Anal. Chem. 1998, 70, 977-984
Sequential Injection Separation System with Stopped-Flow Radiometric Detection for Automated Analysis of 99Tc in Nuclear Waste Oleg Egorov,†,‡ Matthew J. O’Hara,† Jaromir Ruzicka,‡ and Jay W. Grate*,†
Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, and Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195
An automated procedure for the determination of 99Tc in aged nuclear waste has been developed. Using advanced sequential injection (SI) analysis instrumentation, 99Tc(VII) is separated from radioactive and stable interferences using a TEVA resin column that selectively retains pertechnetate ion from dilute nitric acid solutions. The separated 99Tc is eluted with 6 M nitric acid and quantified on-line with a flow-through liquid scintillation detector. A stoppedflow technique has been optimized that improves the analysis precision and detection limit compared to continuous-flow detection, reduces consumption of liquid scintillation cocktail, and increases sample throughput by separating the next sample while the present sample is being counted. The detection limit is 30 pCi, or 2 ng, of 99Tc, using a 15-min stopped-flow period. The analysis time is 40 min for the first sample and is reduced to 20 min for each subsequent sample. Processed nuclear waste samples from the Hanford site were successfully analyzed by this new method. 99Tc is a long-lived radioactive isotope produced in the thermal fission of 235U in nuclear reactors. A high fission yield of ∼6% results in the production of approximately 1 kg of 99Tc for each ton of enriched uranium.1 As a result, 99Tc is present in substantial quantities in stored spent nuclear fuel and in radioactive waste and process streams associated with spent fuel reprocessing. It has been estimated that over 1 ton (over 20 kCi) of 99Tc is present in defense-related nuclear waste currently stored in the underground storage tanks at the U.S. Department of Energy Hanford site.2 Due to the high abundance of 99Tc in these wastes, its long radioactive half-life, and the high mobility of technetium in the environment, 99Tc analysis is important throughout nuclear waste characterization and stabilization activities. 99Tc is a pure β-emitter (β max ) 294 keV) with a half-life of 2.13 × 105 years and specific activity of 629 Bq/µg, decaying to †
Pacific Northwest National Laboratory. University of Washington. (1) Lieser, K. H. Radiochim. Acta 1993, 63, 5-8. (2) Blanchard, D. L.; Brown, G. N.; Conradson, S. D.; Fadeff, S. K.; Golcar, G. R.; Hess, N. J.; Klinger, G. S.; Kurath, D. E. Technetium in Alkaline, HighSalt, Radioactive Tank Waste Supernate: Preliminary Characterization and Removal; PNNL-11386; Pacific Northwest National Laboratory: Richland, WA, 1997. ‡
S0003-2700(97)01121-9 CCC: $15.00 Published on Web 02/03/1998
© 1998 American Chemical Society
stable 99Ru.3 Due to the lack of appreciable γ emissions, direct nondestructive analysis of 99Tc by γ-spectroscopy is not possible. Analytical methods using radioactivity detection (β counting) require separation of 99Tc from inactive matrix constituents and various interfering radionuclides. This separation can be carried out by a variety of methods,4 including ion exchange,5 solvent extraction,6 sorbent extraction,7 and precipitation.8 Frequently, multistep procedures involving a combination of these methods are employed. Practically without exception, 99Tc separation and analysis steps are carried out manually; these analytical methods are time-consuming and labor-intensive and expose the analyst to hazardous chemicals and open sources of radioactivity. Considering the need for 99Tc analysis in a variety of sample matrixes to support nuclear waste characterization and environmental remediation efforts at U.S. DOE sites, the development of an automated 99Tc assay procedure would be advantageous. Sequential injection (SI) analysis9-11 represents the latest and most versatile generation of flow injection (FI) analysis techniques.12,13 These methodologies can be used to automate a variety of sample handling, separation, and analysis steps.13,14 By virtue of being highly reproducible and automated and offering contained solution handling, FI and SI techniques offer a number of potential advantages to the field of analytical radiochemistry.15,16 (3) Browne, E.; Firestone, R. B. Table of Radioactive Isotopes; John Wiley & Sons: New York, 1986. (4) Lavrukhina, A. K.; Pozdnyakov, A. A. In Analytical Chemistry of Technetium, Promethium, Astatine and Francium; Ann Arbor-Humphery Science Publishers: Ann Arbor, MI, 1970; pp 1-92. (5) Nevissi, A. E.; Silverston, M.; Strebin, R. S.; Kaye, J. H. J. Radioanal. Nucl. Chem. Art. 1994, 177, 91-99. (6) Dale, C. J.; Warwick, P. E.; Croudace, I. W. Radioact. Radiochem. 