solution and laser ablation measurements - Springer Link

Anal Bioanal Chem (2008) 390:521–530 DOI 10.1007/s00216-007-1741-x

ORIGINAL PAPER

239, 240, 241

Pu fingerprinting of plutonium in western US soils using ICPMS: solution and laser ablation measurements James V. Cizdziel & Michael E. Ketterer & Dennis Farmer & Scott H. Faller & Vernon F. Hodge

Received: 30 June 2007 / Revised: 6 November 2007 / Accepted: 6 November 2007 / Published online: 30 November 2007 # Springer-Verlag 2007

Abstract Sector field inductively coupled plasma mass spectrometry (SF-ICPMS) has been used with analysis of solution samples and laser ablation (LA) of electrodeposited alpha sources to characterize plutonium activities and atom ratios prevalent in the western USA. A large set of surface soils and attic dusts were previously collected from many locations in the states of Nevada, Utah, Arizona, and Colorado; specific samples were analyzed herein to characterize the relative contributions of stratospheric fallout vs. Nevada Test Site (NTS) plutonium. This study illustrates two different ICPMS-based analytical strategies that are successful in fingerprinting Pu in environmental soils and dusts. Two specific datasets have been generated: (1) soils J. V. Cizdziel (*) Harry Reid Center for Environmental Studies, University of Nevada - Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4009, USA e-mail: [email protected] M. E. Ketterer Department of Chemistry and Biochemistry, Northern Arizona University, Box 5698, Flagstaff, AZ 86011-5698, USA D. Farmer Department of Health Physics, University of Nevada - Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-3037, USA S. H. Faller US Environmental Protection Agency, Radiation and Indoor Environments National Laboratory, Box 98517, Las Vegas, NV 89193-8517, USA V. F. Hodge Department of Chemistry, University of Nevada - Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4003, USA

are leached with HNO3-HCl, converted into electrodeposited alpha sources, counted by alpha spectrometry, then reanalyzed using laser ablation SF-ICPMS; (2) samples are completely dissolved by treatment with HNO3-HF-H3BO3, Pu fractions are prepared by extraction chromatography, and analyzed by SF-ICPMS. Optimal laser ablation and ICPMS conditions were determined for the re-analysis of archived alpha spectrometry “planchette” sources. The best ablation results were obtained using a large spot size (200 μm), a defocused beam, full repetition rate (20 Hz) and scan rate (200 μm s−1); LA-ICPMS data were collected with a rapid electrostatic sector scanning experiment. Less than 10% of the electroplated surface area is consumed in the LA-ICPMS analysis, which would allow for multiple re-analyses. Excellent agreement was found between 239+240Pu activities determined by LA-ICPMS vs. activity results obtained by alpha spectrometry for the same samples ten years earlier. LA-ICPMS atom ratios for 240Pu/239Pu and 241Pu/239Pu range from 0.038–0.132 and 0.00034–0.00168, respectively, and plot along a two-component mixing line (241Pu/239Pu= 0.013 [240Pu/239Pu] – 0.0001; r2 =0.971) with NTS and global fallout end-members. A rapid total dissolution procedure, followed by extraction chromatography and SF-ICPMS solution Pu analysis, generates excellent agreement with certified 239+240Pu activities for standard reference materials NIST 4350b, NIST 4353, NIST 4357, and IAEA 385. 239+240 Pu activities and atom ratios determined by total dissolution reveal isotopic information in agreement with the LA-ICPMS dataset regarding the ubiquitous mixing of NTS and stratospheric fallout Pu sources in the regional environment. For several specific samples, the total dissolution method reveals that Pu is incompletely recovered by simpler HNO3-HCl leaching procedures, since some of the Pu originating from the NTS is contained in refractory siliceous particles.

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Keywords Plutonium isotopes . Laser ablation . ICPMS . Nevada Test Site . Stratospheric fallout . Attic dust

