Development and Performance of the Diffusive Gradients in Thin-Films

Institute, Westlakes Science Park, Moor Row, Cleator Moor, Whitehaven, Cumbria, CA24 3JY, U.K.. A novel technique for obtaining time-integrated 99Tc...
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Anal. Chem. 2005, 77, 135-139

Development and Performance of the Diffusive Gradients in Thin-Films Technique for the Measurement of Technetium-99 in Seawater Megan A. French,†,‡ Hao Zhang,† Jacqueline M. Pates,*,† Stephen E. Bryan,§ and Richard C. Wilson§

Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, LA1 4YQ, U.K., and Westlakes Research Institute, Westlakes Science Park, Moor Row, Cleator Moor, Whitehaven, Cumbria, CA24 3JY, U.K.

A novel technique for obtaining time-integrated 99Tc concentrations in seawater has been developed, using diffusive gradients in thin films (DGT). The performance of TEVA resin as a binding agent for 99Tc was investigated via laboratory experiments. The accumulated 99Tc activity per unit area of resin-gel was proportional to both the bulk solution activity and the exposure time for deployments of up to 2 weeks. The response of DGT was found to be independent of solution chemistries over the pH range 3-8 and ionic strength range 0.01-1.3 M. Seawater has pH 8 and ionic strength of ∼0.7 M; therefore, the potential of the technique for field deployment in seawater was demonstrated. Detection limits of 0.05 and 0.025 Bq L-1, for 2- and 4-week DGT deployments, respectively, were calculated for 99Tc measurement by liquid scintillation spectrometry. Using quadrupole ICPMS to measure bound 99Tc could reduce these detection limits to 0.125 mBq L-1 for a 4-week deployment. These detection limits are sufficiently low for monitoring contaminated environments, including the Irish Sea. This method is simpler and faster than other 99Tc analysis methods and represents the only means of obtaining timeintegrated data. Technetium-99 is a long-lived (t1/2 ) 213 000 years) radionuclide, produced by the fission of 235U. A significant source to the environment in Europe is the British Nuclear Group (BNG) Sellafield site in Cumbria, U.K.. The Enhanced Actinide Removal Plant (EARP) at Sellafield, commissioned in 1994, removes radiologically significant nuclides from medium-active concentrate (MAC) derived from the reprocessing of Magnox fuel. However, the EARP does not separate 99Tc from the waste stream, and this nuclide is discharged, under authorization, to the Irish Sea. The commissioning of EARP and the treatment of previously stored MAC resulted in an ∼30-fold increase in the activity of 99Tc discharged between 1993 and 1995 (i.e., ∼6 to ∼190 TBq y-1).1 * Corresponding author. E-mail: [email protected]. Fax: +44 1524 593985. † Lancaster University. ‡ Current address: School of Environmental Science, University of East Anglia, Norwich, NR4 7TJ. § Westlakes Research Institute. (1) BNFL Annual Report on Discharges and Monitoring of the Environment in the United Kingdom, 1998. 10.1021/ac048774b CCC: $30.25 Published on Web 12/01/2004

