Radionuclide Sensors Based on Chemically Selective Scintillating

Oct 28, 1999 - Geochemistry Group, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington ... Applied Geology and Geochemistry Grou...
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Anal. Chem. 1999, 71, 5420-5429

Radionuclide Sensors Based on Chemically Selective Scintillating Microspheres: Renewable Column Sensor for Analysis of 99Tc in Water Oleg B. Egorov,† Sandra K. Fiskum,‡ Matthew J. O’Hara,§ and Jay W. Grate*,†

Environmental Molecular Sciences Laboratory, Radiochemical Processing Laboratory, and Applied Geology and Geochemistry Group, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352

A method for chemically selective radiometric sensing of non-γ-emitting radionuclides in solution is described. Using scintillating microspheres with selective radionuclide uptake properties, radiochemical separation and radiometric detection steps are integrated within a sensor device. These microspheres are loaded into a renewable minicolumn that serves to capture, preconcentrate, and separate radionuclides. The preconcentrating minicolumn also localizes and retains radionuclides within a detector of well-defined geometry and emits a photometric signal. The sensor material in the column can either be regenerated with eluent chemistries or be renewed by fluidic replacement of the beads. The latter method allows the use of materials that bind analytes irreversibly or are unstable under regeneration conditions. Radionuclideselective scintillating microspheres were prepared by coimmobilization of scintillating fluors and selective organic extractants within the pores of an inert polymeric support. Preparation and characterization of microspheres, and their use for selective quantitative sensing of 99Tc(VII), is described in detail. A sensor-based procedure for 99Tc(VII) analysis was developed and successfully applied toward the determination of 99Tc(VII) in groundwater samples from the Hanford site, using standard addition techniques for quantification. Using a 50mL sample volume and signal accumulation time of 30 min, the detection limit for 99Tc(VII) was 0.37 dpm/mL (9.8 pg/mL). The production of nuclear weapons materials and storage of nuclear wastes at U. S. Department of Energy (DOE) sites has led to the radioactive contamination of soil and groundwater by several routes of intentional and unintentional release. Radionuclide measurement techniques represent an important element in support of nuclear site characterization and remediation. Monitors are required for surface waters and groundwaters, as well as process waters associated with environmental remediation and nuclear waste processing. Thus far, radionuclide contaminants are measured by sampling and costly radiochemical procedures involving manual separations and radioactivity counting performed †

Environmental Molecular Sciences Laboratory. Radiochemical Processing Laboratory. § Applied Geology and Geochemistry Group. ‡

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Table 1. Required Detection Limits for Selected Radionuclide Contaminants analyte 90Sra 99Tca 238Ub 239Pua 241Ama

radioactivity, Bq/mL 10-4

2.96 × 3.00 × 10-2 2.00 × 10-4 4.00 × 10-5 4.00 × 10-5

mass concn, µg/mL 5.74 × 10-11 4.77 × 10-5 2.00 × 10-2 1.93 × 10-8 3.15 × 10-10

a Maximum contaminant level standards to yield an annual dose equivalent of 4 mrem/year. b Drinking water standard.

in centralized laboratories. Detection limit requirements for selected radionuclides in water are presented in Table 1.1 There is a critical lack of radionuclide sensor methodologies suitable for determination of non-γ-emitting radionuclide contaminants such as 90Sr, 99Tc, and Pu, as indicated by comprehensive reviews of environmental sensors.2-4 Radioactivity detection is the method of choice for a sensor because of the ultratrace detection limit requirements for water monitoring and the need to distinguish radioactive isotopes from natural stable isotopes of the same element. However, direct radiometric detection of R- and β-emitters in solution presents a significant challenge for implementation in a sensor device. First, R- and β-radiation is characterized by short ranges and rapid energy dispersion in liquid media. For example, the maximum range of β-particles emitted by 99Tc (Eβ, max ) 294 keV) is ∼750 µm in water.5 Thus, radioactive species are detected only in close proximity to a radioactivity detector or scintillating transduction material. Second, because of the limited energy resolution of conventional scintillating detection techniques and the continuous (1) Hartman, M. J., Dresel, P. E., Eds., Hanford Site Groundwater Monitoring for Fiscal Year 1997, Pacific Northwest National Laboratory, PNNL-11793 UC-402, 403, 702, 1998. (2) Koglin, E. N.; Poziomek, E. J.; Kram, M. L. In Handbook of Vadose Zone Characterization & Monitoring; Wilson, L. G., Everett, L. G., Cullen, S. J., Eds.; Lewis Publishers: Ann Arbor, MI, 1994. (3) Roach, W. H. Literature Search, Review, and Compilation of Data for Chemical and Radiochemical Sensors: Task 2 Report, Advanced Sciences, Inc. for Hazardous Waste Remedial Actions Program, DOE/HWP-133, 1993. (4) Roach, W. H., Literature Search, Review, and Compilation of Data for Chemical and Radiochemical Sensors: Task 1 Report, Advanced Sciences, Inc. for Hazardous Waste Remedial Actions Program, DOE/HWP-130, 1993. (5) Marion, J. B.; Young, F. C. Nuclear Reaction Analysis. Graphs and Tables; John Wiley & Sons: New York, 1968. 10.1021/ac990735q CCC: $18.00