1996, 7, 23-27. (7) Banavali, A. D.; Raimondi, J. M.; Moreno, E. M.; McCurdy, D. E. Radioact. Radiochem. 1995, 6, 26-35. (8) Holm, E.; Rioseco, J.; Ballestra, S.; Walton, A. J. Radioanal. Nucl. Chem. 1988, 123, 167-169. (9) Ruzicka, J.; Marshall, B. D. Anal. Chim. Acta 1990, 237, 329. (10) Christian, G. D. Analyst 1994, 119, 2309-2314. (11) Ivaska, A.; Ruzicka, J. Analyst 1993, 118, 885-889. (12) Ruzicka, J. Analyst 1994, 119, 1925-1934. (13) Ruzicka, J.; Hansen, E. H. Flow Injection Analysis, 2nd ed.; Wiley-Interscience: New York, 1988; Vol. 62, p 498. (14) Fang, Z. Flow Injection Separation and Preconcentration; VCH: Weinheim, 1993. (15) Grate, J. W.; Strebin, R. S.; Janata, J.; Egorov, O.; Ruzicka, J. Anal. Chem. 1996, 68, 333-340.
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Nevertheless, the number of reported applications of FI/SI techniques to automation of radionuclide separations and analyses remains limited.15,17-19 In our previous report in this journal, we described an automated SI method for analysis of 90Sr in aged nuclear waste samples.15 In a closed system operating under computer control, 90Sr was rapidly separated from interfering radionuclides on a minicolumn containing a strontium-selective sorbent extraction material and delivered directly to a flow-through liquid scintillation detector. Nuclear waste samples from the Hanford site were successfully analyzed using the detector in a continuous-flow mode. In this paper, we describe the development of a SI method for separation and stopped-flow radiometric detection of 99Tc. The stopped-flow mode improves the sensitivity of on-line radioactivity detection and thus facilitates the automated analysis of diluted samples with relatively low 99Tc activity. The on-line separation is carried out using sorbent extraction material (TEVA resin, EIChrom Industries, Inc.) comprised of a mixture of quaternary amines (Aliquat 336) immobilized on an inert polymer support.20 This new method is characterized in detail and applied to the analysis of aged, processed nuclear waste samples derived from the underground storage tanks at the Hanford site (henceforth referred to as “tank waste”).
EXPERIMENTAL SECTION Sequential Injection System. A FIALab 3000 (Alitea USA, Medina, WA) sequential injection system was configured with a 24 000-step digital syringe pump (syringe volume, 10 mL), a 10port multiposition Cheminert valve, and a 4-port, two-position Cheminert valve (Figure 1). The 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 Scientific). The inlet of the sorbent column was connected to a port of the multiposition valve via a 50-cm transport line. The column was 4.6 mm × 50 mm (calculated volume, 0.83 mL), constructed of parts from the OmegaChrom column system (Upchurch Scientific) and frits from the QuickSnap column system (IsoLab, Inc., Akron, OH). TEVA resin (EIChroM Industries, Inc., Darien, IL) sorbent extraction material of particle size 20-50 µm was slurried in 0.1 M HNO3 and packed into the column using a 10-mL syringe. The sorbent bed was replaced daily or after 16 analysis cycles. The SI system was controlled via a serial cable using FIALab software (Alitea USA) running on a lap top PC. On-Line Radioactivity Detector. The flow-through radioactivity detector was a b-Ram 2B (IN/US Systems, Inc. Tampa, FL) liquid scintillation counter equipped with a pump for liquid (16) Egorov, O.; Ruzicka, J.; Grate, J. W.; Janata, J. Spectrum 96: Nuclear and Hazardous Waste Management International Topic Meeting, Seattle, WA, 1996. (17) Hollenbach, M.; Grohs, J.; Mamich, S.; Kroft, M.; Denoyer, E. R. J. Anal. Spectrom. 1994, 9, 927-933. (18) Aldstadt, J. H.; Kuo, J. M.; Smith, L. L.; Erickson, M. D. Anal. Chim. Acta 1996, 319, 135-143. (19) Dadfarnia, S.; McLeod, C. W. Appl. Spectrosc. 1994, 48, 1331-1336. (20) Horwitz, E. P.; Dietz, M. L.; Chiarizia, R.; Diamond, H.; Maxwell, S. L.; Nelson, M. R. Anal. Chim. Acta 1995, 310, 63-78.