© 2005 American Chemical Society

Discharges of 99Tc have subsequently fallen (85 TBq in 2002)2 and are set to be reduced again by up to 90% following the successful trial of an abatement technology earlier this year.3 However, it is anticipated that low levels of 99Tc will continue to be discharged, and the environmental fate of historic discharges remains a contentious issue. Many of the concerns surrounding 99Tc stem from questions pertaining to the existing methodologies used to obtain 99Tc activity concentrations in seawater, especially near discharge points, where concentrations may vary markedly with time. Spot sampling techniques can result in an over- or underestimation of average seawater concentrations. Furthermore, the determination of 99Tc in seawater is not straightforward. 99Tc must be first preconcentrated from large volumes (up to 50 L) by precipitation, ion exchange, or both, followed by more detailed purification, using a combination of ion exchange, solvent extraction, or chromatography using TEVA-spec resin.4-6 A yield monitor must be used to account for losses during analysis; stable rhenium or 99mTc are frequent choices, but both have their own problems.4 Rhenium is known to overestimate 99Tc recoveries using standard techniques,4 although recent developments in isotopic dilution ICPMS may overcome this problem.7 Standard γ-spectrometry can be used to determine 99mTc, but its half-life of 6.01 h presents serious time constraints on the analysis. Typically, 99Tc is measured either by mass spectrometric (e.g., ICPMS) or radiometric (e.g., liquid scintillation) techniques. Radiometric techniques generally require 50-L samples and achieve detection limits of ∼0.06 mBq L-1,5 whereas ICPMS techniques can achieve detection limits of 0.012 mBq L-1 with a 5-L sample, although larger samples can be analyzed.6 To meet the requirements of the nuclear industry (e.g., BNG) and regulatory authorities (e.g., the England and Wales Environment Agency (EA) and the Scottish Environment Protection Agency (SEPA)), there is a need to develop measurement techniques that can provide time-integrated concentrations and (2) BNFL Annual Report on Discharges and Monitoring of the Environment in the United Kingdom, 2003. (3) Environment Agency, 2004. Press release 051-04tf, available from http:// www.environment-agency.gov.uk/news. (4) McCartney, M.; Olive, V.; Scott, E. M. J. Radioanal. Nucl. Chem. 1999, 242, 413-418. (5) Leonard, K. S.; McCubbin, D.; McDonald, P.; Service, M.; Bonfield, R.; Conney, S. Sci. Total Environ. 2004, 322, 255-270. (6) Keith-Roach, M. J.; Sturup, S.; Oughton, D. H.; Dahlgaard, H. Analyst 2002, 127, 70-75. (7) Mas, J. L.; Tagami, K.; Uchida, S. Anal. Chim. Acta 2004, 509, 83-88.

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that can ideally be deployed in situ. The new technique of diffusive gradients in thin films (DGT) has been developed for such purpose for trace metals.8 DGT measures solutes by accumulating them in situ in a simple plastic device8 and then measuring the accumulated solute later in well-controlled laboratory conditions. The solute diffuses through a well-defined membrane (usually a hydrogel) prior to accumulation on a selective binding agent, which is chosen according to the solute in question. The known geometry and conditions of the in situ accumulation allow precise calculation of the in situ concentration from the measured amount of accumulated solute. Previous studies have used DGT for a wide range of measurements, including labile metal species in seawater,8 trace metal remobilization fluxes, and concentration profiles at high spatial resolution in sediments9,10 and cesium concentrations in freshwaters.11,12 This paper reports on the development of a method for measuring 99Tc using the DGT technique with a new binding agent, and its systematic testing and validation for the measurement of 99Tc in seawater. DGT offers a simple and rapid method for the routine monitoring of 99Tc in seawater, avoiding the complications of traditional methods, in addition to providing timeintegrated data. EXPERIMENTAL SECTION Instrumentation. All samples were counted by liquid scintillation spectrometry (LSS), using a Packard Tri-Carb 3170 TR/ SL, which uses pulse shape discrimination to reduce background. Background events, external to the instrument, result in longer pulses than those originating in the vial. The background reduction circuitry discriminates between true events and background events on this basis; pulses longer than a preset time (the delay before burst, DBB) are rejected. A bismuth germanate guard aids this process by causing external background events to produce longer pulses than normal.13 The default DBB is 75 ns, which has been optimized for low-energy β-emitters; higher energy β-emitters require the DBB to be adjusted, to reduce the background without seriously compromising the counting efficiency. When counting 99Tc (E max ) 294 keV), this laboratory routinely uses a DBB setting of 700 ns, combined with an energy window of 10-300 keV. Reagents. Liquid scintillation vials were 20-mL low-potassium glass vials with aluminum foil-lined caps (Perkin-Elmer). Ultima Gold LLT (Perkin-Elmer) was the cocktail used throughout. It is biodegradable and has a high flash point, both of which characterize it as a “safe” cocktail, i.e., one that can be worked with on the bench and drain disposed. Additionally, this cocktail is suited to applications using pulse shape discrimination.14 99Tc Analysis. 99Tc was analyzed by LSS in two different matrixes. Tc in test solutions (0.01 M NaCl) was analyzed by (8) Davison, W.; Zhang. H. Nature 1994, 367, 546-548. (9) Zhang, H.; Davison, W.; Miller, S.; Tych, W. Geochim. Cosmochim. Acta 1995, 59, 4181-4192. (10) Zhang, H.; Davison, W.; Mortimer, R. J. G.; Krom, M. D.; Hayes, P. J.; Davies, I. M. Sci. Total Environ. 2002, 296, 175-187. (11) Chang, L.; Davison, W.; Zhang. H.; Kelly. M. Anal. Chim. Acta 1998, 368, 243-253. (12) Murdock, C.; Kelly, M.; Chang, L.; Davison, W.; Zhang, H. Environ. Sci. Technol. 2001, 35, 4530-4535. (13) Noakes, J. E.; Valenta, R. J. In Advances in Liquid Scintillation Spectrometry; Cook, G. T., Harkness, D. D., MacKenzie, A. B., Miller, B. F., Scott, E. M., Eds.; Radiocarbon: Tucson, AZ, 1994; pp 43-58. (14) Thomson, J.; Burns, D. A.; Cook, G. T. In Advances in Liquid Scintillation Spectrometry; Cook, G. T., Harkness, D. D., MacKenzie, A. B., Miller, B. F., Scott, E. M., Eds.; Radiocarbon: Tucson, AZ, 1994; pp 261-264.

