depleted in the fissile isotope 235 U, typically (2 -3) x10 -3 235 U/ 238 U 8 . The atom ratio, 235 U/ 238 U (or n 235 U/n 238 U), of natural uranium has a traditional consensus value 7.253 x10 -3 (1/137.88) 9, 10 , recently recommended as 7.257 x10 -3 11-13 . However, there is evidence of natural isotopic fractionation c. ± 0.009 x10 -3 of this value 14 , and one known example (Oklo, Gabon) of sustained natural fission resulting in 235 U depletion 15, 16 . A useful fingerprint of anthropogenic contamination is the presence of 236 U, which naturally occurs at negligible abundances (in the order of x10 -11 -x10 -10 236 U/ 238 U 17, 18 ). In contrast, DU is typically contaminated by up to 3 x10 24, 25 .
Analytical Methods
Radiometric methods of uranium isotopic analysis include alpha and gamma-ray spectrometry. Due to the long half-lives of the uranium isotopes, long counting times (days) are required for precise determination of the minor isotopes. Furthermore, alpha spectrometry requires laborious chemical separation of the analyte from its matrix.
Analytical Methods
The high-precision analysis of actinide bearing particles by a combination of fission track analysis and thermal ionisation mass spectrometry (FT-TIMS) was described by Dietz 2 , and is the traditional, but laborious method in nuclear forensics 26 . Fission-track analysis is first used to locate actinide-bearing particles for analysis by TIMS. It is also possible to estimate 235 U/ 238 U ratios directly by fission-track analysis 27 . Fissiontrack analysis is time consuming and requires access to neutron irradiation facilities. Digital autoradiography 28 or environmental scanning electron microscopy (SEM) are alternatives for particle location. A disadvantage of TIMS is the requirement for careful chemical separation of the analyte, which may be imperfect and result in poor ionisation and hence precision (risky with only a 'one-shot' analysis per particle). Inductively-coupled-plasma mass-spectrometry (ICP-MS) offers faster analyses when compared to TIMS, and achieves good precision with multi-collector instruments (MC-ICP-MS). However, as for TIMS, particles of interest need to be manipulated and dissolved prior to analysis, and ideally the analyte is chemically separated from its matrix to avoid isobaric interferences. Hydride formation is also an issue when introducing solution samples, e.g. 235 U 1 H on 236 U, but this can be minimised by the use of a desolvating nebuliser. Secondary ion mass spectrometry (SIMS) has been used in the nuclear forensics context for the precise analysis of uranium and plutonium isotope ratios directly from particulates [29] [30] [31] . SIMS offers excellent spatial resolution, enabling particle location and sub-sampling 32 We have 'borrowed' these sample preparation and analytical techniques for this novel application. To the best of our knowledge, this paper demonstrates for the first time, the application of high precision isotope ratio LA-MC-ICP-MS analysis to a large population of individual uranium-oxide grains from environmental samples.
Experimental Sample Preparation
Samples A dust and a surface soil sample were collected from residential properties within 200 m of the former NLI site. Aliquots of these materials were analysed by scanning electron microscopy with an energy dispersive X-ray analyser (SEM-EDX: Hitachi S-3600N with Oxford Instruments Inca x-sight), revealing discrete uranium-oxide particles. Bulk uranium concentration and isotopic composition were estimated by quadrupole ICP-MS (VG Elemental PQ ExCell with Cetac Technologies Aridus II desolvating nebuliser) after total dissolution: for the soil 90 ± 9 mg kg -1 uranium, (2.1 ± 0.1) x10 -3 235 U/ 238 U (2s); and for the dust 385 ± 33 mg kg -1 uranium, (2.2 ± 0.1) x10 -3 235 U/ 238 U. These isotope ratios confirm that the vast majority of the uranium in these samples is from anthropogenic DU contamination. The uranium-oxide particulates were concentrated from the bulk samples using the protocol summarized in Table 1 . Dense-liquid (di-iodomethane, ρ 3.3 g cm -3 ) was used to separate low-density silicates from the bulk samples, recovering dense grains including uranium-oxides (ρ 10.96 g cm -3 ) greater than about 20 µm diameter.