© 1999 American Chemical Society Published on Web 10/28/1999

nature of the β-particle’s spectrum, other radioactive species present in the sample may interfere with the analysis if chemical separations are not performed.6,7 Third, analyte preconcentration is required for reliable detection to desired ultratrace levels within a realistic signal accumulation time. Therefore, a sensor device based on radioactivity detection must provide high levels of preconcentration, it must separate radionuclides of interest from interfering species, it must spatially localize the radionuclides in close proximity to a scintillating material within a detector, and it must retain the radionuclides to accumulate signal. To be useful as a monitor, it must also be reversible, regenerable, or renewable for additional measurements. Recently, analytical methodology based on the use of sequential injection methods for the fluidic manipulation and observation of functionalized microspheres was introduced by Ruzicka and co-workers.8-14 The new technique, referred to recently as bead injection,8 is based on the introduction of a well-defined volume of a bead suspension into a flow system with subsequent temporary capture of the beads in the form of a renewable reactive microbed.8 Changes in the bed properties (e.g., reflectance, absorbance, fluorescence, etc.) induced upon the interaction with the sample solution serve as the basis of an analytical measurement. These changes can be monitored either directly on the reactive microbed (renewable sensing)8,14 or downstream in the effluent liquid phase (renewable chromatography).8,15 After completion of the measurement, the beads are released and replaced with a fresh sample of bead suspension, providing a renewed surface for separation and sensing. Thus far, the applications of bead-based renewable sensing have been directed primarily at biochemical and chemical assays.8 The renewable surface-sensing approach is potentially advantageous for sensing R- and β-emitting radionuclides in aqueous solutions. It provides a means to collect and concentrate analyte on bead surfaces within a detector of specific geometry. We have already demonstrated that beads with selective radionuclide uptake properties can be used in a renewable column format for automated radionuclide separations.15 Chemical selectivity in the separation format was obtained by using macroreticular beads impregnated with selective or semiselective organic extractants. Extraction chromatographic radiochemical separations16-20 were then automated using separation-optimized sequential injection (6) Handbook of Radioactivity Analysis; L′Annunziata, M. F., Ed.; Academic Press: San Diego, 1998. (7) Grate, J. W.; Egorov, O. B. Anal. Chem. 1998, 70, 779A-788A. (8) Ruzicka, J.; Scampavia, L. Anal. Chem. 1999, 71, 257A-263A. (9) Dockendorff, B.; Holman, D. A.; Christian, G. D.; Ruzicka, J. Anal. Commun. 1998, 35, 357-359. (10) Holman, D. A.; Christian, G. D.; Ruzicka, J. Anal. Chem. 1997, 69, 17631765. (11) Willumsen, B.; Christian, G.; Ruzicka, J. Anal.Chem. 1997, 68, 3482-3489. (12) Ruzicka, J.; Ivaska, A. Anal. Chem. 1997, 69, 5024-5030. (13) Mayer, M.; Ruzicka, J. Anal. Chem. 1996, 68, 3808-3814. (14) Egorov, O.; Ruzicka, J. Analyst 1995, 120, 1959-1962. (15) Egorov, O.; O’Hara, M. J.; Grate, J. W.; Ruzicka, J. Anal. Chem. 1998, 71, 345-352. (16) Horwitz, E. P.; Chiarizia, R.; Dietz, M. L.; Diamond, H.; Nelson, D. M. Anal. Chim. Acta 1993, 281, 361-372. (17) Horwitz, E. P.; Chiarizia, R.; Dietz, M. L. Solvent Extr. Ion Exch. 1992, 10, 313-336. (18) Horwitz, E. P.; Dietz, M. L.; Chiarizia, R.; Diamond, H.; Maxwell, S. L.; Nelson, M. R. Anal. Chim. Acta 1995, 310, 63-78. (19) Dietz, M. L.; Horwitz, E. P. LC-GC 1993, 11, 424-436. (20) Maxwell, S. L. Radioact. Radiochem. 1997, 8, 36-44.

Figure 1. Conceptual diagram of the renewable radionuclide sensor column placed within the observation field of two photomultiplier tubes (PMT). Dimensions are not to scale.

techniques for fluid handling.7,21-23 Separated radionuclides were detected downstream by on-line liquid scintillation detection or by other counting methods after fraction collection. Automated extraction chromatographic separations of species such as 90Sr, 99Tc, Pu, and Am have been demonstrated15,21-26 and were recently reviewed.7 The renewable radionuclide separation column could be converted to a renewable radionuclide sensor if a transduction mechanism could be integrated with the column or column material. In this paper, we describe a new sensing approach that integrates radiochemical separations and detection in a single functional unit using scintillating microspheres with selective radionuclide uptake properties. These microspheres are delivered to a minicolumn placed between two photomultiplier (PMT) tubes for light collection, as shown in Figure 1. Sensing based on selective retention of a single radionuclide is illustrated using chemistry for 99Tc(VII). The principle of using elution chemistries to separate co-retained radionuclides is demonstrated in the detection of 90Sr and 90Y. Preparation and characterization of selective scintillating microspheres and their use for quantitative sensing of 99Tc(VII) is described in detail. Technetium is a significant radioactive contaminant at the U.S. DOE sites that was generated from the thermal fission of 235U with a high production yield of 6%.27,28 It is a pure β-emitter with a half-life of 2.13 × 105 years and high mobility in the environment. For these reasons, the development of improved analytical methodologies for this radionuclide is of significant practical value.1 Scintillating microspheres have found applications in a scintillation proximity assay (SPA), a widely used technique for studies of binding interactions of biologically relevant compounds.29,30 This technique uses radiolabeled molecules of high specific activity (21) Grate, J. W.; Egorov, O. B.; Fiskum, S. K. Analyst 1999, 124, 1143-1150. (22) Grate, J. W.; Fadeff, S. K.; Egorov, O. Analyst 1999, 124, 203-210. (23) Egorov, O.; O’Hara, M. J.; Ruzicka, J.; Grate, J. W. Anal. Chem. 1998, 70, 977-984. (24) Egorov, O.; Grate, J. W.; Ruzicka, J. J. Radioanal. Nucl. Chem. 1998, 234, 231-235. (25) Grate, J. W.; Egorov, O. Anal. Chem. 1998, 70, 3920-3929. (26) Grate, J. W.; Strebin, R. S.; Janata, J.; Egorov, O.; Ruzicka, J. Anal. Chem. 1996, 68, 333-340. (27) Browne, E.; Firestone, R. B. Table of Radioactive Isotopes; John Wiley & Sons: New York, 1986. (28) Lieser, K. H. Radiochim. Acta 1993, 63, 5-8. (29) Cook, N. D. Drug Discovery Today 1996, 1, 287-294. (30) L’Annunziata, M. F. In Handbook of Radioactivity Analysis; L’Annunziata, M. F., Ed.; Academic Press: San Diego, 1998; pp 556-565.