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adding 1 mL of solution to a scintillation vial, together with 10 mL of cocktail. Samples and background vials (1 mL of 0.01 M NaCl and 10 mL of cocktail) were counted for 20 min. The counting efficiency, determined by spiking vials containing 1 mL of 0.01 M NaCl and 10 mL of cocktail with 5 Bq 99Tc, was found to be 82.8 ( 0.23%. To determine the 99Tc activity in the resin-gels, the nuclide was eluted from the gel to avoid the reduction in counting efficiency associated with self-absorption effects. The resin-gels were placed in a scintillation vial with 1 mL of 4 M HNO3 and then placed on an automatic shaker for a period of 24 h. Upon removal, each vial was given a brief but vigorous shake by hand until the separation of the resin beads from the gel became obvious, and 20 mL of cocktail was added. Samples and background vials, prepared identically using unexposed resin-gels, were counted for 20 min. To determine the counting efficiency for this second matrix, resin-gels were exposed to 99Tc solutions of a range of activities (see Uptake Efficiency). The resin-gels and the residual solutions were both analyzed by LSS. A mass balance allowed the activity on the resin-gel to be determined, and consequently, the counting efficiency was found to be 74.5 ( 2.0%. DGT Technique. Comprehensive details on the DGT theory and technique have been given previously.8,15 Briefly, the method uses a layer of binding agent beneath a layer of diffusive gel. A membrane filter is placed on top, and the three layers are held in a plastic DGT assembly that has a 2-cm-diameter exposure window. The binding layer usually comprises a layer of gel with the binding agent cast within it. When the binding agent is a resin, this is known as the resin-gel. When DGT is deployed, solutes present in the surrounding media are able to diffuse through the diffusive gel and membrane filter to accumulate on the resin-gel. The concentration gradient that is quickly established within the diffusive layer allows the amount of ions accumulating in the resin-gel layer to be related directly to the concentration in the surrounding media, as given by eq 1,15 where Cb is the solute concentration (or activity) in the

Cb ) M∆g/DAt

(1)

bulk solution, M is the mass (or activity) of ions accumulated on the resin, ∆g is the thickness of the diffusive gel, D is the diffusion coefficient, A is the exposed surface area of a DGT device, and t is the deployment time. Preparation of Diffusive Gel and Resin-Gel. Diffusive gels (0.8 mm thick) were prepared to a standard formula (15 vol % acrylamide and 0.3% agarose-derived cross-linker, DGT Research Ltd.).16,17 They were hydrated in Milli-Q water and then soaked for a minimum of 24 h in a 0.01 M NaNO3 solution. Resin-gels (0.4 mm thick) were prepared using 0.5 g (dry weight) of TEVA resin (Eichrom Technologies), which had previously been hydrated in Milli-Q water, and 10 mL of gel solution. Ammonium persulfate initiator (120 µL of 10% solution) and TEMED catalyst (60 µL) were subsequently added, and the resin-gels were cast between glass plates using 0.25-mm spacers. (15) Zhang, H.; Davison, W. Anal. Chem. 1995, 67, 3391-3400. (16) Zhang, H.; Davison, W. Anal. Chim. Acta 1999, 398, 329-340. (17) Davison, W.; Fones, G.; Harper, M.; Teasdale, P.; Zhang, H. In In Situ Analytical Techniques for Water and Sediments; Buffle, J., Horvai, G., Eds.; Wiley: New York, 2000; pp 495-569.