Grain Mounts
Aliquots of the concentrates were mounted in epoxy resin, ensuring separation of grains, and then ground and polished to reveal cross sections (alternatively, they could be adhered whole to the surface of a suitable mount). The mounts were scanned using SEM-EDX to map the uraniferous grains, see Figure 2 . Sample preparation and the grain mapping are moderately time consuming. However, the methodology does produce robust grain mounts with a good density of uraniferous grains, which can be quickly located using the laser ablation system's optical microscope. 
Solutions
Solutions were prepared from uraniferous grains from these and two other soil dust samples collected from the vicinity of NLI, for comparison with the laser ablation dataset. Spherical grains that appeared metallic or glassy (anthropogenic in appearance) under an optical microscope were picked from the concentrates (under ethanol) using fine tweezers, and then transferred to a low-tack adhesive (Glue Dots Repo ™), as shown in Figure 3 . These mounts were scanned using SEM-EDX, but fewer than 1 in 40 proved to be uraniferous (the others were mostly lead, tin or lead glass). SEM-EDX analyses show the uraniferous grains were typically comprised of only uranium and oxygen (elemental LLD c. 1 %), for this reason, it was not considered necessary to chemically separate the uranium from these solutions. Subsequent analysis of similar grains shows that they are typically polycrystalline UO 2 (unpublished data), and they often include cavities. Twenty-four uranium-oxide grains were successfully transferred by tweezers into individual pre-leached micro-centrifuge tubes, and then dissolved in ultrapure double-distilled concentrated nitric acid (within a class 100 clean room, typical digest blanks <100 fg U). This method of sample preparation is by comparison with the previous, relatively time-consuming.
Analysis

Mass Spectrometry
Analysis was made using a double-focussing MC-ICP-MS instrument (VG Elemental Axiom), coupled with a desolvating nebuliser (Cetac Technologies Aridus) to reduce hydride interference from solutions. Following peak centring, low abundance 236 U was measured on a secondary electron multiplier, 235 U and 238 U on Faraday cups. Abundance sensitivity and mass bias were quantified at the start and end of each analytical run, using a natural uranium solution (U950a). Solutions The solutions were diluted in ultrapure 2% HNO 3 (aq) to approximately 25 ng g -1 uranium. The sample analyses were bracketed by analyses of a solution of enriched uranium standard reference material U010 that includes 236 U (New Brunswick Laboratory).
Laser Ablation of grain mounts
The grain mounts were sampled by laser-ablation (New Wave Research LUV266x), using a c. 25 x 14 µm spot, 1 Hz repetition rate at a fluence of c. 68 mJ cm -2 (sufficient to give a stable signal within detector range). The output from the desolvating nebuliser provided the carrier-gas flow (c. 1 l min -1 Ar 2 ) for the ablation cell, and was used for the introduction of solution reference materials U950a and U010 at the start and end of each analytical run. integrations. These data were output as the mean and standard error of the mean, after rejection (10%, 2σ). The large volume (c. 30 cm 3 ) of the ablation cell attenuates the pulses of sample from the ablation, and thereby minimises the effects of the detector response delays between Faraday cups and electron multiplier. The 238 U signal was then monitored for approximately 30 seconds, to allow the passing of 'spikes' from previously ablated material and the return to baseline values, before the next analysis was started. The ablation protocol used produced irregular conical pits, approximate dimensions 25 x 14 x 1 µm in uranium-oxide sample grains (measured using SEM and Caminex Enterprises Alicona infinite focus microscope). The sampling volume is roughly equivalent to a 9 µm diameter uranium oxide sphere, or 4 ng uranium. Ablation of the resin gave negligible 238 U detector responses (c. 3 x10 -5 V using a 10 11 ohm resistor, c.f. 1.6 V from typical samples). Sample grains were bracketed by analyses of natural uraninite grains for quality control. Of the 115 sample grains, 68 were analysed in replicate (up to 21 repeats from a single grain).
Data Processing
Corrections were made to the data using U950a as a primary reference material: abundance sensitivity ( 238 U on 236 U, c. 38 (approximated by an exponential mass-bias function 39 ). A secondary reference material, U010, was used to correct for bias between the ion counter and Faraday cups. Estimates of uncertainty were propagated from the analytical standard error of the mean (σ m ) and the relative standard deviations of the corrected primary and secondary reference materials. Fig. 4 235 U/ 238 U ratios for natural uraninite grains by LA-MC-ICP-MS, from four analytical runs. Nine of the data have been used as reference materials (RM) to correct an observed bias in the second half of run 4.