978 Analytical Chemistry, Vol. 70, No. 5, March 1, 1998
Figure 1. Schematic diagram of the sequential injection 99Tc analyzer. C, carrier (water); SP syringe pump; HC, holding coil; S; sample line; E, eluent lines; W, waste; MPV, multiposition valve; SC, TEVA resin column; PL, purge line; DV, two-way diverter valve; LSC, liquid scintillation cocktail; CP, cocktail pump; DC detector flow cell; MC, mixing coil.
scintillation cocktail delivery. Two flow cells of 0.5- and 2.5-mL volumes were used. The detector was operated by Win-Flow software (IN/US) running on a laptop PC connected to the detector via a serial line. The detection cycle was initiated externally by a signal from the laptop PC controlling the SI system. The detector integration time, ti (time to accumulate counts for each data point reported), was 6 s. Off-Line Radioactivity Measurements. Off-line liquid scintillation measurements were performed with a Tri Carb 2550 TR/ AB liquid scintillation spectrometer (Packard Instrument Co., Meriden, CT). γ spectroscopy was carried out using HPGe detectors (EG&G Ortec, Oak Ridge, TN) equipped with Canberra electronics and data acquisition system. For the manual analysis of tank waste samples, β-counting was carried out using an LB 5100 gas proportional counter (Oxford Instruments Inc., Oak Ridge, TN) Reagents and Standards. All chemicals used were of analytical grade. Deionized water (MilliQ-Plus, 18.1 MΩ) was used as a carrier solution without degassing. Low-viscosity liquid scintillation cocktail Ultima-Flo AP (Packard) was used for online radioactivity measurements. Ultima Gold (Packard) cocktail was used for all static liquid scintillation measurements. Nitric acid solutions of 99Tc(VII), 239Pu(IV), and 90Sr/90Y were prepared by dilution of standard stock solutions obtained from an in-house standards laboratory. Activity of the prepared standards was verified by liquid scintillation counting. Tank Waste Samples. Tank waste samples were obtained as processed diluted solutions derived from the nuclear waste storage tanks at the Hanford site. The types of samples used in this study included dissolved salt cakes and sludge leachates. The salt cake slurries were dissolved in 0.75 M NaOH by stirring overnight. The sludge leachates were prepared by stirring the sludge with 10 M NaOH for 5 h at 100 °C, centrifuging, and decanting the liquid. The resulting solutions were treated with crystalline silicotitanate cation exchanger overnight and filtered. This step is used to remove the bulk of 137Cs and 90Sr activity,
while technetium remains in solution.2,21,22 After this treatment, the samples can be removed from the hot cell, and further handling steps can be carried out in a hood. Aliquots of 100 µL of the resulting solutions were diluted to 10 mL with water and used for subsequent analysis by standard manual and our new SI procedures. Prior to analysis by the SI method, the samples were acidified to pH 1 with concentrated nitric acid. One-milliliter sample aliquots were treated overnight with 100 µL of 0.1 M KMnO4 solution. Next, 5 µL of 30% H2O2 solution was added, and the samples were filtered. Permanganate will oxidize any reduced technetium to pertechnetate.4 Hydrogen peroxide efficiently reduces excess permanganate, which interferes with pertechnetate separation. Conventional Procedure for 99Tc Analysis.23,24 A tank waste sample was evaporated to near dryness after adding an aliquot of 95m Tc solution as a yield tracer. To ensure that all Tc in the residue was present as pertechnetate, a 0.1 M ceric ammonium nitrate solution was added in 100-µL increments until the yellow color persisted. Several milliliters of concentrated nitric acid were added, and the sample was evaporated to near dryness. Then 5 mL of water was added, the sample was again evaporated to near dryness, and the residue was dissolved in 3 mL of water. This sample was loaded on a 500-µL column containing AG-50W × 8 cation-exchanger in H+ form, which was then washed with 4 mL of water. Pertechnetate anions are unretained by this column, while fission products and other cationic radionuclies are retained. Aliquots of 100 µL of 1 M tartaric acid, 200 µL of 0.01 M tetraphenylarsonium chloride, and 1 mL of 10 M sodium hydroxide were added to the combined sample and column wash fractions. Then 5 mL of methyl isobutyl ketone was added to