Introduction Plutonium (Pu) is a radioactive element widely distributed in the Earth’s surface environment as a result of atmospheric fallout from nuclear weapons testing. In some local and regional settings, Pu in soil, sediment, and dust originates from other specific point sources related to nuclear weapons production or the nuclear fuel cycle. Atmospheric fallout from weapons tests, however, accounts for the vast majority of the Earth’s surficial Pu inventory. This fallout can be broadly classified into two classes: “global” or “stratospheric” fallout from high-yield tests conducted mainly by the USA and the former Soviet Union, and “tropospheric fallout” produced by lower-yield explosions conducted by several nations. The stratospheric fallout was deposited globally after injection into the stratosphere, after which it returned to the Earth’s surface, subsequently being removed as wet and dry deposition as a predictable function of latitude and precipitation. Most of this deposition occurred between 1952 and 1964. Tropospheric fallout had a shorter residence time in the lower atmosphere, and was removed on a local or regional scale by interaction with prevailing weather conditions. In the USA, the observed fallout is a combination of stratospheric fallout and tropospheric fallout from the Nevada Test Site (NTS). Plutonium atom ratios, especially 240Pu/239Pu and 241Pu/ 239 Pu, have been used as a fingerprint to resolve global fallout from other environmental Pu components [1]. This is effective because Pu is of synthetic origin, and its isotopic composition varies widely according to its mode of production and neutron capture history. Pu atom ratios and 237Np/239Pu are well defined for global fallout; deviations outside of the 2σ global fallout atom ratio ranges (northern hemisphere, mid-latitude regions) of 0.180±0.014 for 240Pu/239Pu and similar constraints for other ratios are indicative of sources other than global fallout [2]. For example, Beck and Krey [3] used Pu atom ratios in reconstructing radiation exposures in Utah due solely to Nevada nuclear tests. Ketterer et al. [4] showed that soils in northeastern Poland, containing Chernobyl deposition, exhibited 240Pu/239Pu and 241Pu/239Pu ratios in excess of the stratospheric fallout ranges. Kelley et al. [5] used Pu atom ratios to examine sources and migration of Pu in groundwater at the Savannah River Site. Perturbations in the Pu global fallout ratios were also observed in local sediments where Pu bombs were mechanically destructed following aircraft accidents in Thule, Greenland, and Palomares, Spain [6, 7]. In this study, we have evaluated a large set of surface soils and attic dusts, previously collected by the authors, from

Anal Bioanal Chem (2008) 390:521–530

many locations in the states of Nevada, Utah, Arizona, and Colorado. Our objectives are to use rapid inductively coupled plasma mass spectrometry (ICPMS)-based approaches to “fingerprint” or apportion the relative contributions of global fallout and tropospheric fallout from the NTS. Although the relative contributions of NTS and global fallout Pu in the environment have been previously investigated by mass spectrometry [3], these studies utilized thermal ionization mass spectrometry (TIMS), which is highly specialized, time consuming, and not widely available among Pu researchers. It follows that ICPMS would be well suited to performing the necessary Pu activity and atom ratio measurements in a rapid, pragmatic fashion. Specific samples have been analyzed in the present work to characterize the relative contributions of stratospheric fallout vs. Nevada Test Site plutonium. Two datasets have been generated: (1) HNO3-HCl leaching, followed by alpha spectrometry (AS) and re-analysis of the alpha sources by laser ablation sector field (SF)-ICPMS—referred to as dataset 1, and (2) HNO3-HF-H3BO3 total dissolution, followed by solution analysis of Pu fractions by SF-ICPMS— referred to as dataset 2. Measurements of Pu activity and atom ratios 239

Pu and 240Pu, the most abundant Pu isotopes in the environment, are alpha emitters, and their activity in a sample is traditionally measured by AS. This analytical process involves extraction of Pu from the sample (typically by acid digestion after sample ashing or more thoroughly by total dissolution techniques), isolation by anion exchange, AS source preparation by electrodeposition or by co-precipitation with NdF3, and detection of alpha particles in an evacuated chamber using passivated implanted planar silicon (PIPS) detectors or surface barrier detectors. This process is labor intensive and time consuming, placing severe constraints on the speed and scope of data generation. A major shortcoming of alpha spectrometry is that typically it is not possible to resolve 239Pu and 240Pu because of their similar alpha energies (5.157 MeV and 5.168 MeV, respectively). Independent quantification of 239Pu and 240Pu in high-activity samples can be accomplished with high-resolution AS and spectral deconvolution [8]; however, sample activities are more commonly reported combined as 239+240Pu, i.e., lacking this crucial isotopic signature. Further, 241Pu, which is deposited with the other Pu isotopes, cannot be measured by AS because it is a beta-emitting nuclide requiring an entirely different procedure for measurement, usually liquid scintillation spectrometry or AS following 241Am in-growth [9]. There are undoubtedly thousands of electroplated or coprecipitated alpha sources in laboratories worldwide that have been archived after activity measurements by AS. These sources contain Pu isotopes already separated from


