Anal. Chem. 2006, 78, 2254-2261
Evaluation of Flow Cell Detector Configurations Combining Simultaneous Preconcentration and Scintillation Detection for Monitoring of Pertechnetate in Aqueous Media Lara D. Hughes* and Timothy A. DeVol
Department of Environmental Engineering and Science, Clemson University, Clemson, South Carolina 29634-0919
Flow cell detectors were developed for simultaneous concentration and scintillation detection of technetium99 in water. Evaluated flow cell geometries consisted of a coil and a fountain flow cell design; the latter is based on radial solution flow through a resin bed interfaced with a photomultiplier tube through a polycarbonate window. The sorptive scintillating media investigated were (1) an extractive scintillator combining a porous polystyrene resin with the extractant Aliquat-336 and fluor 2-(1naphthyl)-5-phenyloxazole, (2) a mixed bed of organic scintillator (BC-400) and Tc-selective resin (TEVA), and (3) a mixed bed of inorganic scintillator particles (CaF2Eu) with either TEVA resin or strong base anion-exchange resin (Dowex 1 × 8-400(Cl)). Depending on flow cell geometry and medium, the detection efficiencies for 99Tc ranged from 7.26 (BC-400/TEVA in coil geometry) to 50.20% (CaF2(Eu)/Dowex 1 × 8-400(Cl) in fountain flow cell geometry). The configuration with the highest sensitivity, CaF2(Eu)/Dowex 1 × 8-400(Cl) in coil geometry, can detect 99Tc as low as 3.78 Bq L-1 for a 100-s count interval and a 200-mL sample, which is below the current regulatory level of 33 Bq L-1. The issue of sensor reusability was addressed in this research, and its potential application at near neutral pH was demonstrated. The optimal sensor design was evaluated with a 99Tc-spiked synthetic groundwater matrix. Radionuclides are a major contaminant group at U.S. Department of Energy sites, and efforts are under way to implement monitoring and remediation programs to contain the spread of the contamination. Radionuclides of interest are those which are potentially mobile in the environment such as 90Sr, 99Tc, 129I, and various actinides (plutonium, uranium, neptunium).1,2 The detection of non-γ-emitting radionuclides in situ or at the site is challenging because of the short range of R and β particles and the difficulty in identifying a nuclide of interest, particularly β * To whom correspondence should be addressed. Phone: 864-656-1530. Fax: 864-656-0672. E-mail:
[email protected]. (1) United States Department of Energy. Groundwater/Vadose Zone Intergration Project Science and Technology Summary Description; 2999; DOE/RL-98-48 Vol. III, Revision 3. (2) United States Department of Energy. Hanford Science and Technology Needs Statements; DOE/RL 98-01, Revision 3, 2001.
2254 Analytical Chemistry, Vol. 78, No. 7, April 1, 2006
particles, by the energy of the radiation emission. Moreover, environmental concentrations of these contaminants are often low, which requires analyte concentration to achieve sufficient signal strength for quantification.1,3 Current research aims to reduce the standard sampling and analysis procedures by implementing fielddeployable monitoring devices, so that the presence of contamination can be detected at the site.1,2 A major focus of efforts is currently on the fission product 99Tc (E , β max 294 keV, t1/2 213 000 yr), a pure β emitter. The predominant chemical form in nuclear waste and nonreducing environments, the oxygenated anion pertechnetate (99TcO4-), does not easily partition to the predominantly negatively charged soil matrix and therefore migrates readily through soil and into groundwater.4 The current scientific research interest for detection of 99Tc focuses on the reagent-free use of a sensor, its