extract tetraphenylarsonium pertechnetate. The organic phase was transferred to a stainless steel planchette and evaporated to dryness, and the samples were counted in a gas-proportional β-detector. The analysis recovery was estimated by counting 95mTc activity in a γ spectrometer. The 99Tc activity in the tank waste samples was calculated by correcting the results of β counting for detection efficiency, analyte recovery, and the contribution of 95mTc activity to the β-detector background. CAUTION! Highly radioactive tank waste samples used in this work present severe radiological hazards. RESULTS AND DISCUSSION Flow System Design and Operation. The SI system was set up to perform the automated solution handling steps required for the separation of 99Tc from interfering radionuclides and delivery of the separated technetium fraction to a flow-through liquid scintillation detector. The system design shown schemati(21) Marsh, S. F.; Svitra, Z. V.; Bowen, S. M. Distribution of 14 elements on 63 Absorbers from Three Simulant Solutions (Acid-Dissolved Sludge, Acidified Supernate, and Alkaline supernate) for Hanford HLW Tank 102-SY; LA-12654; Los Alamos National Laboratory, Los Alamos, NM, 1994. (22) Anthony, R. G.; Dosch, R. G.; Gu, D.; Philip, C. V. Ind. Eng. Chem. Res. 1994, 33, 2702-2605. (23) Harvey, C. O., Separation of technetium by cation exchange and solvent extraction prior to measurement by beta counting, PNNL Technical Procedure PNL-ALO-432; Pacific Nortwest National Laboratory: Richland, WA, 1993. (24) Fadeff, S. K. Tecnetium Analysis Using the Ceric(+4) Amonium Nitrate and Nitric Acid Oxidation. PNNL Technical Procedure; Pacific Northwest National Labortatory, 1997 (in preparation).
Table 1. Reagent Delivery Sequence for Automated 99Tc Separation Procedure step no. 1 2 3
4 5 6 7
description
reagent
flow rate (mL/min)
column conditioning sample load column wash (matrix and actinides-IV removal) column wash Tc elution column cleanup column cleanup
2 mL of 0.1 M HNO3 1 mL of sample or standard 8 mL of 0.1 M HNO30.2 M HF
1.5 1.5 1.5
10 mL of 0.1 M HNO3 10 mL of 6 M HNO3 3 mL of 8 M HNO3 3 mL of water
1.5 0.5 1.5 1.5
cally in Figure 1 is similar to that used previously for SI separation and analysis of 90Sr,15 but there are some significant changes in components and operating method. The system features a highresolution digital syringe fluid drive, which represents an improvement over the peristaltic pump used in our previous report. A four-port, two-position valve is used as the diverter valve prior to the detector. This valve, combined with a purge line, provides a path for sending wash solutions directly through the detector to purge samples from the detector independent of the sorbent column (see below). The holding coil consists of a wide-bore tubing with an inside diameter of 1.6 mm rather than the narrower tubing (0.8-mm i.d.) used in the 90Sr analyzer. The use of a widebore holding coil tubing allows solution aspiration steps to be performed at high flow rates (15 mL/min) without outgassing. This change was desirable because solution aspiration operations constituted a significant portion of the overall analysis time in the previous 90Sr analyzer. However, severe dispersion can occur in this wide-bore coil, so solution handling operations were changed. An air segment is used to separate aspirated solutions from the carrier solution, and zones are not stacked in the holding coil. Rather, each solution is aspirated and dispensed before loading and dispensing the next solution. In this manner, the holding coil functions as a zero-dispersion volumetric extension of the syringe, and the syringe contacts only inert carrier solvent. Each solution delivery operation begins by aspirating a 75-µL air segment into the holding coil, followed by aspiration of the required volume of the solution plus an additional 50-µL portion of the same solution. Following the aspiration step, the main line, which contains the holding coil, is connected to the sorbent column line, and the solution is dispensed from the holding coil to the sorbent column at a specified flow rate (see Table 1 for examples). After the required volume of solution has been dispensed, the remaining 50 µL of the solution and the 75-µL air segment are expelled to waste, followed by 200 µL of water carrier. Reproducibility of this solution delivery