The sheets were incubated at 60 ( 2 °C for 1.5 h. The resin-gel was then hydrated in Milli-Q for a period of at least 24 h before use. Standard DGT solution deployment devices were used (DGT Research Ltd.).16,17 A protective 135-µm-thick cellulose nitrate membrane filter (0.45-µm pore-size Whatman) separated the diffusive gel from the solution. Characterization of DGT Performance. Uptake Efficiency. Samples comprising 10 mL of 0.01 M NaCl solution were spiked with a range of 99Tc activities (0, 2, 4, 6, and 10 Bq), in triplicate, and a TEVA resin disk added to each. The samples were left to equilibrate on a shaker for 24 h, and then the resin disks removed and analyzed by LSS. A 1-mL aliquot was taken from each of the residual solutions and analyzed by LSS. This experiment was subsequently repeated with a new batch of TEVA resin, to ensure consistency and reproducibility between batches. Kinetics of 99Tc Binding to TEVA Resin-Gel. A total of 24 samples (10 mL of 0.01 M NaCl) were spiked with ∼3 Bq of 99Tc each. A TEVA resin-gel disk was added to each sample at the start of the experiment, and then triplicate samples were shaken for each of the following times: 0.5, 1, 2, 5, 15, 30, 60, and 120 min. Three blank samples were also prepared. At the end of the time period, the resin disks were analyzed by LSS. Effect of Diffusive Layer Thickness. An experiment was performed to determine whether the activity accumulated by DGT increases linearly with increasing solution 99Tc activities. Eight 1-L 0.01 M NaCl solutions were spiked with 99Tc at the following bulk activities: 0, 200, 400, and 600 Bq in duplicate. For each activity, three DGT devices with 0.8-mm diffusive gels were deployed in one solution and three devices without diffusive gels (filter membranes only) were deployed in another. All devices were deployed for 24 h with the solutions being constantly and vigorously stirred. A 1-mL aliquot was taken from each bulk solution at the beginning and end of the experiment to confirm the activity concentrations in the solutions. Exposure Time. Four 2-L 0.01 M NaCl solutions were prepared and spiked with ∼ 800 Bq of 99Tc before being sealed and left for a period of ∼3 days to equilibrate. Six devices were deployed in each of three containers, with three devices being removed at the end of each of the following times: 3, 6, 16, 24, 30, and 44 h. Three devices were deployed in the remaining container for 2 weeks. Influence of pH. To investigate DGT performance over the pH range 3-8, six 1-L 0.01 M NaCl solutions were each spiked with ∼400 Bq of 99Tc, and left for 3 days to equilibrate. A pH buffer, 100 mL of 50 mM 2-(N-morpholino)ethanesulfonic acid) (MES) was added to each bulk solution, followed by incremental additions of 1 M HNO3 or 1 M NaOH to achieve the desired pH (pH 3, 4, 5, 6, 7, and 8). Three devices were deployed at each pH for 4 h. Ionic Strength. The performance of DGT was also investigated over a range of ionic strengths. A series of 1-L NaCl solutions were made up at the following concentrations: 0.01, 0.1, 0.7, and 1.3 M. Each solution was spiked with ∼400 Bq of 99Tc and then left to equilibrate for 3 days. Three DGT devices were deployed for a period of 4 h in each solution. RESULTS AND DISCUSSION Uptake Efficiency. The primary objective of this experiment was to quantify the uptake efficiency of 99Tc to the TEVA resingel. Analysis of resin-gel samples and the respective residual solutions allowed a mass balance to be performed. Of the initial