Results

Quality Control
Natural uraninite grains were ablated 155 times throughout the four analytical runs for quality control. The data are presented in Figure 4 (with 8 outliers removed). Following mid-session calibration, the data in the fourth analytical run drift from a significant low bias. Nine of the uraninite data have been used to bracket the remaining sample and quality control data for that interval (as a tertiary reference material). These self-corrected data are highlighted in Figure 4 , and are excluded from the following quality control statistics. The remaining data (n=138) are normally distributed about an arithmetic mean 235 U/ 238 U (7.259 ± 0.002) x10 -3 (2σ m ). The relative precision for these data is 0.22 % (2σ). The mean is within uncertainty of the recently recommended value 12 , within the range of natural variability 14 , or slightly biased when compared to the traditional consensus 9 . The mean square weighted deviation (MSWD) for these QC data was 1.2, demonstrating that the propagated uncertainty had probably been slightly underestimated 40 . Therefore, the uncertainties for 235 
Data
The analytical data for the laser ablation of uranium-oxide grains are presented in Figure 5A , alongside those from analyses of solutions. The solution data show a similar spread of isotopic compositions to the laser ablation data. All these data are expressed as atom ratios. Figure 5B shows the data for soils and dusts, and the spread of isotopic compositions from these samples are similar. Particle-solution exchange in the wet soil environment does not explain the spread of data.
Discussion

Case-study Interpretation
The data confirm the hypothesis that the individual particles of uranium-oxide record a variety of anthropogenic isotopic compositions, which are averaged in bulk soil and dust samples. All of the uranium-oxide grains analysed are from DU, with 235 U/ 238 U less than 2.4 x10 -3 . Enriched uranium grains were not observed; these may be very scarce as comparatively little enriched uranium was handled by NLI, and it may have been recycled because of its value. Enriched uranium was evident in one former employee's urine 6 , implying dispersal of some of this material within the plant and possibly further afield. NLI reduced uranium tetrafluoride (UF 4 processes at NLI could also have mixed these isotopic compositions: feedstock storage, reduction to uranium metal (derby), castings, machining, shop-floor debris, scrap storage, and finally chip burning (conversion) in the furnace releasing uranium-oxide particulates to the environment. There is a large spread in 236 U abundance, with a reasonably well defined mixing-line from (5 -31) x10 -6 236 U/ 238 U. These data range from (2.05 -1.99) x10 -3 235 U/ 238 U with increasing 236 U/ 238 U. The data cluster around 2.7 x10 -5 236 U/ 238 U, 2.0 x10 -3 235 U/ 238 U. We interpret these ratios to follow either a mixing line between two isotopically discrete batches, or an evolving series of compositions. The former hypothesis seems less likely, as there does not appear to be a second cluster. There is a scatter of ratios up to 5 x10 -5 236 U/ 238 U, and up to 2.4 x10 -3 235 U/ 238 U. These ratios are explained by inhomogeneous mixing (possibly in the NLI conversion furnace) of a continuation of the previous trend with a third component of slightly less depleted uranium. This hypothesised process also appears to affect some of the grains from the clustered region ( Figure Aii) , drawing them away from the mixing line. collected by air filters in April and May 1979 and analysed by FT-TIMS 2 ( Figure 6 ). These ratios fit into the scattered region of out dataset, and support the continuation of the trend of increasing 236 U to at least 6 x10 -5 236 U/ 238 U. These are most likely to have been from emissions at that time. However, the scrap metal may have accumulated for several months before conversion. The isotopic compositions of the uranium materials processed by NLI appear to have been more variable during this period. It was reported that in 1980, 150 drums of waste uranium had accumulated over several months, and nearly 2 tonnes were converted to oxide in March and April of that year, with the release of only 7.5 g of uranium, thanks to operation of, and improvements to a filtration system, following enforcement action 41 . Extensive uranium contamination of soils is evident by 1980 3 , estimated in the order of 5 tonnes uranium deposited on soils within 1 km 2 6 . The vast majority of the contamination from NLI pre-dates 1980, therefore the sample grains analysed in this study probably also pre-date 1980. Based on the number of grains loosely tied to 1979 (c. 20%), a significant portion of the contamination appears to have been emitted during that period. We speculate that the feedstock received by NLI evolved through a series of compositions, from 2.05 x10 The spread of 235 U/ 238 U isotope ratios revealed by this study is matched by those in Figure 1 , but we are able to resolve more information from this large and precise dataset. Some of these DU grains comprise more 236 U than previously reported. These data show that at least some of the DU processed at NLI had low levels of 236 U. Therefore, 236 U cannot be used as a defining fingerprint of DU contamination if, as for quadrupole ICP-MS, the lower limit of detection of the analytical technique approaches these ratios.