method was estimated gravimetrically and was found to be better than 1.5% rsd (n ) 10) for volumes ranging from 100 µL to 10 mL. This approach is a departure from conventional SI analysis solution handling procedures; it is, however, well suited to SI separations requiring substantially larger solution volumes than a typical SI analysis. Automated 99Tc Separation Using TEVA Resin. Automated analysis using a SI instrument with on-line liquid scintillation detection requires that the 99Tc be completely and reliably separated from other radioactive interfering species. The separation of Tc(VII) using TEVA resin sorbent extraction material has Analytical Chemistry, Vol. 70, No. 5, March 1, 1998
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been described previously.5,7,25,26 The pertechnetate ion is nearly selectively retained on TEVA resin from dilute nitric acid with capacity factor k′ ≈ 104 in 0.1 M nitric acid solution.20 In a typical separation, the sample is loaded on the column in dilute nitric acid (e.g., 0.1 M), and additional dilute nitric acid is used to wash the column to remove other stable and radioactive ions. Horwitz et al. evaluated the retention behavior of over 30 metal ions typically present in high-level nuclear wastes and found that only tetravalent actinides are retained along with Tc(VII) from 2 M nitric acid solution; other ions were removed with 20 free column volumes of dilute nitric acid wash.20 Pertechnetate retention decreases with increasing nitric acid concentration (k′ ≈ 2 in 8 M HNO3),20 and the retained Tc(VII) can be eluted using 6-12 M nitric acid.5,7,17,25,26 Tetravalent Pu is the most strongly retained actinide species, and its retention on TEVA resin is significant, even in dilute nitric acid (capacity factor ∼50 in 0.1 M HNO3).20 Plutonium is a typical constituent of high-level tank wastes and may interfere with subsequent 99Tc quantification by liquid scintillation. Retention of tetravalent actinides by a sorbent material based on immobilized liquid anion exchanger (TEVA resin) can be lowered by incorporating a suitable complexing reagent. In a series of preliminary experiments with on-line detection, we observed that Pu(IV) is unretained in 0.1 M nitric acid solution containing 0.2 M HF and can be efficiently removed from 4.6- × 50-mm TEVA resin column with less than 5 mL (10 free column volumes) of 0.1 M HNO30.2 M HF complexing eluent. We developed the automated SI separation procedure described in Table 1 to condition the TEVA resin column, load the sample, wash the column, and elute the separated technetium. The progress of the separation experiments was followed by monitoring the radioactivity of the column eluates using a 0.5-mL flow cell and 0.8 mL/min cocktail flow rate. The detector trace in Figure 2 corresponds to the separation procedure applied to 1 mL of the processed tank waste sample (see Experimental Section) with the highest content of fission products. Peak A corresponds to radionuclides not retained by the resin and is predominantly due to 137Cs. The second peak (peak B, Figure 2 inset), observed using 6 M HNO3 eluent, corresponds to 99Tc(VII) present in the sample. Due to the short half-lives or low fission yields of other Tc isotopes, they are not found in aged tank wastes;2 therefore, the only Tc isotope present is 99Tc. In further experiments, the separation procedure using 6 M nitric acid as technetium eluent was carried out without on-line detection, and the eluent fraction corresponding to Tc elution was collected for subsequent off-line radiochemical analysis. The liquid scintillation analysis of the Tc fraction obtained in a separation run using 99Tc(VII) standard indicated separation recovery of 99 ( 2%. Using a tank waste sample, only 99Tc activity was discernible by liquid scintillation spectrometry of the Tc fraction. Only 137Cs was detectable by γ spectroscopy of the same fraction; however, its activity level was 2 orders of magnitude lower than that of 99Tc. The separation procedure provided a 1.5 × 104 decontamination factor for 137Cs, the most abundant matrix radionuclide present in the series of the tank waste samples used (25) Technetium-99 in Water, Analytical Procedure, TCWO1; EIChrom Industries, Inc., Darien, IL, 1995. (26) Technetium-99 in Soil, Analytical Procedure, TCSO1; EIChrom Industries, Inc., Darien, IL, 1995.
