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Rapid Sample Preparation for Alpha Spectroscopy with Ultrafiltration Membranes Christine Elizabeth Duval, Abenazer W. Darge, Cody L. Ruff, Timothy A DeVol, and Scott Michael Husson Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00135 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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Analytical Chemistry

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Rapid Sample Preparation for Alpha Spectroscopy with Ultrafiltration Membranes

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Christine E. Duval†§, Abenazer W. Darge†, Cody Ruff†, Timothy A. DeVol‡, and Scott M.

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Husson*†

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†

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Clemson, SC 29634 USA

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‡

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Computer Court, Anderson, SC 29625 USA

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§

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Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106

Department of Chemical and Biomolecular Engineering, Clemson University, 127 Earle Hall,

Department of Environmental Engineering and Earth Sciences, Clemson University, 342

Current address: Department of Chemical and Biomolecular Engineering, Case Western

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*Corresponding author. Current address: Department of Chemical and Biomolecular

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Engineering, Clemson University, 127 Earle Hall, Clemson SC 29634, USA. Tel: +1 (864) 656-

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4502, Fax: +1 (864)-656-0784. Email: [email protected]

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Abstract: This contribution describes a rapid, fieldable alpha spectroscopy sample preparation

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technique that minimizes consumables and decreases the nuclear forensics timeline. Functional

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ultrafiltration membranes are presented that selectively concentrate uranium directly from pH 6

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ground water and serve as the alpha spectroscopy substrate. Membranes were prepared by

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ultraviolet-grafting of uranium-selective polymer chains from the membrane surface. Membranes

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were characterized by Fourier-transform infrared spectroscopy before and after modification to

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support functionalization. Membrane performance was evaluated using uranium-233 or depleted

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uranium in both deionized and simulated ground water at pH 6. Functionalized membranes

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achieved peak energy resolutions of 31 ± 2 keV and recoveries of 81 ± 4% when prepared

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directly from pH 6 simulated ground water. For simulated ground water spiked with depleted

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uranium, baseline energy resolution was achieved for both isotopes (uranium-238 and uranium-

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234). The porous, uranium-selective substrate designs can process liters per hour of uranium-

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contaminated ground water using low-pressure (< 150 kPa) filtration and a 45 mm diameter

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membrane filter, leading to a high throughput, one-step concentration, purification, and sample

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mounting process.

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Keywords: EGMP, ethylene glycol methacrylate phosphate, nuclear forensics, phosphoric acid 2-

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hydroxy-ethyl methacrylate ester, phosphate ligands, uranium selective membrane

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Manipulation of special nuclear material (SNM) for the express purpose of assembling a weapon

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of mass destruction is likely to contaminate the environment with trace-levels of SNM (i.e.

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plutonium, uranium-233, and uranium enriched in the isotopes uranium-233 or uranium-235).

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Today, no fieldable technique exists for the direct isotopic analysis of environmental samples

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containing SNM. A fieldable technique should be portable, robust and minimize consumables.

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Presently, the limiting factor is not the isotopic detection method (alpha spectroscopy), but rather

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the unavailability of separation materials to prepare samples for analysis directly from neutral pH

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environmental waters.

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Traditional methods of sample preparation for alpha-spectroscopy require a two-step process:

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(1) sample isolation and purification followed by (2) sample mounting1. Methods of chemical

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separation include co-precipitation, purification by solid-phase separation agents, and liquid-

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liquid extraction (LLE)—all of which have drawbacks when considering a fieldable protocol.

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Co-precipitation is not necessarily a selective process and requires that the analyte is purified by

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removing interfering ions that can lower spectral resolution. Actinide selectivity can be achieved


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with LLE; however, it requires organic solvents, ligands, and acid while creating mixed

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radioactive-hazardous waste. Solid-phase separation agents, like extractive resins, produce less

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waste than LLE; however, commercial resins, like UTEVA® and Diphonix® (Eichrom

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Technologies) require solutions to be acidified to pH < 2 to achieve uranium-selectivity.

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After isolation and purification, samples are mounted onto a substrate for analysis. The state-

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of-the-art technique for sample mounting is electrodeposition. This technique can achieve a high

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resolution of ~20 keV in the pulse-height spectrum2; however, mounting takes 1-2 h and requires

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an expensive platinum anode in an electrolytic cell. Other sample mounting methods for lower

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resolution needs are evaporation, which yields resolutions of 40-70 keV3, and a combination of

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precipitation and filtration, which yields resolutions of 75-100 keV4. Mounting purified samples

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by precipitation with neodymium fluoride yields resolutions comparable to electrodeposition5.