Figure 1. Uptake of 99Tc by the TEVA resin-gel as a function of time. Error bars indicate the standard deviation of the mean, for triplicate samples. The dashed line is an indicative uptake curve for 99Tc to the TEVA resin-gel over 2 h.

activity added to the solutions, 97.1 ( 2.5% was removed from the solution by the resin-gel. The count rate varied linearly as a function of the activity on the resin-gel, with a zero intercept. Replicate experiments showed excellent agreement (i.e., y ) 1.0134x, R2 ) 0.9994 and y ) 1.0105x, R2 ) 0.9995). The high degree of reproducibility between these experiments shows the resin-gel to be of a consistent quality, both within and between batches. Kinetics of 99Tc Binding to TEVA resin-Gel. Figure 1 shows the uptake of 99Tc by the resin-gel over time. The initially steep curve illustrates the rapid binding of 99Tc to the TEVA resin. This effective and rapid binding satisfies the basis of the DGT theory, which requires that the dissolved analyte concentration at the resin surface is effectively zero. As only a very small fraction of the technetium in solution binds within the few minutes it takes to establish the linear gradient, the fact that 100% removal is not attained until after 2 h is, contrary to the previous belief,18 not relevant. The activity of 99Tc accumulated by the DGT device can be found by rearranging eq 1. The maximum activity accumulated in a 30-s time interval can be calculated for a typical concentration, Cb (400 Bq L-1), used in the experiments in this work. Known values were used for the area of exposed gel, A (3.142 cm2), the diffusive gel thickness, ∆g (0.092 cm), and the experimentally derived diffusion coefficient, D ((8.64 ( 0.01) × 10-6 cm2 s-1) derived from the replicate deployments (n ) 18) of DGT devices in standard solutions at different times (see Exposure Time). The resultant activity theoretically taken up by DGT (3.5 × 10-3 Bq) is 2 orders of magnitude less than the mean activity obtained in the uptake experiment for the same 30-s time period, i.e., 0.2 Bq (Figure 1). The binding rate is therefore more than sufficient to satisfy the DGT demand. These results demonstrate that the effectiveness of DGT should not be judged by the time taken for complete removal of metal ion from solution as stated previously,18 but rather by the time taken to remove a sufficient quantity to sustain the DGT demand. Effect of Diffusive Layer Thickness. Sensitivity is increased with DGT by reducing the thickness of the diffusive layer. The (18) Zhang, H.; Davison, W.; Gadi. R.; Kobayashi, T. Anal. Chim. Acta 1998, 370, 29-38.

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Figure 2. Uptake of 99Tc taken up by the TEVA DGT devices as a function of solution activity (I ) 0.01 M NaCl, pH ∼6.5, deployment time 24 h). Regression equations: diffusive layer absent (y ) 0.0482x, R2 ) 0.9986); diffusive layer present (y ) 0.0103x, R2 ) 0.9787). Error bars indicate the standard deviation of the mean, for triplicate samples. (open circles, diffusive layer present; closed squares, diffusive layer absent).

maximum sensitivity is achieved by using a device that uses only a 135-µm-thick filter as the diffusive layer. The activities accumulated by such devices were compared to the activities accumulated by conventional devices containing 0.8-mm diffusive gels and a filter. There was a linear response in both instances (Figure 2). The uptake of 99Tc to the TEVA resin-gel disks was increased by a factor of ∼5 when the diffusive layer was removed. However, although both data sets showed good linearity, the samples without a diffusive layer are less precise, probably because with a thinner diffusive layer the measurements are more sensitive to small variations in the hydrodynamic regime. For example, at a bulk solution activity of 400 Bq of 99Tc, the mean bound activity in the presence of a diffusive layer is 3.73 ( 0.33 Bq, whereas the mean bound activity in the absence of a diffusive layer is 19.51 ( 2.67 Bq. Consideration of this factor might be important for future field applications if devices are constructed without a diffusive layer. Exposure Time. To investigate the effects of exposure time on the performance of DGT for the measurement for 99Tc, devices were deployed in solutions containing 800 Bq of 99Tc over the course of a 44-h period and additionally for 2 weeks (334 h). The activity bound to the resin-gel increased linearly with exposure time (Figure 3). These data are consistent with those obtained from the solution activity experiment (Figure 2). The bound activities of 99Tc were found to be 8.24 and 8.72 Bq by extrapolating to equivalent conditions for the activity and exposure time experiments, respectively. The response seen confirms the principle and mechanism of the DGT technique using TEVA resin-gel for the measurement of 99Tc and validates the application of eq 1 for the calculation of the 99Tc activity in solution. The results from the 2-week deployment (Figure 3) offer encouragement for the use of DGT as an in situ long-term monitoring tool for 99Tc. They show that the TEVA resin successfully and continuously removes 99Tc from solution over longer deployment periods and that the capacity of each TEVA resingel disk is in excess of 90 Bq. This capacity corresponds to a bulk solution activity of ∼250 Bq L-1, which is 2-4 orders of magnitude greater than typical 99Tc concentrations reported for the Irish Sea (i.e., 0.014-1.33 Bq L-1 in 1999).5 138