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Figure 2. Detector trace showing separation procedure applied to 1-mL aliquot of the oxidized tank waste sample. Detection cycle is started simultaneously with the beginning of the column wash sequence; liquid scintillation cocktail flow rate, 0.8 mL/min. (a, c) The column wash steps performed at 1.5 mL/min flow rate; (d) technetium elution step performed at 0.5 mL/min; (b) reagent aspiration step.
in this study. In addition, decontamination from 90Sr/90Y was evaluated by applying the separation procedure to a high-activity 90Sr/90Y standard. Off-line liquid scintillation analysis of the collected 6 M nitric acid fraction indicated decontamination factor of 9.4 × 105 for the sum of 90Sr/90Y activities. 99Tc Quantification. (i) Radioactivity Quantification in Automated Separation/Detection System. Once 99Tc is separated, its activity is quantified using the on-line liquid scintillation detector. The radioactivity detection step can be carried out either in a continuous-flow format or using stopped-flow detection.15 Assuming no secondary mixing in the flow cell, the residence time, tr, of the eluted radionuclide zone in the detector flow cell in a continuous-flow measurement, is equivalent to the counting time in a static radioactivity measurement:
tr ) Vc/F
(1)
where Vc is the volume of the flow cell and F is the combined eluent/cocktail flow rate through the flow cell. One limitation of the radioactivity detection in the continuous-flow format is a short counting time (typically under 5 min), which impairs the accuracy and detection limits of the radioactivity quantification for lowactivity samples. By stopping the flow, sample residence time in the detector flow cell can be increased indefinitely, and both the detection limit and counting error can be reduced by providing the signal acquisition time required to generate adequate counting statistics. The error of radioactivity measurement (counting) is proportional to the square root of the number of detected decay events (Poisson statistics). In a continuous-flow measurement, the detector peak maximum occurs when the maximum amount of radioactivity is present in the flow cell. Assuming no secondary mixing or phase separation effects in the flow cell, the fraction of the sample zone present in the detector flow cell at peak maximum, Dm, can be
obtained from the transient continuous-flow peak signal using the following equation:
Dm )
Cmaxtr t iC n
(2)
Cmax is the number of net counts at peak maximum, Cn is the net peak area counts, tr is sample residence time, and ti is detector integration or update time. If the flow is stopped at this time, the background-subtracted (net) count rate, Ccpm, can be related to the sample activity, Adpm, as follows:
Ccpm ) DmEdEsAdpm
(3)
The fraction of the sample zone present in the flow cell at peak maximum is given by the parameter Dm, while Ed and Es are the detection and separation efficiencies, respectively. (Separation efficiency indicates the fraction of activity loaded on the column that is recovered in the elutions step.) The stopped-flow analysis system will have an effective efficiency as given by eq 4, that can
Ees ) DmEdEs
Figure 3. Detector traces corresponding to 99Tc elution experiments using various concentrations of nitric acid (0.5-mL flow cell). Detection cycle was initiated simultaneously with the beginning of the 99Tc elution step. Eluent flow rate, 0.5 mL/min, liquid scintillation cocktail flow rate, 0.8 mL/min.