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and is designed to discriminate between bound and unbound molecules. The requirements of the SPA as an analytical technique are substantially different from those of environmental radiochemical analysis applications. Preparation of polymeric ionexchange beads with scintillating properties was reported 30 years ago, but the collection of radionuclides from the sample and subsequent counting were performed manually in separate steps.31 This work resulted in practically no follow-up work relevant to radiochemical analysis.32 Recently, a flow cell containing scintillating glass beads derivatized with chelating ligand for the extraction and detection of radionuclides was described in a proceedings paper by DeVol and co-workers.33 These investigators subsequently reported impregnation of scintillating polymer beads with extractants for radionuclide sensing.34 The latter materials work by the same mechanism as those that we shall describe and are suitable for analytical characterization and implementation in renewable minicolumn sensors. EXPERIMENTAL SECTION Radionuclide Sensor Instrument. The sensor column flow cell (Figure 1), with an internal channel for the sensor column and ports for fluid inlet, fluid outlet, slurry delivery, and slurry waste, was fabricated from a Plexiglas block (3 cm wide × 4 cm long × 1 cm thick). Ports were machined with 1/4-28-threads for fluidic connections. The central sensor column channel was 3 cm long × 3 mm i.d. (calculated volume 212 µL). The slurry delivery and slurry waste channels were 11 mm long × 1.6 mm i.d. A 5-mm-diameter, 25-µm-pore size polypropylene frit disk (Alltech Associates, Inc., Deerfield, IL) was used to retain scintillating sorbent beads within the sensor flow cell. The frit disk was placed directly into the threaded outlet of the column bed channel (Figure 1) and held in place with a flat-bottom fitting. The sensor flow cell was placed into an aluminum flow cell holder and held in place with the fittings (Upchurch) used for fluidic connections. This assembly was positioned between the two PMTs of a Packard flow-through scintillation counter. An automated fluid handling system shown in Figure 2 was configured with a 24 000-step digital syringe pump (syringe volume 10 mL) (Alitea USA, Medina WA), a 10-port multiposition valve (Valco Instrument Co., Houston, TX), and two four-port twoway valves. An auxiliary two-way valve (Valco) and a 25-mL syringe pump (Alitea USA) were added to the system (as shown in the figure) to deliver large sample volumes in the analysis of groundwater. The SI system holding coil was constructed from 1.6-mm-i.d. FEP Teflon tubing (Upchurch Scientific, Oak Harbor, WA) of 6-m length (calculated volume 12 mL). All transport and reagent lines were made of 0.8-mm-i.d. FEP Teflon tubing (Upchurch). To prevent stray light from entering the sensor flow cell assembly, black FEP Teflon tubing (Upchurch) was used to construct inlet and outlet lines of the sensor flow cell. The sensor instrument was controlled via a serial cable using FIALab software (Alitea USA) running on a laptop PC. (31) Heimbuch, A. M.; Gee, H. Y.; DeHaan, A. J.; Leventhall, L. Radioisotope Sample Measurement Techniques in Medicine and Biology, International Atomic Energy Agency Symposium, Vienna, May 24-28, 1965. (32) Li, M.; Schlenoff, J. B. Anal. Chem. 1994, 66, 824-829. (33) DeVol, T. A.; Roane, J. E.; Harvey, J. T. IEEE Nucl. Sci. Symp. Conf. Rec. 1997, 1, 415-419. (34) DeVol, T. A.; Roane, J. E.; Williamson, J. M.; Duffey, J. M.; Harvey, J. T. 44th Annual Conference on Bioassay, Analytical, and Environmental Radiochemistry, Albuquerque, NM, November 15-20, 1998.

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Figure 2. Schematic diagram of the fluidic sensor instrument for analysis of 99Tc(VII) in water: C, carrier (0.02 M HNO3); SP, syringe pump (10-mL syringe); HC, holding coil; S, sample line; R, reagent line; W, waste line; MPV, multiposition valve; ATPV, auxiliary twoposition valve; ASP, auxiliary syringe pump (25-mL syringe); SL, solution line; BDL, bead delivery line; BWL, bead waste line, SWL, solution waste line; BS, bead slurry; TPV A, two-position valve A; TPV B, two-position valve B; FD, frit disk; SFC, sensor flow cell.

Radioactivity Measurements. A Radiomatic 515A (Packard Instrument Co., Meriden, CT) on-line flow-through scintillation detector was configured with the sensor flow cell assembly positioned in the place of a conventional flow cell. The detector integration time (time to accumulate counts for each data point reported) was 6 s. Off-line scintillation measurements were performed with a Tri Carb 2550 TR/AB liquid scintillation spectrometer (Packard Instrument Co.). Static measurements using selective scintillating beads were performed according to the technique described by Ross.35 A 3-mm-i.d. glass capillary was flame-sealed at one end and glued to the center of a 20-mL liquid scintillation vial. Bead suspension and an aliquot of a radioactive standard were introduced into the glass capillary and mixed by resuspending the bead slurry. Water was added to the space between the capillary and the vial, and the vial was capped and gently centrifuged prior to counting. Materials and Standards. Primary scintillator fluor 2,5diphenyloxazole (PPO) was obtained from Sigma (St. Louis, MO). Secondary scintillator fluor 1,4-bis(2-methylstyryl)benzene (bisMSB), liquid organic anion-exchange extractant tricaprylylmethylammonium chloride (Aliquat-336), and bis(2-ethylhexyl)phosphoric acid (HDEHP) extractant were from Aldrich (Milwaukee, WI). All reagents were used as received. Acrylic ester macroporous polymer beads (20-50-µm particle size) were obtained from Eichrom Industries, Inc. (Darien, IL). Ultima Gold (Packard) cocktail was used for static liquid scintillation measurements. All other chemicals used were of analytical-reagent grade. Deionized (DI) water (18 MW/cm) was obtained from a Barnstead E-pure Series 582 water purification system (Thermolyne Corp., Dubuque, IA) and was used for all dilutions. Solutions of 99Tc(VII), 90Sr/ (35) Ross, H. H. In Liquid Scintillation Counting and Organic Scintillators; Ross, H. H., Noakes, J. E., Spaulding, J. D., Eds.; Lewis Publishers: Chelsea, MI, 1991; pp 195-209.