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Figure 3. Mean 99Tc taken up to the TEVA DGT devices over 2 weeks (y ) 0.2794x; R2 ) 0.9955) (bulk solution activities ∼800 Bq, I ) 0.01 M NaCl, pH ∼6.5). Inset is the first 44 h (y ) 0.3635x; R2 ) 0.9884). Error bars indicate the standard deviation of the mean, for triplicate samples.

A theoretical capacity for each TEVA resin-gel disk can also be derived on the basis of the capacity of the TEVA resin, which, according to the manufacturers of the resin, can take up in the order of 106 Bq g-1 resin. Given the weight of TEVA resin used in preparation of the resin-gel, a corresponding capacity of ∼5000 Bq per resin-gel disk can be estimated. With this theoretical capacity, the most sensitive DGT (i.e., without a diffusive gel layer) could potentially be deployed for up to 3 months in a solution of 250 Bq L-1 before reaching saturation. The diffusion coefficient (D), for use in eq 1, was determined from the slope of the regression line (s) (Figure 3) using eq 2.

D ) s∆g/CbA

(2)

The values used were as follows: slope of the regression line, s (0.3635 Bq h-1 ) 1.01 × 10-4 Bq s-1); diffusive gel thickness, ∆g (0.092 cm); bulk solution activity, Cb (0.342 Bq cm-3); and area of exposed gel, A (3.142 cm2). A value of (8.64 ( 0.01) × 10-6 cm2 s-1 was derived using the bound activity of 99Tc at 20 °C for 18 replicate samples (i.e., Figure 3 regression). Influence of pH. The performance of DGT was tested over the pH range 3-8. The results are presented as the ratio of the solution activity concentration of 99Tc obtained from the DGT measurement (Cdgt) to the known solution activity (Csolution) (Figure 4). The predicted value has been corrected for any variation in temperature by using a diffusion coefficient value that corresponds to the experimental conditions.15 The results show that the activity determined by DGT is independent of pH. This observation is consistent with other studies, despite their use of entirely different binding agents. For example, DGT performance is independent of pH for Cu over the range 2-10 and for Co, Mn, and Zn over the range 3.5-10.19 Ionic Strength. The experiments discussed in previous sections tested the performance of DGT for a fixed ionic strength (I) of 0.01 M NaCl. To test the effect of ionic strength on DGT performance, experiments were carried out over a range of I, including that of seawater, i.e., ∼0.7 M (Figure 5). The resingel activities obtained for 0.01, 0.1, 0.7, and 1.3 M agreed well, (19) Gimpel, J.; Zhang, H.; Hutchinson, W.; Davison, W. Anal. Chim. Acta 2001, 448, 93-103.

Figure 4. Ratio of solution concentration predicted by DGT (Cdgt) against known solution concentration (Csolution), over pH range 3-8. Dashed lines indicate 10% deviation above and below the theoretical value of unity. Error bars indicate standard deviation of the mean, for triplicate samples.