(4)
be determined from the slope of the calibration line or from the continuous-flow detector trace by dividing the net peak maximum counts by the detector update time and by the decay rate of the sample. Similar to low-level static radioactivity measurements, optimization of the automated separation system with stoppedflow detection involves selection of experimental conditions that maximize the figure of merit, FM:27
FM ) Ees2/B
(5)
where Ees is the effective efficiency of the separation/detection system (eq 4) and B is the detector background count rate (cpm). (ii) Effects of the Flow Cell, Eluent, and Eluent/Cocktail Ratio. We performed 99Tc(VII) analysis experiments (Table 1) with continuous-flow detection in order to compare the effective efficiencies and figures of merit for the count rate at peak maximum using 0.5- and 2.5-mL flow cells and using several concentrations of nitric acid as technetium eluents. In these experiments, the detection cycle was initiated simultaneously with the beginning of technetium eluent delivery, while all prior column effluents were diverted to waste. The cocktail-to-eluent ratio was 1.6:1. Detector traces in Figure 3 correspond to 99Tc(VII) elution profiles using 4, 5, 6, and 8 M nitric acid eluents (0.5-mL flow cell). This figure indicates that efficient Tc elution is possible with all eluents tested. The elution profiles are less dispersed with more concentrated nitric acid, which results in a larger fraction of eluted technetium present in the detector flow cell at peak maximum. At the same time, however, increasing nitric acid concentrations are known to quench liquid scintillation, which results in lower detection efficiency, Ed, and lower detector background, B. The nitric acid quenching effect is evident in the smaller peak areas with increasing nitric acid concentration in (27) Knoll, G. F. Radiation Detection and Measurement; John Wiley & Sons: New York, 1979.
Figure 4. Plots of effective efficiency (A) and figure of merit (B) at peak maximum versus the concentration of nitric acid eluent obtained using 0.5- and 2.5-mL flow cells. Cocktail-to-eluent ratio, 1.6:1. The error bars indicate (3 standard deviations.
Figure 3. Figure 4 shows the effective efficiency (plot A) and figure of merit (plot B) for the count rate at peak maximum as a function of nitric acid concentration, using both 0.5- and 2.5-mL flow cells. The 2.5-mL flow cell accommodates a larger percent of the eluted technetium zone, which results in about 3 times Analytical Chemistry, Vol. 70, No. 5, March 1, 1998
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Figure 5. Plots of effective efficiency (A, left y-axes) and figure of merit (B, right y-axes) at peak maximum versus the cocktail-to-eluent ratio obtained using 6 M nitric acid as the Tc eluent. Flow cell volume, 2.5 mL. The error bars indicate (3 standard deviations.
higher effective efficiency relative to that of the 0.5-mL flow cell. Detector background levels, however, were also approximately 3 times higher using the 2.5-mL flow cell. The figures of merit in Figure 3, plot B, show that the use of the larger flow cell is advantageous for the quantification of peak maximum. From 5 to 8 M nitric acid, the figures of merit did not vary significantly when using the larger flow cell. Nitric acid of 6 M concentration is suitable for elution and optimized the tradeoff between zone width and nitric acid quench for either size flow cell. The effect of the cocktail/eluent ratio on peak maximum quantification was evaluated using 6 M nitric acid eluent. Radioactivity detection of the eluted Tc zone was carried out in a continuous-flow format using the 2.5-mL flow cell. Figure 5 A shows the plot of the effective efficiency for the peak maximum versus the cocktail/eluent ratio. Higher cocktail ratios result in an increase in detection efficiency. However, the eluted technetium zone is diluted with cocktail. As a result, the effective detection efficiencies for the peak maximum are not significantly different for cocktail/eluent ratios above 1.5. Figure 5B shows the effect of the cocktail/eluent ratio on the figures of merit for the effective efficiency at the peak maximum. Higher instrument backgrounds observed at higher cocktail/eluent ratios resulted in figures of merit that begin to decrease above a 2:1 cocktail/ eluent ratio. Therefore, high cocktail-to-eluent ratios do not improve quantification of peak maximum. (iii) Analysis Using Stopped-Flow Detection. The results of stopped-flow detection of 99Tc in a 2.5-mL flow cell are shown in Figures 6 and 7, using 6 M nitric acid to elute the Tc and a 1.6:1 cocktail/eluent ratio. The A traces in Figure 6 illustrate continuous-flow detection of duplicate runs of a high-activity (2.502 × 104 dpm) 99Tc(VII) standard, which established the time position of the peak maximum. In addition, these runs demonstrated that 89% of the eluted technetium zone resides in the detector flow cell at peak maximum (as determined using eq 3), and the detection efficiency was 53% (based on analysis of the peak area counts corrected for the residence time and assuming 100% Tc 982 Analytical Chemistry, Vol. 70, No. 5, March 1, 1998
Figure 6. Detector traces illustrating 99Tc analysis with stoppedflow detection. The error bar indicates 3 standard deviations of the peak maximum counts. See text for more details.