90Y,

and 137Cs were prepared by dilution of the certified standard stock solutions obtained from an in-house standards laboratory. Preparation of the Selective Scintillating Microspheres. Acrylic ester polymer support beads (nominally 1 g) were thoroughly mixed with the benzene solution (nominally 4 mL) containing the required amounts of dissolved primary scintillator (PPO), secondary scintillator (bis-MSB), and organic extractant. The solvent was allowed to evaporate under continuous vigorous mixing, yielding polymeric beads coimpregnated with the known amounts of scintillating fluors and organic extractant. The beads were air-dried, suspended in DI water, filtered using Whatman 41 paper, and rinsed with additional water. The filtered beads were resuspended in DI water and stored in the dark for 24 h prior to use to eliminate sporadic chemoluminescence signals often observed when freshly prepared sensor beads are used. The following reagent loadings (calculated as percent ratio of the organic reagent weight to the weight of the polymeric support beads) were used in this study: PPO 20%, bis-MSB 2%, and Aliquat-336 15%. The density of the reagent-loaded beads measured via water displacement17 was 1.17 g/mL. The sensor bed density was 0.34 g/mL (gravimetric measurement). Fluor and extractant loadings for HDEHP-impregnated beads, used for capturing Sr and Y, were the same. The free column volume (FCV) of the sensor column bed was calculated as the difference between the bed volume and the volume of the sensor beads in the bed;17 for the pertechnetate-selective scintillating beads, the FCV was measured to be 0.71 mL/mL bed. Distribution Ratio/Capacity Factor Measurement. Test solutions containing known quantities of 99Tc(VII) activity and known amounts of sensor beads were equilibrated along with the blank solutions for at least 2 h under continuous mixing in an orbital shaker. (Blank solutions were identical to the test solutions but with no sensor material added.) Following the equilibration, the test and blank solutions were passed through a 0.2-µm filter and analyzed by liquid scintillation spectrometry. Weight distribution ratios, Dw, were calculated according to the formula Dw ) ((A0 - As)/W)/(As/V), where A0 is the activity of the blank solution after equilibration, As is the activity of the test solution after equilibration with beads, W is the weight of the beads (g), and V is the volume of the equilibrated solution (mL). Capacity factors, k′, were calculated as k′ ) ADw(Vs/Vm), where A is the conversion factor from Dw to volume distribution ratio (8.0 in this study) and Vs/Vm is the stationary/mobile phase volume ratio (0.06 in this study). Renewable Column Packing Procedure. To expedite experimentation, bead slurries were delivered to the column manually. Two-position valve A in Figure 2 was positioned to connect the slurry syringe to the flow cell via the bead delivery line (BDL). Two-position valve B was positioned to connect the solution waste line (SWL) to the waste, with the bead waste line (BWL) connected to the capped port. To ensure uniform column packing and prevent beads from entering the solution line (SL), the carrier solution (0.02 M HNO3) was pumped at 3 mL/min flow rate simultaneously with the delivery of the bead slurry. After the column channel was entirely filled with the packed bed (calculated bed volume 212 µL), two-position valve A was switched to connect the bead slurry delivery line to the capped port.

Table 2. Determination of Samplesa

99Tc

in Hanford Groundwater

sample no.

sample matrix

sensor result, dpm/mL

manual result, dpm/mL

effective effic, %

GW1 GW2 GW3 GW4 GW5 GW6

0.02 M HNO3 0.02 M HNO3 0.01 M HCl 0.01 M HCl 0.01 M HCl 0.01 M HCl

5.94 ( 0.34 14.93 ( 0.57 24.65 ( 1.01 8.50 ( 0.51 7.94 ( 0.61 5.80 ( 0.42

6.13 ( 0.67 12.84 ( 0.72 25.53 ( 2.8 8.37 ( 0.98 6.68 ( 0.75 5.48 ( 0.62

44 ( 3 40 ( 2 34 ( 2 35 ( 2 25 ( 2 30 ( 2

a

Uncertainties are propagated 2σ counting errors.

To dispose of the sensor column, two-position valve B was switched to connect the slurry waste line to an open waste port, with the sample waste line connected to the capped port. The sensor material was expelled by delivering 6 mL of carrier solution at 8 mL/min flow rate. Additional wash steps with flow reversal and air segment delivery were used to ensure complete removal of the beads from the sensor flow cell.15 Groundwater Samples and Reference Methods. Groundwater samples were obtained from the sampling wells located at the U.S. DOE Hanford site. Water samples were filtered and acidified to pH ∼2 with hydrochloric or nitric acids (as indicated in Table 2) at the sampling location. Aliquots of the filtered acidified samples were analyzed using newly developed sensor procedures. Sample aliquots were also analyzed by standard manual radiochemical procedures1 in an independent laboratory. The manual 99Tc analysis procedure involved an initial sample treatment using nitric acid and H2O2 to ensure that all of the 99Tc was present as pertechnetate. Radiochemical separation involved a sequence of hydroxide and carbonate precipitation steps, followed by 99Tc(VII) purification using a column of a strongly basic anion-exchange resin. The radiochemical determination was performed by liquid scintillation counting. CAUTION! Radioactive solutions used in this work present radiological hazards. RESULTS AND DISCUSSION Pertechnetate-Selective Scintillating Microspheres. For the development of a renewable radionuclide-sensing technique, we required sensor materials in bead form that combined chemically selective radionuclide uptake with scintillating properties. Our general approach was to convert macroreticular polymer beads that are suitable for impregnation with extractant to scintillating beads by co-depositing scintillating fluors with the extractant. A primary scintillator fluor (PPO) and a fluorescencewavelength-shifting fluor (bis-MSB) were utilized in this study. These reagents are inexpensive and have good light output characteristics and good solubility in aromatic solvents. The solvent evaporation technique described in Experimental Section was straightforward to implement, provided precise control over the amounts of reagents immobilized, and yielded sensor materials with effective, reproducible, sorptive/scintillating properties. In addition, the resulting reagent-loaded polyacrylic beads were readily suspendable in water without agglomeration. This is an important requirement for reliable fluidic renewal of the sensing column. Other techniques for coimmobilization of scintillator fluors and extractants within the polymeric beads (e.g., bead swelling) Analytical Chemistry, Vol. 71, No. 23, December 1, 1999