Analytical Methodology
Laser ablation allows for the rapid collection of data, when compared to TIMS or solution ICP-MS. A typical sample grain analysis took less than two minutes, and the instrumental productivity (including set-up, reference materials, and particle location), was around 16 minutes per sample grain. Modern MC-ICP-MS systems offer faster set-up times, which could further improve productivity. Pre-concentration of particles of interest using dense liquid separation was quick and effective, and allows for efficient analysis by laser ablation (or SIMS). However, the methodology does bias the sample by excluding particulates and grains smaller than approximately 20 µm. Smaller grains could be recovered using heavy-liquids with centrifugation 43 , froth-floatation 44 , or inertial separation. Alternatively raw samples may be analysed, but requiring more time searching for the grains of interest and exchanging sample mounts. It is not necessary to embed and polish the particles; they could be adhered to a mount with a clean adhesive, or sampled directly from a swipe sample. The volume of sample consumed per analysis is small when compared to the volume of the grains of interest, and allows for replicate analyses. The sampling area is similar to the extent of the grains presented on the mount surface. Smaller particles can be analysed, when sufficiently separated from each other, as the uranium content of the resin is indistinguishable from detector noise. However, there is potential for minor additional 235 U 1 H formation with hydrogen liberated from the epoxy resin. Clean mounting material would be necessary for fine particulates, for example carbon planchets. Modern laser systems can also achieve better spatial resolution. An observed bias in the quality control data for part of one analytical run was corrected by using some of these data as a tertiary standard. There are variations in the uraninite data, and the uncertainties were slightly underestimated, both of which can also be explained by changes in instrument bias between external corrections. This demonstrates the need for more frequent monitoring, preferably by laser ablation of a solid reference material that includes 236 U (e.g. New Brunswick Laboratory U005-A). These minor issues were not significant to our interpretations of the sample data. Accuracy was demonstrated by repeat analyses of natural uraninite grains; the mean value agrees with the 'natural ratio'. Relative precision over four analytical runs of 0.2 % (2σ) for 235 U/ 238 U is better or at least comparable to the current methodologies used for nuclear forensic applications. Relative uncertainties (2σ) for the sample grains ranged between 0.2 -1.8 % for 235 U/ 238 U, and 2.3 -4.0 % for 236 U/ 238 U. The precision of this method was more than adequate to resolve differences in the isotopic composition of microscopic grains of DU-oxide. The precision of LA-MC-ICP-MS compares favourably with LA-SF-SC-ICP-MS 36 , and is similar for SIMS analyses of particles from environmental samples 29, 31 . It is hard to judge from the literature that achieved by FT-TIMS, although TIMS may offer better analytical precision. However, our method allows the acquisition of large datasets that may be more representative of the samples, the precision is fit-for-purpose, and appears to be a significant way forward for nuclear forensics. For this case-study, a larger dataset could resolve further details regarding the history of uranium processing at NLI (perhaps twice as large, and including sample from other locations for representativity).
Conclusions
We have demonstrated the use of LA-MC-ICP-MS to rapidly analyse a large population of microscopic uranium-oxide grains for an environmental case-study. It is clear from these and other data that the isotopic compositions of depleted uranium are variable, especially with respect to 236 U. The accuracy and precision analysing 235 U/ 238 U for natural uranium was excellent. Typical relative uncertainties (2σ) of 0.4% for 235 U/ 238 U and 2.7 % for 236 U/ 238 U, are well-suited to nuclear forensic applications, and are an improvement over single-collector (quadrupole or sector-field) LA-ICP-MS. LA-MC-ICP-MS offers several advantages to nuclear forensics, including fast analysis time, minimal sample preparation, and partial ablation of the sample.

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