Figure 5. Ratio of solution concentration predicted by DGT (Cdgt) over known solution concentration (Csolution), across ionic strength range 0.01-1.3 M NaCl. Dashed lines indicate 15% deviation above and below theoretical value of unity. Error bars indicate standard deviation of the mean, for triplicate samples.

yielding an overall mean of 1.43 ( 0.05 Bq. The DGT measurement of 99Tc is independent of ionic strength in the range 0.011.3 M since the ratios (Cdgt/Csolution) fall within 15% of unity. Deviations from unity can be explained primarily by an overestimation of the diffusion coefficient (D) at the higher ionic strength values, since it is known that viscosity effects cause D to be reduced by ∼10% in such solutions,20 thus resulting in an underestimation of Cdgt. Evaluation of the Technique. The blank samples used within this study provide a means of determining the minimum detectable activity (MDA), based on the definition of the lowest limit of detection by Currie21 (eq 3), where µ is the background (in

MDA ) (2.71 + 4.65xµ)/tE

(3)

counts), t is the count time, and E is the counting efficiency. The background obtained over the course of this work was 7.85 cpm. Assuming a count time of 400 min, and a counting efficiency of 74.5%, the MDA is 0.02 Bq of 99Tc. This MDA corresponds to a (20) Li, Y. H.; Gregory, S. Geochim. Cosmochim. Acta 1974, 38, 703-714. (21) Currie, L. A. Anal. Chem. 1968, 40, 586-593. (22) Fiskum, S. K.; Riley, R. G.; Thompson, C. J. J. Radioanal. Nucl. Chem. 2000, 245, 261-272.

seawater concentration of 0.05 Bq L-1 for a DGT device with a diffusive layer deployed for 2 weeks or 0.025 Bq L-1 for a device deployed for 4 weeks. Although these concentrations are significantly higher than the detection limits achievable with more traditional methods, they are consistent with those recently observed in the Irish Sea (i.e., 0.3-1.6 Bq L-1 in 2002).2 Furthermore, the DGT technique is unique in being a time-integrating method. As the DGT used here is quite small, further increases in sensitivity can be easily achieved. Increasing its radius from 1 to 3.3 cm would increase the exposure surface area 10 times to 31.4 cm2; alternatively bulking resin-gel disks from 10 individual standard DGT devices would give the same increase in sensitivity. The corresponding detection limit for DGT would be 5 mBq L-1 for a 2-week deployment and 2.5 mBq L-1 for a 4-week deployment. Additionally, sensitivity can be improved by 5 times by removing the diffusive layer, although the precision may suffer. 99Tc can also be measured using ICPMS, which has detection limits an order of magnitude lower than β-counting.4,6 The detection limit of a quadruple ICPMS for 99Tc is typically 0.1ng L-1. When DGT is used, the detection limit can be reduced to 2 × 10-4 ng L-1 corresponding to 0.125 mBq L-1 for a standard DGT device (0.8-mm diffusive gel) deployed for 4 weeks. Although this study has focused on the analysis of 99Tc in seawater, there are other potential applications of the DGT technique to the determination of 99Tc in environmental waters. Groundwater in the United States has been contaminated with 99Tc as a consequence the nuclear weapons program. Consequently, there is a demand for rapid and simple analytical techniques to monitor and evaluate this contamination,22 and DGT could offer an alternative to current methods. CONCLUSIONS The laboratory investigations demonstrate the validity of DGT, using TEVA resin, for the measurement of 99Tc in seawater. It has been shown that uptake of 99Tc by DGT is independent of ionic strength and pH for seawater conditions, and uptake is proportional to solution activity and deployment time. Thus, the potential of the technique for the in situ monitoring of 99Tc is supported. Although detection limits are higher than for traditional techniques for 99Tc analysis, there is scope for considerable improvement by simple optimization of the DGT design. The use of DGT in combination with ICPMS to determine 99Tc offers even lower detection limits in future. The DGT technique can provide time-integrated measurements with considerable savings in analytical time. ACKNOWLEDGMENT M.A.F. acknowledges NERC studentship support (Grant NER/ S/I/2002/10925) and the guidance and financial support provided by Westlakes Research Institute in this collaborative study.

Received for review August 17, 2004. Accepted October 12, 2004. AC048774B Analytical Chemistry, Vol. 77, No. 1, January 1, 2005

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