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appear possible35,36 but were not investigated in this initial proofof-principle study. We selected Aliquat-336, a long-chain quaternary ammonium ion extractant, for selective pertechnetate capture. Extraction chromatographic materials based on the immobilized Aliquat-336 (Eichrom TEVA-resin) have been investigated in detail by Horwitz et al.18 Pertechnetate ion is strongly and nearly selectively extracted by the immobilized Aliquat-336 organic phase from neutral or dilute acid solutions. Efficient separation of 99Tc(VII) from interfering radionuclides using TEVA-resin has been described previously for manual37-40 and automated procedures.15,23,41 A sample pretreatment method to ensure that all Tc is oxidized to the Tc(VII) state prior to TEVA-resin column separations has also been described.23 In addition, TEVA-resin has been used in a renewable column format.15 On this basis, this separation chemistry is well suited for the development of 99Tc(VII)-selective sensor material. Several basic parameters were investigated in the preparation of the pertechnetate-selective microspheres. Scintillation efficiency was found to increase with increasing amounts of PPO loading up to ∼20% weight. The use of the bis-MSB wavelength-shifting fluor (optimal loading ∼2%) was essential in providing good scintillation efficiency, which was typically reduced by a factor of 2 in the absence of bis-MSB. We observed no improvement in scintillation efficiency when naphthalene was added to the scintillating fluor mixture (up to 20% loadings tested). The scintillation efficiency was not affected by the amounts of Aliquat-336 extractant loaded into the beads (10-40% loadings were tested). We used 15% extractant loading in all further studies to maintain the overall reagent loading under 40% (as typically used in extraction chromatography with these types of polymeric supports).16-19,42 Pertechnetate uptake and scintillating properties of the sensor microspheres are illustrated in Figure 3 and Figure 4. Figure 3 shows the plot of the measured pertechnetate capacity factor, k′, as a function of nitric acid concentration. These results are in agreement with the previous studies of the Aliquat-336-based extraction chromatographic material.18 Strong pertechnetate uptake at low acidities (k′ > 104 in 0.01 M nitric acid) indicates that high degrees of analyte preconcentration are feasible using miniature columns of the pertechnetate-selective sensor material. Plot A in Figure 4 corresponds to a static instrumental pulse height spectrum of 99Tc(VII) standard (20 µL of 1.38 × 105 dpm/ mL 99Tc(VII) in 0.02 M HNO3) obtained using sensor material and counting procedures described in the Experimental Section. Plot B in Figure 4 shows the pulse height spectrum of the same standard obtained in a homogeneous liquid scintillation (LS) cocktail and conventional counting geometry. A significant degree of quenching is evident for the 99Tc spectrum obtained using the (36) Janata, J.; Bezegh, A. Anal. Chem. 1988, 60, 62R. (37) Nevissi, A. E.; Silverston, M.; R. S., S.; Kaye, J. H. J. Radioanal. Nucl. Chem. Articles 1994, 177, 91-99. (38) Banavali, A. D.; Raimondi, J. M.; Moreno, E. M.; McCurdy, D. E. Radioact. Radiochem. 1995, 6, 26-35. (39) Technetium-99 in Water. Analytical Procedure. TCW 01, Eichrom Industries, Inc., 1995. (40) Technetium-99 in Soil. Analytical Procedure. TCSO1, Eichrom Industries, Inc., 1995. (41) Hollenbach, M.; Grohs, J.; Mamich, S.; Kroft, M.; Denoyer, E. R. J. Anal. Atom. Spectrom. 1994, 9, 927-933. (42) Horwitz, E. P.; Dietz, M. L.; Chiarizia, R.; Diamond, H.; Essling, A. M.; Graczyk, D. Anal. Chim. Acta 1992, 266, 25-37.

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Figure 3. 99Tc(VII) capacity factor on the sensor material as a function of nitric acid concentration.

Figure 4. Instrumental 99Tc β-particle energy spectra obtained using selective sensor material (plot A) and using liquid scintillation cocktail (plot B). Detection efficiences are listed.

sensor material: the pulse high spectrum is shifted to lower energies, and absolute 99Tc detection efficiency (peak area) is reduced from 96 (LS cocktail measurement, plot B) to 56% (plot B). Nevertheless, the 99Tc detection efficiency using sensor beads is sufficiently high for practical quantification applications. Preconcentrating Minicolumn Sensor System. A computercontrolled sequential injection fluidic system shown in Figure 2 and described in the Experimental Section was set up to investigate the performance of the scintillating microspheres and to develop analytical procedures. This system was set up to deliver samples and reagents to the 212-µL-volume sensor minicolumn (Figure 1) and to facilitate the fluidic packing and disposal of the sensing material. The schematic diagram shown in Figure 2 includes subsystems for manually delivering bead slurries to the column and for loading large sample volumes using an auxiliary two-position valve and syringe pump. Sample volumes of less than 10 mL could be loaded into the holding coil; the auxiliary pump and valve were included in the system only for 50-mL Hanford groundwater samples. To expedite our experimentation in this study, bead slurries were delivered to the minicolumn manually using a 5-mL syringe as described in the Experimental Section. We published a detailed implementation of a fully automated SI renewable column instrument previously in this journal,15 where

Figure 5. Detector traces illustrating response of the renewable 99Tc sensor to the analyte and potentially interfering species. Following the injection, the sensor was perfused with 10 mL of 0.02 M HNO3. Flow rate was 1 mL/min. Traces A, duplicate 100 µL injections of 1.38 × 105 dpm/mL 99Tc(VII) standard; trace B, reagent blank; trace C, 100-µL injection of 4.2 × 105 dpm/mL 137Cs standard. Time zero corresponds to the beginning of the sample injection/sensor wash step.