complex environmental matrices. If the Pu is stripped from the sources and analyzed by mass spectrometry to determine Pu atom ratios, additional information can be garnered on the likely sources of Pu in the sample, even if the difference from global fallout is subtle. ICPMS, with high sensitivity and low detection limits, is particularly suited to quantitative determination of individual Pu isotope activities as well as atom ratios [1]. However, removal of Pu from alpha sources involves additional preparation steps, adding analytical time and generating waste solutions. Dissolution of sources potentially introduces undesired matrix components, particularly U, into the solution. The presence of U results in the well-known, undesirable formation of 238 1 + U H ; the signal from 238U1H+ overlaps with the 239Pu peak and interferes with the measurement. Although U can be removed from the solution by preparative chromatography, this complicates and lengthens the procedure. Alternatively, Pu can be introduced directly from alpha sources under relatively dry plasma conditions by laser ablation. Each Pu isotope can then be quantified regardless of its decay mode. Boulyga et al. [10] used laser ablation ICPMS with isotope dilution to measure Pu isotopes on the surface of stainless steel targets after electroplating and found good agreement with results from alpha spectrometry. In a later study, Boulyga et al. [11] performed direct Pu analysis of soils, without dissolution or separation, using laser ablation (LA)-ICPMS. The direct analysis approach is only feasible for soils containing relatively elevated Pu levels. In isotopic fingerprinting, it of great value to generate more than one isotope ratio, where possible, so that a twocomponent “common denominator mixing plot” can be obtained. Mixing plots allow for qualitative recognition of the presence of outlying components from additional contributing sources, and permit quantitative apportionment of up to (n+1) sources when n independent common denominator ratios are used. Hence, simultaneous analyses of 240 Pu/239Pu and 241Pu/239Pu is much more powerful than determination of 240Pu/239Pu by itself. In our studies, we have extended the use of LA-ICPMS in Pu analysis, demonstrating that high-quality results for both 240Pu/239Pu and 241Pu/239Pu can be obtained from straightforward LAICPMS measurements performed on alpha sources. These results are presented as dataset 1. Total dissolution for Pu-containing refractory particles In a second segment of this study, we address the need to prepare specific samples using dissolution procedures more aggressive than acid leaching. Stratospheric fallout is known to generate Pu in soils that is recoverable by simple acid leaching (i.e., not involving a complete dissolution of the silicate component); however, a portion of the NTS Pu is also incorporated into fused vitreous particles that are

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resistant to acid leaching, requiring complete dissolution with HF or molten salt fusions. The case for complete dissolution in many situations has been made strongly [12]. With increasing interest in generating high-throughput, low-cost Pu data by ICPMS methods, previous studies have developed total dissolution procedures in conjunction with ICPMS (e.g., see [13]). These procedures, however, involve lengthy evaporation and re-dissolution steps and/or complex multiple-step column separation methods which tend to nullify the sample throughput advantages of ICPMS. As a simple alternative, we present a simple dissolution procedure involving HNO3/HF, which is neutralized with H3BO3; thereafter, Pu is isolated in a rapid single-column step involving no evaporation and reconstitution of the resulting Pu fraction. Herein, we apply the use of the total dissolution ICPMS procedure in the determination of 239+240 Pu and 240Pu/239Pu in standard reference materials and environmental soils (dataset 2).

Experimental Samples An extensive set of attic dust and soil samples were collected by one of the authors (JVC) during 1996 and 1997 from communities in Nevada and Utah as part of previous environmental studies; details of sample collection and AS source preparation are given elsewhere [14, 15]. The JVC soils represent the top 1 cm of surface soil; dusts were obtained by vacuuming, following which the dust was sieved (2.36 mm) to eliminate large debris. The JVC soils have been used in both datasets of this study. Additional soils have been collected by another author (MEK) as part of more recent work surveying the activities and isotopic compositions of Pu in western US soils; these were obtained as 0- to 10-cm-depth cores from undisturbed surface soils. The MEK soils were composites of three to ten such cores obtained from areas of 100–200 m2, and were sieved (2.36 mm) and pulverized with an alumina mortar/pestle prior to analysis. The MEK soils have been used in the total dissolution (dataset 2) segment of this study. Figure 1 illustrates the approximate locations in the western USA where the JVC and MEK samples were obtained. Leaching and total dissolution procedures For dataset 1, alpha sources were prepared by two sequential aqua regia leaches of 20-g sub-samples; the sample was spiked with 242Pu tracer (NIST 4334d, ca. 0.05 Bq) prior to the leaching procedure. Pu was isolated by anion exchange,