easy regeneration/renewal, field applicability, the exploration of new sensor geometries, and improved methodologies for sensor use.2 The current regulatory level of 99Tc is 33 Bq L-1 based on its contribution to the gross β annual equivalent dose rate for ingestion of 0.04 mSv yr-1.3 To detect 99Tc at low environmental concentrations, a method of analyte separation is required, which concentrates the analyte and removes it from the interfering matrix of the water sample. The detection method will have to rely on radiometric detection, since mass spectrometric methods are not well suited for a fieldapplicable sensor. The use of ion exchange for the removal of pertechnetate from aqueous solutions has been discussed in ionexchange theory,5 and pertechnetate selective resins have been developed.6,7 The Dowex 1 × 8 anion-exchange resin has been studied for pertechnetate uptake from acidic solutions8 and is expected to have a sufficient selective bias for pertechnetate over other groundwater matrix ions.5 By combining very small particle (3) Hartman, M. J.; Dresel, P. E. Hanford Site Groundwater Monitoring for Fiscal Year 1997; Pacific Northwest National Laboratory, Richland, WA, 1998. (4) Pourbaix, M. Atlas of Electrochemical Equilibria, 1st ed.; Pergamon Press: Oxford, 1966. (5) Moyer, B. A.; Bonnesen, P. V. In Supramolecular Chemistry of Anions; Bianchi, A., Bowman-James, K., Garcia-Espana, E., Eds.; Wiley-VCH: New York, 1997; pp 1-44. (6) Bonnesen, P. V.; Brown, G. M.; Alexandratos, S. D.; Bates Bavoux, L.; Presley, D. J.; Patel, V.; Ober, R.; Moyer, B. A. Environ. Sci. Technol. 2000, 34, 3761-3766. (7) Gu, B.; Brown, G. A.; Bonnesen, P. V.; Liang, L.; Moyer, B. A.; Ober, R.; Alexandratos, S. D. Environ. Sci. Technol. 2000, 34, 1075-1080. (8) Kawasaki, M.; Omori, T.; Hasegawa, K. Radiochim. Acta 1993, 63, 53-56. 10.1021/ac051878h CCC: $33.50
© 2006 American Chemical Society Published on Web 02/23/2006
size with a high degree of cross-linking, it was chosen as a suitable medium for the use in a sorptive scintillator. A related method for analyte separation from environmental matrixes is extraction chromatography. Extraction chromatographic resins are available for separating a number of different radionuclides from environmental media.9-11 An extractant consisting of tricaprylylmethylammonium chloride (Aliquat-336) is very effective for technetium separations from weak nitric and hydrochloric acid (97%. The background count rates for the coil geometry ranged from 0.5 counts s-1 for the CaF2(Eu)/Dowex to 1.49 counts s-1 for the TcES resin. The predicted MDA ranged from 6.52 Bq for the BC-400/ TEVA to 0.74 Bq for the CaF2(Eu)/Dowex, which is sufficient for
Table 2. Summary of Test Results for Fountain and Coil Flow Cells sensor material
geometrya
nb
TcES BC400/TEVA CaF2(Eu)/TEVA CaF2(Eu)/Dowex TcES BC400/TEVA CaF2(Eu)/TEVA CaF2(Eu)/Dowex
F F F F C C C C
3 (1) 11 (4) 3 (1) 4 (1) 6 (2) 5 (2) 4 (4) 14 (2)
av det (%)
back ground (counts s-1)c
av loading efficiency (%)
LLD 100 s (counts)
MDA 100 s (Bq)
MDC for 200 mL (Bq/L)
elution/ reuse possible?
10.33 ( 0.36 11.52 ( 1.62 22.33 ( 3.24 50.20 ( 1.48 26.32 ( 1.53 7.26 ( 1.94 27.35 ( 3.52 48.29 ( 1.03
2.98 1.57 3.38 9.88 1.49 0.92 0.59 0.5
98.92 ( 0.57 99.30 ( 0.79 98.79 ( 0.14 98.34 ( 0.92 99.81 ( 0.20 99.66 ( 0.19 99.80 ( 0.10 97.38 ( 3.57
83.03 60.92 88.25 148.97 59.51 47.34 38.41 35.61
8.04 5.29 3.95 2.97 2.26 6.52 1.40 0.74
40.62 26.63 20.00 15.09 11.33 32.70 7.04 3.79
no no no yes no no no yes
a F, fountain flow cell; C, coil flow cell. b n indicates the total number of tests of that detection geometry; the number in parentheses is the number of flow cells used. c Background counting uncertainty