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Recent studies have focused on the use of selective polymer films as alpha spectroscopy

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substrates to avoid the expense associated with electrodeposition6-11. These substrates combine

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the steps of chemical separation and sample mounting while overcoming the poor resolution

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associated with non-selective films used for complex sample matrices12. Pantchev et al.7

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functionalized nylon-6,6 membranes with amidoxime ligands to concentrate uranium from

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potable water. Spectral resolutions of 40 keV were achieved, but uranium was loaded on the

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membranes through batch-sorption (not by filtration), which required a long 24-h equilibration

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time. Surbeck et al.9 impregnated spin-coated polyacrylonitrile films with MnO2 for the

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concentration of radium from potable water. After a 20-h equilibration time, resolutions of 40

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keV were achieved in the alpha spectrum. Gonzales et al.8 developed polymer films containing

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the commercial Dipex® ligand for the simultaneous separation and mounting of americium and

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plutonium from urine samples. Despite excellent resolutions of 20-30 keV and a shorter



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equilibration time (1 h) than Pantchev and Surbeck, sample preparation required the acidification

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of urine to pH 1 and addition of sodium nitrite. Paul et al.13 functionalized polyether sulfone

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(PES) membranes with a thin phosphonate-sulfate bifunctional layer. Uranium and plutonium

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were loaded from 3-4 M nitric acid solutions and achieved high resolutions of 20-35 keV from

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dissolved spent fuel. Membranes were loaded with activity in a batch system (not by filtration)

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and required an equilibration time of 1 h. While selective polymer films have proven effective in

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achieving high resolutions, the current designs are not ideal for rapid, field-based measurements

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due to the need for consumable chemicals and/or long isolation, purification and mounting times.

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The objective of this work was to develop a rapid, fieldable, alpha spectroscopy sample

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preparation technique that minimizes consumables and decreases the nuclear forensics timeline.

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In this work, we present functional ultrafiltration membranes that (1) selectively and rapidly

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concentrate uranium directly from neutral pH ground water and (2) serve as the alpha

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spectroscopy substrate. Uranium selectivity was introduced to the membrane through UV-

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grafting an ethylene glycol methacrylate phosphate-based polymer. Membranes were evaluated

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for permeability and peak energy resolution in the pulse height spectrum. The new substrate

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design allows for the filtration of uranium-contaminated ground water through the uranium-

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binding membrane material, leading to a high throughput separation and sample mounting

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process.

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EXPERIMENTAL SECTION Materials. The following reagents were used as received: 2,2’-Azobis(2-methylpropionitrile)

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(AIBN, 98%, Aldrich), ethanol (reagent grade, Sigma Aldrich), nitric acid (90%, Fisher

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Scientific), N,N’-methylenebis(acrylamide) (N-MBA, 99%, Sigma Aldrich), ethylene glycol


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methacrylate phosphate (EGMP, 90%, Aldrich), sodium hydroxide (NaOH, Sigma Aldrich),

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UltimaGold AB liquid scintillation cocktail (PerkinElmer), uranium-233 (Eckert & Ziegler), and

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uranyl nitrate hexahydrate (depleted uranium, PerkinElmer). Water was distilled and then

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deionized with a SuperQ Water System (DDI water, Millipore, Molsheim, France).

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Membrane preparation by UV-initiated polymerization. Poly[(ether sulfone)-graft-

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(ethylene glycol methacrylate phosphate)-graft-( N,N’-methylenebis(acrylamide))] membranes

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(UVMs) were synthesized by UV-initiated free radical polymerization according to Scheme 1.

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Biomax® PES membranes of varying molecular weight cut-offs (MWCO) were purchased from

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MilliporeSigma and rinsed with ethanol to remove pore fillers prior to functionalization. The

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polymerization solution was prepared in a 20 mL glass vial by dissolving 0.4 g EGMP (uranium-

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binding ligand), 24 mg AIBN (photo-initiator) and 0.01 mg of N-MBA (cross-linker) in 6 mL of

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ethanol. Membranes were placed in a petri dish with the active layer (shiny side) face up and 2

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mL of the polymerization solution was pipetted onto each membrane. A second petri dish was

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placed directly on top of the membrane. Care was taken to assure that there were no air bubbles

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between the membrane surface and the second petri dish. The membrane-containing petri dishes

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were positioned underneath an 8 W UVLS-28 EL Series UV lamp (UVP, Upland, CA).