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Fig. 1 Map of the southwestern USA, depicting the locations of sampled areas relative to the Nevada Test Site

and Pu sources were prepared by electroplating on stainless steel planchette disks. Additional analytical details are given in previous papers [14, 15]. Total dissolution (dataset 2) was performed using 2–3 g pulverized soil. Material was dry-ashed for 4 h at 600 °C, then spiked with 0.007 Bq (ca. 50 pg) of 242Pu. 242Pu tracer was prepared from NIST 4334 g stock solution. Dissolution was accomplished in 125-mL fluorinated ethylene–propylene bottles with 20 mL of 29 M HF and 20 mL of 16 M HNO3. Sample bottles were heated at 75 °C in a convection oven for 1–3 days until dissolution was complete. Thereafter, 20 mL additional 16 M HNO3 and 30 mL water were added; samples were treated with a slight stoichiometric excess of H3BO3, thereby converting unreacted HF to BF4 . The resulting aqueous solution contains approximately 7 M HNO3. Sodium nitrite (2.0 g) was added to each sample to reduce Pu to Pu(IV); thereafter 0.15 g TEVA resin (EIChrom, Darien, IL, USA, 100–150 μm) was added, and the mixtures were gently agitated for 2–3 h. The TEVA material effectively retains Pu(IV) from a wide range of aqueous HNO3 concentrations, since the capacity factor, k′, exceeds 103 in the range 0.3–10 M [16, 17]. On standing, the TEVA resin that settled to the top of the solution was carefully removed with a transfer pipet without disturbing any solids present on the bottom of the container. It is noted that upon standing, and after cooling, addition of boric acid, NaNO2, and TEVA resin, some hydrated SiO2 did re-precipitate, though this would not affect sample–spike equilibration already achieved after the initial heating step and complete dissolution. The resin fractions, containing recovered Pu, were loaded onto 10-mL pipet tip “columns” equipped with tightly packed glass wool plugs; these completely retained all of the relatively large 100- to 150-μm TEVA resin

particles. The columns were rinsed with five 5-mL portions of 2 M HNO3 (discarding U), and two 5-mL portions of 8 M HCl (discarding Th). Pu was eluted using the sequence: 2 mL H2O, 2 mL 0.05 M aqueous ammonium oxalate, and 1 mL H2O. This Pu fraction is suited for direct solution analysis by ICPMS without further preparations, and contains adequate recovered Pu along with being sufficiently decontaminated of U and Th for SF-ICPMS measurements. Laser ablation SF-ICPMS measurements for dataset 1 An LSX-213 laser ablation system (Cetac Technologies, Inc., Omaha, NE, USA) was coupled to a sector field ICPMS located at UNLV (Axiom; VG Elemental, now Thermo Fisher Scientific, Inc., Waltham, MA, USA). Instrumental conditions for the current study are provided in Table 1. The RF power used in this study (1,200 W) was similar to the optimal setting found by Boulyga et al. [10] (1,150 W), whereas our optimal nebulizer flow rate was slightly higher, 1.2 L min−1 versus 1.1 L min−1. The laser was operated using the largest spot size available (200 μm), a defocused beam (setting 1,250), a rapid scan rate (200 μm s−1), and near full energy (80%) to maximize the width of the laser beam and produce a relatively large stable signal. The nebulizer gas was passed through a CETAC Aridus II prior to entering the laser ablation unit. The Aridus II consists of a PFA nebulizer followed by a semipermeable desolvating membrane and allows optimization of plasma conditions with a dry aerosol. A U tuning solution was also used for investigating hydride formation and instrumental mass bias (see below). Data for the planchettes were collected in low resolving power mode (R=420 at 10% height) using electrostatic


525

Table 1 Laser and ICPMS operating conditions Parameter Laser ablation parameters Laser type Wavelength Pulse width Laser energy Pulse rate Spot size Scan rate Energy setting Defocus setting Plasma parameters RF power Cool gas flow Auxiliary gas slow Nebulizer gas flow Data acquisition Dwell time Points/peak Masses monitored Sweeps Total acquisition time per run Mass spectrometer settings Mass resolution Ion energy Transfer lens 1 voltage Transfer lens 2 voltage X1 deflection X2 deflection Curve lens X lens Multiplier voltage