renewable columns of up to several hundred microliters in volume were described. Fluid-handling methods for delivering solutions using the separation-optimized SI technique have also been described in detail before.15,22,23 Sensing Based on Selective Retention on Scintillating Microspheres. The separation chemistry for 99Tc(VII) on Aliquatimpregnated scintillating microspheres illustrates selective retention of a single radionuclide for detection and quantification, with the option of washing away other unretained radionuclides if they are present in the sample. Detector traces illustrating this approach are shown in Figure 5. Responses to 99Tc(VII) and other potentially interfering species 137 ( Cs and 90Sr/90Y) were evaluated by injecting 100 µL aliquots of radionuclide standards in 0.02 M HNO3 and perfusing the sensor column with an additional 10 mL of 0.02 M nitric acid wash solution. These experiments were performed at 1 mL/min flow rate using freshly packed sensor columns preconditioned with 3 mL of 0.02 M nitric acid. The A traces in Figure 5 illustrate the sensor response in duplicate experiments using 100 µL of 1.38 × 105 dpm/mL 99Tc(VII) standard. Injected pertechnetate is captured by the sensor column giving a continuous signal which persists as the sensor column is washed with 0.02 M nitric acid. The detector signals of duplicate runs using a renewed sensor bed for each measurement are reproducible. The detector count rate corresponded to ∼45% detection efficiency, Ed (assuming quantitative analyte capture). Using the renewable column approach, no analyte carryover was evident in a blank run performed after an analysis of a high activity standard (Figure 5, trace B). The C trace in Figure 5 corresponds to the injection of a 100 µL of 4.2 × 105 dpm/mL 137Cs standard. Cesium ions show no affinity for the sensor material and pass through the column yielding a sharp transient peak signal. Similar results were obtained for 90Sr/90Y standard (data not shown). In this manner, unretained 137Cs and 90Sr/90Y species can be rapidly removed from the sensor column with a small volume of wash eluent. They will not interfere with 99Tc(VII) quantification procedures based on

Figure 6. Detector traces corresponding to the 90Sr/90Y separation experiments using HDEHP-based sensor material. Sample was 100 µL of 1.57 × 104 dpm 90Sr/90Y standard in 0.001 M HCl. Flow rate was 1 mL/min: (a) sample injection and sensor wash step using 6 mL of 0.001 M HCl; (b) 90Sr elution step using 6 mL of 0.2 M HCl; (c) 90Y elution using 6 mL 4 M HCl. See text for more details. Time zero corresponds to the beginning of the sample injection/sensor wash step.

the signal level after the wash. This is significant because both 137Cs and 90Sr are mobile in the environment and may be present in radiologically contaminated groundwaters. Horwitz et al. evaluated the elution behavior of 31 elements from 2 M nitric acid on Aliquat-336-based TEVA-resin.18 Only tetravalent actinides (i.e., Pu, Np, and Th) were found to be coretained with Tc(VII) in 2 M nitric acid. Retention of Th(IV) and Np(IV) decreases sharply with decreasing nitric acid concentration; retention is insignificant for both species at low acidities (k′ ∼ 1 in 0.01 M HNO3). Tetravalent Pu is more strongly retained than Th(IV) or Np(IV). Nevertheless, Pu is not expected to be present in any significant quantities or to interfere with 99Tc(VII) determination. Moreover, if so desired, complexing eluents can be used to facilitate efficient removal of the Pu species without affecting the pertechnetate retention.23 On this basis, our sensing approach using Aliquat-impregnated scintillating beads is selective toward 99Tc(VII). Selective Retention and Elution of Co-Retained Radionuclides. When multiple radionuclide species are co-retained on the sensor column, selective elution chemistries can be employed to obtain individual analyte responses. We demonstrated this approach in a model experiment using scintillating microspheres impregnated with HDEHP extractant, which has a high affinity for metal ion species in neutral and low acidity solutions.43 Experimental results using an injection of 100 µL of 1.57 × 104 dpm/mL 90Sr/90Y standard in 0.001 M HCl and an HDEHP-based sensor column are shown in Figure 6. The flow rate used was 1 mL/min. Both 90Sr and 90Y are extracted from 0.001 M HCl solution by the organic phase and remain on the sensor column during the subsequent wash with 6 mL of 0.001 M HCl (Figure 6, step a). Next, the sensor column was washed with 6 mL of 0.2 M HCl eluent. 90Sr elutes while 90Y remains on the sensor (Figure 6, step b). Finally, 90Y was eluted using 6 mL of 4 M HCl with the (43) Horwitz, E. P.; Bloomquist, A. A. J. Inorg. Nucl. Chem. 1975, 37, 425-434.

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Figure 7. Selected detector traces from the 99Tc(VII) sensor calibration experiments. Activities of the standards are listed. Flow rate was 1 mL/min: (a) sample load step (sample volume 10 mL); (b) sensor column wash with 10 mL of 0.02 M HNO3; (c) ejection of the sensor bed. Time zero corresponds to the beginning of the sample load step.

sensor response rapidly returning to the baseline (Figure 6, step c). Assuming 100% efficiencies of the capture and elution procedures, the difference between count rates during steps a and c was used to calculate individual absolute detection efficiencies, Ed, for 90Sr (Ed ) 46%) and 90Y (Ed ) 99%). In this manner, selective elution steps can be used to quantify individual radionuclide species that are co-retained by the sensor column during the sample load/preconcentration steps. The sensor can be used repeatedly either by the renewable sensing technique that replaces bead material or by regenerating the bead material already localized within the column. Calibration Methods and Detection Efficiency. Six 99Tc(VII) standards with activities ranging from 103 to 1381 dpm/mL (sample volume 10 mL) were analyzed using Aliquat-impregnated scintillating microspheres. Following the column conditioning (3 mL of 0.02 M HNO3) and sample delivery steps, the sensor column was washed with 10 mL of 0.02 M nitric acid. The flow rate was 1 mL/min, and each experiment was performed using freshly packed sensor columns. Selected sensor signals are shown in Figure 7. Selective retention of a single radionuclide provides two measurements for quantification: (1) the slope of the sensor response (cpm/mL) obtained during the sample load step (Figure 7, step a) and (2) the net integrated count rate (cpm) obtained after sample load and removal of interfering species. The slope can be used if the count rate for the analyte of interest is sufficient and the activities of other unretained radioactive species passing through the column do not interfere. The net integrated count rate method can be carried out with flowing wash solution or in a stopped-flow mode after a sufficient wash step. In the 99Tc calibration experiments, the net integrated count rate was calculated as the sum of the detector counts during the wash step from a standard analysis minus the sum of the detector counts during the wash step from a blank analysis, divided by the length of the counting interval (10 min). Both slope and integrated count rates gave linear calibration curves when plotted against the sample activity. 5426