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Membranes were irradiated with 365 nm UV light for 10 min at a distance of 7.6 cm from the

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UV lamp. After the reaction, the UVMs were rinsed with DDI water, submerged in a 50% (v/v)

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mixture of ethanol and DDI water, and then placed in a reciprocal shaking bath (Precision

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Scientific Inc., Winchester, IL) at 100 RPM and 25oC overnight to remove any residual reactants

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and/or physically adsorbed polymer from the membrane. Membranes were rinsed with DDI

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water then stored in a 50% (v/v) mixture of ethanol and DDI water between synthesis and use.



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Scheme 1. UV-grafting of poly(EGMP)-co-(N-MBA) from a PES membrane.

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Membrane characterization by infrared spectroscopy. Attenuated total reflectance

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Fourier-transform infrared spectroscopy (ATR-FTIR) was used to characterize the membranes

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before and after functionalization. Samples were analyzed using a nitrogen purged Nicolet Nexus

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870 FTIR Spectrometer (Thermo Scientific, USA) with a zinc sulfide ATR crystal for 64 scans at

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a resolution of 4 cm-1. Data were collected and analyzed using Omnic 8.3.103 software (Thermo

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Scientific).

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Direct-flow water flux measurements. Pure water flux experiments for pristine PES

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membranes and UVMs were conducted according to a previously described procedure15.

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Experiments were performed with pressures from 50 to 138 kPa using a 50 mL Amicon direct

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flow ultrafiltration cell (Amicon Biosepartaions, Jaffrey, NH) connected to an air cylinder.

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Membranes were compressed at 138 kPa for 15 min before data collection. Permeate fractions

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were collected in 30 s intervals and weighed to determine the mass flowrate through the 6 ACS Paragon Plus Environment

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membrane. Flux data were used to calculate the pure water permeability coefficient of the

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membranes. Permeability coefficients are reported as the average from triplicate measurements

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of different membranes.

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Simultaneous purification and mounting for alpha spectroscopy. Samples were prepared

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for alpha spectroscopy by mounting the membranes in a 50 mL Amicon ultrafiltration cell and

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filtering the uranium-containing solutions at 138 kPa, shown in Scheme 2.

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Scheme 2. Alpha spectroscopy substrates were prepared in a 50-mL ultrafiltration cell (left),

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removed from the cell (center) and mounted in a custom sample holder (right).

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Alpha spectroscopy. Isotopic analysis was performed by alpha spectroscopy using the

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ultrafiltration membranes as the substrate for analysis. Immediately after sample purification and

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mounting, membranes were placed on a stainless-steel plate, immobilized with a custom 3-D

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printed sample holder and inserted in a Canberra Model 7401 alpha spectrometer (Canberra

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Industries, Inc., Oak Ridge, TN). The 3-D printed sample holder positioned the sample under the

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detector and prevented curling of the membrane edges during measurement. Samples were

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counted under vacuum at a distance of 9 mm from the detector surface. Counting time was 1 h

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for uranium-233 and 48 h for depleted uranium samples. The vacuum chamber pressure for all

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measurements was 500 LMH/bar), demonstrating the potential for

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high-throughput analysis. Peak energy resolutions were found to be independent of the MWCO

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of the membrane and the sample matrix for the conditions tested, indicating that still higher

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MWCO membranes could be used to achieve higher permeabilities (faster processing time). 16 ACS Paragon Plus Environment

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Experiments with depleted uranium in simulated ground water yielded resolutions for uranium-

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238 and uranium-234 of 27 ± 5 keV and 26 ± 3 keV, with fully resolved peaks, as needed for the

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isotopic analysis of uranium in nuclear forensics applications. The results of this research offer

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the first evidence of a high-throughput sample preparation method for isotopic analysis with

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alpha spectroscopy that directly concentrates uranium from pH neutral ground water.

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Furthermore, they lay the groundwork for the development of ultrafiltration alpha spectroscopy

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substrates that target other radioisotopes of interest.

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ASSOCIATED CONTENT

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Supporting information

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The Supporting Information is available free of charge on the ACS Publications website.

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Example water flux data used to calculate permeability coefficients, alpha spectra of

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UVMs with MWCO 30 kDa and 50 kDa exposed to 10 Bq of uranium-233, and pulse-

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height spectrum of unmodified 50 kDa PES membrane after filtering 10 mL of 500 Bq L-

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uranium at pH 6.

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AUTHOR INFORMATION

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Corresponding Author

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*Email: [email protected]. Tel: +1 (864) 656-4502, Fax: +1 (864)-656-0784.

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ACKNOWLEDGMENTS

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This work was supported by the Defense Threat Reduction Agency, Basic Research Award

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#HDTRA1-16-1-0016, to Clemson University.



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REFERENCES

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