Value

Nd:YAG 213 nm 5 ns >4.0 mJ per pulse 20 Hz 200 μm 200 μm s−1 80% 1,250 1,200 W 14 L min−1 1.2 L min−1 1.2 L min−1 10 ms 1 239, 240, 241, 242 750 30 s 420 (m/Δm at 10% height) 4,972 V 4,345 V 4,040 V −6 36 21 −1,737 V −2,200 V

scanning (1 point per peak, 10-ms dwell, 750 sweeps) to rapidly peak hop between masses 239, 240, 241, and 242. Mass 238 was not monitored during Pu data collection to negate possible detector effects from large changes in signal intensity between m/z 238 (e.g., U) and higher m/z values (e.g., Pu masses). 238U was monitored in a separate scan for hydride correction; UH+/U+ ratios were 6–8×10−6, and the correction for UH+ at m/z 239 was 0.997. The Pu ion intensities also increased with increasing laser energy, whereas the 240Pu/239Pu ratio decreased slightly. The 240Pu/239Pu decrease may be a result of some 238 1 + U H formation in the plasma because the uranium signal also increased as a function of laser energy. Although the change in the ratio was small, we decided to sacrifice some signal and use the 80% E setting as a compromise between the two opposite trends. The highest laser repetition settings (Hz) were found to generate the highest 239Pu+ ion intensities. A defocused beam, which spreads the energy out more evenly over the beam width, also yielded greater ion intensities. In summary, the following LSX-213 settings were chosen for sample analysis: pulse rate 20 Hz, energy 80%, defocus 1,250, and scan rate 200 μm s−1. Dataset 1: 240Pu/239Pu atom ratios and 239+240Pu activity ratios for blanks and standards by LA-ICPMS Using the laser conditions determined above, nine ablation scans over three separate days were performed on a planchette prepared with only reagents (procedural blank). The detection limit (assuming a 20-g sample) for 239+240Pu was 0.012 Bq kg−1, sufficient for most surface soils. To test for accuracy, a series of planchettes were analyzed and the 240 Pu/239Pu atom ratio and 239+240Pu activity per mass ratio were compared with certified or consensus values or data obtained from other methods. Seven ablation scans on two separate days of a planchette prepared from a NIST standard reference material (SRM 4353) gave a 240Pu/239Pu atom ratio of 0.061±0.002 (one standard deviation). This is close to NIST’s uncertified 240Pu/239Pu TIMS atom ratio of 0.0553 (no uncertainty provided). The small difference between the laser ablation result and the uncertified value is perhaps explained by a notation on the NIST 4353 certificate, which states that approximately 8% of the 239+240Pu in NIST 4353 is resistant to HNO3-HCl leaching. The


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absence of this refractory Pu in the planchettes (based on an acid leach) would tend to produce a higher 240Pu/239Pu atom ratio, in the direction of global fallout. Dataset 1: comparison of planchette re-analysis

239+240

Pu by AS vs. LA-ICPMS

Although conducted a decade apart, measurements of Pu activity by AS and by laser ablation ICPMS of the same planchette produced nearly identical results (Fig. 4). This is perhaps not surprising given the analyses were conducted on the same deposit, same sub-sample, and the planchette was made from a solution that was isotopically homogeneous. The only likely errors in this scenario would be counting statistics for the two independent methods and any unaccounted biases in atom ratio measurements by ICPMS (e.g., mass discrimination, presence of interfering polyatomic ions, abundance sensitivity, etc.). These particular sources of error were either accounted for in the calculations or are relatively small. In any case, the good agreement over a wide range of 239+240Pu activities between the methods supports the validity of LA-ICPMS re-analysis of alpha planchettes. 239+240

Dataset 1: 240Pu/239Pu and 241Pu/239Pu ratios determined by LA-ICPMS planchette re-analysis The LA-ICPMS planchette re-analysis also generated 240Pu/ Pu and 241Pu/239Pu atom ratios of paramount importance in fingerprinting Pu sources. While individual nuclear tests conducted at the NTS produced fallout with various specific Pu ratios, the integrated fallout 240Pu/239Pu and 241Pu/239Pu atom ratios at most sites in Utah are 0.032±0.003 and 0.000445 (no uncertainty provided, 241Pu reference date

239

11 10 Activity by Alpha Spectrometry (Bq/kg)

Fig. 4 Comparison of the 239+240 Pu activities for three soil and six attic dust samples determined by AS and LA-ICPMS of the same electroplated alpha sources. Included in the regression but not shown on this scale is a data point with x=24.48 and y=24.79