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The slopes of the calibration curves yielded effective analytical efficiencies, Ee, of 44 ( 3 and 47 ( 3% for response slope and integrated count rate measurements, respectively. The efficiency of the analyte capture (i.e., separation recovery), Es, was assessed by collecting analysis effluents and determining unretained 99Tc by LS spectroscopy. Using the highest activity standard, no 99Tc was detectable in the collected fractions (Es > 99.9%). Given quantitative analyte capture, the detection efficiency, Ed, and effective analytical efficiency, defined as Ee ) EsEd,23 are equivalent. Detection Limits. Using the net integrated counts method of quantification, the detection limit and measurement error are a function of signal acquisition or counting time, background count rate, effective efficiency, and sample volume. The detection limit in counts, Ld, of the radioactivity measurement (95% confidence) is given as44

Ld ) 2.71 + 4.65xCbt

(1)

where Cb is the background (reagent blank) count rate and t is the signal acquisition time. For the radionuclide sensor, the detection limit in activity units (dpm/mL) or minimal detectable activity (MDAdpm/mL) can be calculated using the following equation:

MDAdpm/mL )

2.71 + 4.65xCbt Ld ) VtEe VtEe

(2)

where V is the sample volume (in mL) and Ee is the effective efficiency of the analysis (in decimal units, as opposed to percentage). Thus, detection limits can be lowered by increasing signal acquisition time and/or sample volume. Using the column sensor approach, the radionuclide species are captured and localized within the radioactivity detector. By stopping the flow, the analyte residence time within the sensor system can be increased indefinitely and signal acquisition time can be selected as necessary to achieve required detection limits or measurement precision. Moreover, using the sorbent sensor beads, the analyte is preconcentrated on the sensor column. The maximum sample volume, Vms, that can be delivered to the sensor column without the breakthrough analyte losses ( 99.9%) at any tested flow rate. In addition, both the slopes and net integrated count rates during the wash step were consistent within 3% (2σ) experimental error at all flow rates. These results indicate that 99Tc(VII) can be quantitatively captured at relatively high flow rates of 7 mL/min, which corresponds to 99 mL cm-2 min-1 linear velocity through the sensor column bed. High flow rates are advantageous if large sample volumes are required for analyte preconcentration. Higher flow rates are possible but were not tested because they lead to increasing back pressure in the flow system using the current narrow-bore column configuration. Sensor Regeneration vs Renewal. The 99Tc(VII) uptake results in Figure 3 indicate the possibility of eluting retained pertechnetate using a nitric acid solution with concentrations of g4 M. When we attempted washing the sensor material with 5 mL of 4 M HNO3, an intense chemoluminescence signal was observed. Subsequent analysis runs on the same sensor material indicated a complete loss of the scintillating properties. In general, the chemoluminescence was evident when the sensor material was exposed to nitric acid solutions with concentrations exceeding 0.5 M; the signal intensity increased with increasing acid concentrations. In this manner, scintillating fluors precipitated in the bead pores are affected by nitric acid and possibly destroyed when exposed to relatively concentrated solutions of nitric acid. No chemoluminescence was evident using hydrochloric acid solutions at up to 6 M concentrations. Nevertheless, 6 M HCl solution was not efficient in removing retained pertechnetate, which is expected to be strongly retained by quaternary ammonium organic phase even from concentrated HCl solutions.18 On this basis, sensor regeneration via elution chemistry is not feasible for the 99Tcselective microspheres obtained by co-deposition of Aliquat and

Figure 9. Detector traces from the analysis of GW 1 sample. Flow rate used was 2 mL/min: (a) sample load step (50 mL); (b) sensor wash step using 5 mL of 0.05 M HNO3; (c) 30-min stopped-flow counting interval. Time zero corresponds to the beginning of the sample load step.

scintillating fluors. However, using the renewable column approach,15 there is no need to regenerate the sensor material, since fresh sensor material can be delivered to the column for each measurement. Typical concerns associated with sensor material reuse, including reversibility, long-term stability, degradation in corrosive environments, and potential for analyte carry-over into subsequent analyses, are eliminated. Analysis of Hanford Groundwater Samples. To validate the applicability of our 99Tc-sensing approach in actual environmental samples, six groundwater samples from the Hanford site were analyzed. In Hanford groundwater, 99Tc is mainly present as highly mobile pertechnetate species. The analysis was performed using 50-mL aliquots of the acidified filtered samples (see Experimental Section) and a standard addition technique for quantification. To accommodate larger sample volumes, a second syringe pump (25mL syringe volume) was used for sample delivery. Sample solutions were loaded directly into the syringe without the use of the holding coil. To avoid cross-contamination, the syringe and sample transport lines were rinsed with 10 mL of 0.02 M HNO3 between each run. The following sample solutions were prepared and used in the analysis: (1) reagent blanks (0.02 M HNO3 and 0.01 M HCl for samples preserved in nitric and hydrochloric acids, respectively), (2) acidified groundwater samples, and (3) identical acidified groundwater samples with known amounts of 99Tc(VII) added (nominally 20.0 dpm/mL levels). Each analysis was performed using a freshly packed sensor column and a 2 mL/min flow rate for solution delivery. Columns were preconditioned with 3 mL of 0.02 M HNO3 or 0.01 M HCl (depending on the sample matrix) prior to delivering the 50-mL sample volume. After a sensor column wash with 5 mL of 0.02 M HNO3, the flow was stopped and the analytical signal was accumulated for 30 min. The samples were analyzed in the following order: (1) reagent blank, (2) sample solution, and (3) spiked sample solution. Detector traces for the analysis of sample GW1 (reported value 6.13 dpm/mL 99Tc) are shown in Figure 9. The net integrated count rate as described above (see Calibration) was determined for the stopped-flow Analytical Chemistry, Vol. 71, No. 23, December 1, 1999