Figure 7. Selected detector traces from the analysis of 99Tc(VII) standards using 15-min stopped-flow detection. The activities of the standards are listed. The error bar corresponds to 3 standard deviations of the background count rate.
recovery). The instrument was then reprogrammed to direct the eluent diverter valve to waste and simultaneously stop the cocktail pump at the time coinciding with peak maximum. Detector traces B in Figure 6 show a 15-min stopped-flow analysis of the same standard performed in duplicate and demonstrate that the stoppedflow detection is reproducible. It can be seen from Figure 6, traces B, that high nitric acid content in eluent/cocktail mixture does not result in the deterioration of the detection efficiency with time. Using stopped-flow detection, liquid scintillation cocktail consumption is reduced by a factor of 3. Five 99Tc(VII) standards with activities ranging from 268 to 4600 dpm were analyzed to calibrate 15-min stopped-flow detection. Selected traces are shown in Figure 7. The utility of the stopped-flow technique is exemplified by the analysis of the lowest activity standard (268 dpm of 99Tc). The count rate for each data point during stopped-flow, which corresponds to the peak maximum in a continuous-flow measurement, is not statistically distinguishable from the background. However, signal accumula-
tion during the 15 min stopped-flow interval allows reliable quantification of the 268-dpm standard with 8% (3σ) counting error. The net detector count rate, Ccpm, was calculated from the calibration data as the sum of the detector counts during the stopped-flow interval minus the sum of the background counts, divided by the length of the stopped-flow interval in minutes. The background counts were obtained from the analysis of a reagent blank performed under identical analysis conditions, except that no sample was loaded. The net stopped-flow count rates gave a linear calibration curve (Ccpm ) 0.476 dpm - 4.38, R ) 1.000). The slope of the calibration curve corresponds to 48% effective efficiency for the automated separation/stopped-flow detection procedure. Since the experimental conditions are highly reproducible (see following section), once the effective efficiency of the analysis is established, a single standard can be used to verify the system performance. The limit of detection (LOD) was calculated as 3 standard deviations of the background count rate corrected for the effective efficiency of the analysis. The LOD was 67 dpm (29 pCi), or 2 ng, of 99Tc using 15-min stopped-flow detection. Further reductions of the detection limit are possible by shielding the detector and employing a longer stopped-flow count interval. During the stopped-flow detection interval, the SI system continues to execute column wash steps and can proceed with the separation of the next sample, if so desired. The original sample is expelled from the detector prior to elution of the next sample by delivering 10 mL of 6 M nitric acid solution to the detector through the purge line (see Figure 1). Using this technique, the sample throughput is determined by the sample counting time. With 15-min stopped-flow detection, the analysis time was approximately 40 min for the first sample and 20 min for each subsequent sample. The sample throughput of the continuous-flow measurement is 40 min. Thus, the stopped-flow detection technique can actually improve sample throughput as well as detection limits and analysis precision. Column Life, Carryover, and Reproducibility. Automated SI analysis procedures for radiochemical analysis rely on a high degree of reproducibility of every step, including the separation. Reproducibility of the Tc elution profile is particularly important for reliable stopped-flow detection, so that the peak maximum position and the fraction of sample in the flow cell is consistent from run to run. The feasibility of TEVA resin column reuse has been demonstrated previously in a FI inductively coupled plasma mass spectrometry (ICP-MS) application, but the consistency of peak position, peak shape, and sample recovery was neither indicated nor necessary because an internal standard was used.17 In our analysis, we evaluated the reproducibility of technetium elution by conducting a series of consecutive continuous-flow analyses of a high-activity technetium standard. After 15 runs on a single column, no shift in the position of peak maximum was evident. Also, both the peak maximum counts (effective efficiency) and peak areas (99Tc recovery) remained reproducible within a 3σ counting error of 8.3% and 2.3% for peak maxima and peak areas, respectively. No Tc activity was detectable in the blank run performed immediately following the analysis of highactivity standards (