1980), respectively [3]. In contrast, the global fallout atom ratios for 240Pu/239Pu and 241Pu/239Pu in the northern hemisphere are 0.180±0.014 and 0.00224±0.00032, respectively, the latter decay corrected to 1 January 1997 [2]. These two points represent end-members expected to describe twocomponent mixing of these sources in environmental samples. It is essential to note that archived planchettes are affected by the beta decay of 241Pu (t1/2 =14.4 years) to 241Am (t1/2 = 432 years). The planchettes contained only 241Pu and no ingrown 241Am at the time of source preparation; however, 241 Pu decay will affect the archived sources, which thereafter contain a mixture of 241Pu and 241Am. Therefore, the LA-ICPMS is, in essence, measuring a (241Pu+241Am)/ 239 Pu atom ratio; this measurement closely reflects the actual 241Pu/239Pu ratio at the time of source preparation. While Pu and Am may have slightly different ablation behavior, and different ionization and oxide formation characteristics in the plasma, these effects are believed to be relatively small compared to the inherent precision with which (241Pu+241Am)/239Pu can actually be measured in most samples. It is noted that Pu and Am have very similar first ionization energies (6.07 and 6.00 eV, respectively) and similar melting and boiling points; hence, in a dry plasma LA-ICPMS, the ion production and transmission of 241 Pu+ and 241Am+ would be anticipated to be very similar. Therefore, we interpret the (241Pu+241Am)/239Pu measurement as representing 241Pu/239Pu when the sources were prepared (reference date 1 January 1997). Ratio results are plotted in Fig. 5; only samples that generated at least double the 241Pu signal produced by ablation of the procedural blank were used; samples producing less than this threshold did not produce reliable and interpretable results for 241Pu/239Pu. Despite relatively large uncertainties in the 241Pu/239Pu atom ratios, and the

9 8 7 6 5 4

y = 1.0057x - 0.099 R2 = 0.998

3

Error bars are 2 S.D.

2 1 0 0

1

2

3

4 5 6 7 Activity by LA-ICP-MS (Bq/kg)

8

9

10

11

528 0.0026

y = 0.013x - 0.0001

0.0024

R2 = 0.971

Global

0.0022 0.0020 0.0018 Enterprise, UT 241Pu/239Pu

Fig. 5 Mixing plot of 241Pu/239Pu vs. 240Pu/239Pu for attic dust and soil from Nevada and Utah (circles) compared with NTS and global fallout source terms (squares). The sources were prepared with an approximate date of 1 January 1997, and the measured (241Pu+241Am)/239Pu proxy atom ratio (see text) represents the 241Pu/239Pu present in the sample at that time. Error bars are ± two standard deviation


0.0016 0.0014 0.0012 0.0010 0.0008 0.0006 Beatty, NV 0.0004 Error bars are 2 S.D. 0.0002 NTS 0.0000 0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

240Pu/239Pu

fact that the samples were prepared with an acid leach and not total dissolution, the data clearly describe a mixing line governed by the NTS and global fallout end-members. There is no evidence of lack-of-fit or points outlying either end member; therefore, no distinguishable third component is likely to be present. The samples themselves, excluding the end-members, are described by a straight line (241Pu/239Pu= 0.013·[240Pu/239Pu] – 0.0001; r2 =0.971). Dataset 2: determination of 239+240Pu activities and 240Pu/239Pu in standard reference materials by SF-ICPMS analysis of solutions To validate the total dissolution approach, aliquots of four different standard reference materials with certified 239+240Pu activities were analyzed. Results for this comparison, along with the 240Pu/239Pu atom ratios obtained, are given in Table 2. These results show good agreement between the found and certified activities. Our 239+240Pu activity results for two separate aliquots of NIST 4353, 10.95±0.05 and 8.66±0.10 Bq kg−1, are the least consistent with the certified values. This is likely due to the presence of “hot” PuO2 in Rocky Flats Soil, as discussed on the certification sheet. The analyses of the other reference materials all are within 10% relative of the certified 239+240Pu activity. While none of these standard reference materials are certified for 240 Pu/239Pu atom ratios, the 240Pu/239Pu results are consistent with the sources of Pu in each sample. NIST 4353 (Rocky Flats Soil) contains mainly Pu from weapons manufacturing at the Rocky Flats site, and an uncertified value of 0.0553 is obtained from TIMS results presented in the certification sheet. Our 240Pu/239Pu results of 0.055±0.001

and 0.060±0.002 (1 SD) for two independently dissolved NIST 4353 aliquots are consistent with the TIMS data and expectations for weapons-grade Pu handled at Rocky Flats. The ICPMS result of 0.118±0.003 (1 SD) for NIST 4350b is somewhat higher than the uncertified value of 0.105 obtained from the TIMS data presented in the certification sheet. The 4350b material (Columbia River Sediment) is known to consist of a mixture of Hanford weapons-grade Pu and stratospheric fallout, consistent with both the SF-ICPMS and TIMS results. For NIST 4357 (Ocean Sediment) two independently dissolved aliquots produced 240Pu/239Pu results of 0.235±0.003 and 0.238±0.003 (1 SD); this material contains Pu mainly originating from the Sellafield facility, and our ratios are similar to values of 0.22–0.23