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counting interval (Figure 9, step c). The analytical results for groundwater samples were calculated using a standard addition formula, A ) NAs/(Ns - N), where A is the sample activity (dpm/ mL), N is the net analytical signal for the unspiked sample, and Ns is the net analytical signal for the spiked sample. The analysis results obtained by standard manual methods and renewable sensor procedures are compared in Table 2. Manual and sensor procedure results are in good agreement and indistinguishable by the t-test at a 95% confidence level. Calculated effective efficiencies, Ee, of the automated sensor procedure are also included in Table 2. For groundwater samples treated with nitric acid, the Ee values are in satisfactory agreement with calibration experiments. However, when HCl was used for groundwater acidification, the effective efficiency was noticeably reduced. No significant reduction in effective efficiency was observed for the analysis of 50 mL of 99Tc(VII) standard prepared in 0.01 M HCl (Ee ) 42 ( 2%). Further studies are needed to elucidate these observations and to address the effects of the sample matrix on analyte quantification. The standard additions method addresses matrix effects and provides excellent quantification results in a laboratory environment. Addition of radioactive standards is less desirable for field applications. The detection limit of the sensor procedure is estimated to be 0.37 dpm/mL (167 pCi/L) or 9.8 ppt for the 50-mL groundwater samples acidified with nitric acid, using average Cb and Ee values. These values are below the maximum permissible 99Tc level in drinking water of 2.0 dpm/mL (900 pCi/L). DISCUSSION In current practice, radiochemical analyses of non-γ-emitting radionuclides involves a sequence of chemical separation and radiometric detection procedures. The preconcentrating minicolumn configuration with radionuclide-selective scintillating microspheres described in the present work integrates radionuclide separation and detection steps within a single functional sensor device. This approach overcomes the fundamental limitations of the direct detection of non-γ-emitting radionuclides in solution and it meets all the functional requirements for a radionuclide sensor described in the introduction. The packed-column format provides for efficient fluidic processing of the sample for preconcentration, with a large surface area for interaction between sample components and the selective material. The detection method is radiometric via the process of scintillation. Selective extractants localize radionuclides in very close proximity to the scintillation material and retain the radionuclides for counting. Sample volumes and counting times can be adjusted to meet the detection limit requirements. The sensing material can be regenerated or renewed fluidically. These new minicolumn sensors represent a novel and advantageous approach for detection of radionuclides. The ultratrace level detection limits required for radionuclide determinations (Table 1) dictate the use of radioactivity detection and drive the need for preconcentration. The sample volumes that must be processed for preconcentration are traded off with the sample counting time. In Table 3, we indicate sample volumes required to obtain regulatory detection limits given 30-min or 12-h counting times. Volumes range from a few milliliters for Tc to hundreds of milliliters or liters for other species. Except for uranium, which can be detected to ppb or ppt levels using 5428 Analytical Chemistry, Vol. 71, No. 23, December 1, 1999

Table 3. Sample Volumes Required To Achieve Regulatory Detection Limitsa sample vol, mL analyte 90Sr 99Tc 238U 239Pu 241Am

30-min counting time

12-h counting time

338 3 500 2500 2500

67 0.7 100 500 500

a Quantitative analyte capture, a detection efficiency of 1, and a background count of 50 cpm are assumed in these calculations.

stripping voltametry,45,46 it would be extremely challenging to detect these radionuclides to the required levels by chemical rather than radiometric-sensing techniques, or without preconcentration. Uranium is tractable as a chemical-sensing problem because the required detection limits are many orders of magnitude less stringent. Because of the limited energy resolution capabilities associated with the scintillation detection,47 the selectivity of the radionuclidesensing procedure is largely determined by the selectivity of the separation chemistries incorporated into the sensor material. Given the vast array of selective and semiselective liquid extraction and solid-phase extraction chemistries available for the separation of metal ions,19,48-52 development of scintillating sensor materials specific for a wide range of radionuclides is conceivable. Using the renewable sensing approach, it is possible to take advantage of the high-affinity separation chemistries, which may not be reversible under practical conditions (e.g., bis(2-ethylhexyl)methanediphosphonic acid extractant for the preconcentration of actinides48). Separation chemistries with high binding affinities are advantageous for analyte preconcentration in environmental applications.53 In this manner, the radionuclide-sensing approach using a renewable preconcentrating minicolumn may be adapted to the analysis of a range of important radionuclide species for environmental or nuclear waste analysis applications. We believe that with further development of radionuclideselective scintillating materials and advances in scintillation light detection and signal-processing techniques,33,54-56 this radionuclide-sensing approach will represent a useful platform for the development of effective radiochemical probes for field analytical instruments and process control applications. (45) Wang, J.; Lu, J.; Wang, J.; Luo, D.; Tian, B. Anal. Chim. Acta 1997, 354, 275-281. (46) Wang, J.; Wang, J.; Tian, B.; Jiang, M. Anal. Chem. 1997, 69, 1657-1661. (47) Knoll, G. F. Radiation Detection and Measurement; John Wiley & Sons: New York, 1989. (48) Horwitz, E. P.; Chiarizia, R.; Dietz, M. L. React. Funct. Polym. 1997, 33, 25-36. (49) Izatt, R. M.; Bradshaw, J. S.; Bruening, R. L. Pure Appl. Chem. 1996, 68, 1237-1241. (50) Cortina, J. L.; Warshawsky, A. Ion Exch. Solvent Extr. 1997, 13, 195-293. (51) Izatt, R. M.; Bradshaw, J. S.; Bruening, R. L.; Bruening, M. L. Am. Lab. 1994, 26, 28C. (52) Izatt, R. M. J. Inclusion Phenom. Mol. Recognit. Chem. 1997, 29, 197-220. (53) Burnett, W. C.; Corbett, D. R.; Schultz, M.; Horwitz, E. P.; Chiariza, R.; Dietz, M.; Thakkar, A.; Fern, M. J. Radioanal. Nucl. Chem. 1997, 226, 121-127. (54) DeVol, T. A.; Fjeld, R. A. IEEE Trans. Nucl. Sci. 1995, 42, 959-963. (55) Hastie, K. H.; DeVol, T. A.; Fjeld, R. A. Nucl. Instrum. Methods Phys. Res., Sect. A 1999, 422, 133-138. (56) Chotoo, S. D.; DeVol, T. A.; Fjeld, R. A. IEEE Trans. Nucl. Sci. 1997, 44, 1630-1634.

ACKNOWLEDGMENT The authors gratefully acknowledge stimulating discussions and guidance from Jaromir Ruzicka on sequential injection and bead injection techniques, stimulating discussions on environmental radionuclide sensing with Timothy DeVol, groundwater samples and information on environmental monitoring at the Hanford site from Jeffrey Serney, and continued interest in the development of radioanalytical methods for legacy nuclear wastes from Jiri Janata. This work has been supported with funding from

the Office of Biological and Environmental Research of the U.S. Department of Energy. The Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute.

Received for review July 7, 1999. Accepted September 10, 1999. AC990735Q

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