Table 2 Results for the determination of 239+240Pu activities and 240 Pu/239Pu atom ratios by total dissolution and solution analysis by SF-ICPMS (dataset 2) Material

239+240

Pu activity (Bq kg−1)

240

NIST 4350b NIST 4353

0.55±0.01 (0.51±0.03) 10.95±0.06, 8.66±0.1a (8.1±0.6) 11.14±0.03, 10.15±0.02a (10.4±0.2) 3.10±0.04, 3.03±0.03a (2.98±0.16)

0.118±0.004 0.055±0.001, 0.060±0.002 0.235±0.003, 0.238±0.003 0.193±0.003, 0.192±0.004

NIST 4357 IAEA 385

Pu/239Pu atom ratios

Certified activities are given in parentheses. The quoted uncertainties of the ICPMS results are one standard deviation of the experimental results (n=3–5) a Two separate sub-samples were independently prepared and analyzed


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observed by Taylor et al. [18] in multiple collector ICPMS studies of Sellafield-impacted sediments from other Irish Sea locations. Finally, the material IAEA 385 (Radionuclides in Irish Sea Sediment) has information values for activities of 239Pu and 240Pu [19]; these generate an approximate 240 Pu/239Pu atom ratio of 0.17. Our IAEA 385 results for 240 Pu/239Pu were 0.193±0.003 and 0.192±0.004 (1 SD). The collective SF-ICPMS results for 240Pu/239Pu atom ratios indicate that the technique can be confidently used in environmental studies, while underscoring the need for soils and sediments with actual certified Pu atom ratios. Dataset 2: comparison of 239+240Pu activities and 240Pu/239Pu atom ratios by the total dissolution procedure with dataset 1 (acid leaching) results A comparison between the LA-ICPMS results for dataset 1 (planchettes, acid-leaching sample preparation) and the total dissolution method is shown in Table 3. Note that this comparison involved the analysis of separate subsamples. In general terms, the agreement is relatively close for some samples and poor for others. For three samples (S6, H44, H29), the LA-ICPMS results are significantly more precise than the solution ICPMS results, and this may arise from the use of a relatively noisy ultrasonic nebulizer system in single collector solution ICPMS measurements. In several situations (e.g., H42, H39, H22, S4, H6) the differences in the activity results exceed the quoted uncertainties (one standard deviation) of each analysis. For these specific disparities, the 240Pu/239Pu atom ratios tend to be lower and the 239+240Pu activities higher via the total dissolution procedure. This finding is consistent with

Table 3 Plutonium atom ratios and activities for separate sub-samples of attic dust and soil from near the NTS as determined by the LAICPMS method (dataset 1) and total dissolution with solution analysis by SF-ICPMS (dataset 2) Sample IDa

H40 (UT) H42 (UT) S2 (NV) S6 (UT) H44 (UT) H39 (UT) H22 (NV) S4 (UT) H6 (NV) H29 (NV) H23 (NV)

240

Pu/239Pu atom ratio

Pu (Bq kg−1)

239+240

Dataset 1

Dataset 2

Dataset 1

Dataset 2

0.126±0.004 0.114±0.001 0.104±0.007 0.102±0.005 0.101±0.002 0.093±0.002 0.092±0.002 0.080±0.002 0.072±0.001 0.065±0.003 0.053±0.002

0.125±0.005 0.104±0.002 0.108±0.006 0.108±0.013 0.118±0.012 0.043±0.003 0.067±0.003 0.070±0.001 0.058±0.001 0.098±0.008 0.058±0.002

1.95±0.02 1.74±0.03 0.567±0.008 0.217±0.007 0.486±0.008 1.45±0.01 2.49±0.01 2.78±0.05 9.97±0.08 2.84±0.06 6.06±0.04

1.98±0.04 2.91±0.02 0.65±0.02 0.28±0.01 0.43±0.01 3.58±0.06 6.6±0.1 3.75±0.04 38.9±0.2 2.23±0.03 6.79±0.03

All uncertainties are ± one standard deviation UT Utah locations, NV Nevada locations, H attic dusts, S soils

a

Table 4 Summary statistics for 240Pu/239Pu atom ratio results (dataset 2) determined by total dissolution and solution analysis by SF-ICPMS Location

Number

Nevada Utah Arizona Colorado

10 20 6 3

Average 0.065 0.104 0.144 0.144

SD

Max

Min

0.022 0.045 0.035 0.005

0.108 0.187 0.192 0.149

0.041 0.030 0.096 0.141

the presence of spurious, high-activity particles containing NTS-derived Pu in a fused silicate matrix that cannot be dissolved completely by acid leaching. The refractory particles are, however, fully dissolved by the total dissolution method. Tamura [20] discusses the presence of vitreous, fused silicate Pu-containing particles at the NTS and its immediate vicinity. Hence, a total dissolution approach, which could also involve alpha source preparation and LA-ICPMS of planchettes, would be preferred in situations where refractory particles are expected. Nevertheless, in all cases, acid leaching does recover a portion of the NTS-derived Pu (presumably along with all stratospheric fallout Pu) and results in 240Pu/239Pu atom ratios that are interpreted as resulting from mixing of NTS and stratospheric fallout Pu. Dataset 2: geographic trends in in western US locations

240

Pu/239Pu fingerprints

Soil and dust samples were analyzed from both the JVC and MEK collections using the total dissolution procedure, and these samples are interpreted in terms of several groups by locations: Nevada, western Utah, northern Arizona, and the Colorado Front Range. Summary statistics for 240 Pu/239Pu in each group are given in Table 4. In general, the prevailing trend is that the lowest 240Pu/239Pu ratios are found in Nevada soils and dusts; Colorado Front Range soils exhibit the highest ratios, although even these (about 1,000 km northeast of NTS) exhibit 240Pu/239Pu atom ratios slightly lower than the stratospheric fallout range, indicating a small NTS contribution. Samples from western Utah and northern Arizona, in closer proximity to the NTS, exhibit lower but widely varying 240Pu/239Pu atom ratios. A detailed description and interpretation of all of these results is beyond the scope of this paper; nevertheless, our findings are consistent with previous studies of Pu in Utah soils [3], and previous fingerprinting studies based upon the activity ratio 137Cs/239+240Pu [15].

Conclusion Laser ablation-ICPMS analysis of archived AS sources provides a very simple, effective method of obtaining 239+240Pu

530

activity and Pu atom ratio data. Once these sources have been counted by AS, the LA-ICPMS method allows for an independent confirmation of 239+240Pu activity. The information obtained through Pu atom ratios is very effective for discriminating Pu sources; if more than one ratio is obtained, two-component mixing plots can be constructed with the results. As an archived alpha source is stored over time, 241Pu decays to 241Am, yet the LA-ICPMS measures the sum of 241Am and 241Pu ion intensities, a proxy for the 241 Pu/239Pu present when the source was prepared. Excellent agreement is obtained between 239+240Pu activities measured independently by AS and LA-ICPMS. Mixing plots of 241Pu/239Pu vs. 240Pu/239Pu for soils and dusts from Nevada and Utah indicate two-component mixing between NTS and global fallout Pu. The NTS end-member is well described by an average 240Pu/239Pu of 0.032±0.003; while specific devices detonated at NTS produced varying 240 Pu/ 239Pu, the cumulative NTS deposition is described adequately by this single composite value [3, 21]. A simple method of total dissolution, involving fewer steps than existing published procedures, has been developed. Total dissolution produced excellent agreement with certified 239+240Pu activities for four different standard reference materials. 240Pu/239Pu atom ratios resulting from the total dissolution procedure are in reasonable agreement with uncertified TIMS values, and conform to the Pu sources contained in the materials. A comparison between acid leaching and total dissolution demonstrated systematically higher 239+240Pu activities, and lower 240Pu/239Pu atom ratios for the latter; this results from the presence of varying amounts of insoluble Pu, originating from the NTS, in many of the samples. Subtle but definite contributions of NTS Pu are observed in soils from northern Arizona and the Colorado Front Range, based upon 240 Pu/239Pu atom ratios being systematically lower than global fallout. This work has illustrated two effective approaches for rapid determination of 239+240Pu and Pu atom ratios in environmental samples; these approaches are suited for large-scale fingerprinting studies of Pu in environmental media.

Anal Bioanal Chem (2008) 390:521–530 Acknowledgements Portions of this work were supported by Harry Reid Center for Environmental Studies (Chemistry Division) at the University of Nevada - Las Vegas. MEK acknowledges support from Intel Corp. for the donation of the VG PQII; NSF CHE-0118604 for the Axiom ICPMS; and NSF EAR-0125934 for support of portions of the experimental work.

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