Investigation and Optimization of On-Column Redox Reactions in the

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Anal. Chem. 1998, 70, 3920-3929

Investigation and Optimization of On-Column Redox Reactions in the Sorbent Extraction Separation of Americium and Plutonium Using Flow Injection Analysis Jay W. Grate* and Oleg B. Egorov

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352

A flow injection (FI) system with on-line liquid scintillation detection has been used to investigate and automate the separation of americium and plutonium on a sorbent extraction column (Eichrom TRU-resin). The FI system delivers various reagent solutions under controlled conditions to perform reactions that control the speciation of Pu, adjusting it between the III and IV states on the column. Am(III) is eluted in hydrochloric acid, while Pu(IV) remains on the column. The Pu is then selectively eluted by an on-column reduction reaction to Pu(III), which is not retained in hydrochloric acid. Remaining tetra- and hexavalent actinides are removed from the column using complexing eluent. Studies of the oncolumn oxidation of Pu with nitrite revealed that nitrite is slightly retained and partially converted to other species, both processes influencing subsequent reduction reactions. Mild reducing agents such as hydroquinone effected a slow and incomplete reduction of Pu(IV) on the column; examination of several other reducing solutions revealed that Ti(III) solutions gave rapid quantitative oncolumn reductions for selective Pu elution. The effects of column size and eluent flow rates were examined: flow rates as high as 3 mL/min could be used without significant change in elution peak volume. The remediation of radioactively contaminated sites and processing of stored wastes from the production of nuclear weapons materials will require characterization procedures throughout all phases of these activities.1 Radiochemical analysis undoubtedly represents one of the most important parts of the waste characterization effort.2,3 Determination of actinide elements, including Am and Pu isotopes in various sample matrixes, is routinely performed in radioanalytical laboratories throughout the U.S. Department of Energy complex and is of great concern in the nuclear industry.4-9 (1) Campbell, J. A.; Stromatt, R. W.; Smith, M. R.; Koppenaal, D. W.; Bean, R. M.; Jones, T. E.; Strachan, D. M.; Babad, H. Anal. Chem. 1994, 66, 1208A1213A. (2) Erickson, M. D.; Aldstadt, J. H.; Alvarado, J. S.; Crain, J. S.; Orlandini, K. A.; Smith, L. L. J. Haz. Mater. 1995, 41, 351-358. (3) Crain, J. S.; Smith, L. L.; Yaeger, J. S.; Alvarado, J. A. J. Radioanal. Nucl. Chem. 1995, 194, 133-139.

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Determination of Am and Pu isotopes by radioactivity detection techniques (typically R-spectroscopy) requires that both Am and Pu are separated from the large quantities of stable matrix and potential radioactive interferences. In addition, individual separations of Am and Pu from each other are required because 241Am and 238Pu R-particles have unresolvable energies. The required separations can be carried out by a variety of classical and chromatographic methods, including ion exchange,10-15 liquidliquid extraction,10,11,16 extraction chromatography,10,11,17 HPLC, and ion chromatography.18-20 Often, a combination of these methods is required in order to achieve the desired degree of preconcentration and separation.21 Recently, methodology using actinideselective extractants immobilized on solid supports has been developed to simplify and improve radiochemical separations.6,22-24 (4) Smith, L. L.; Crain, J. S.; Yaeger, J. S.; Horwitz, E. P.; Diamond, H.; Chiarizia, R. J. Radioanal. Nucl. Chem. 1995, 194, 151-156. (5) Kaye, J. H.; Strebin, R. S.; Orr, R. D. J. Radioanal. Nucl. Chem. 1995, 194, 191-196. (6) Horwitz, E. P.; Dietz, M. L.; Chiarizia, R.; Diamond, H.; Maxwell, S. L.; Nelson, M. R. Anal. Chim. Acta 1995, 310, 63-78. (7) Nevissi, A. E.; Strebin, R. S. J. Radioanal. Nucl. Chem. 1995, 197, 211218. (8) Maxwell, S. L.; Nelson, M. R. Institute of Nuclear Material Management 35th Annual Meeting, Naples, FL, 1994. (9) Goldstein, S. J.; Hensley, C. A.; Armenta, C. E.; Peters, R. J. Anal. Chem. 1997, 69, 809-812. (10) Penneman, R. A.; Keenan, T. K. The Radiochemistry of Americium and Curium; National Academy of Science: Springfield, VA, 1960. (11) Coleman, G. H. The Radiochemistry of Plutonium; National Academy of Science: Springfield, VA, 1965. (12) Plutonium Handbook. A Guide to Technology; Wick, O. J., Ed.; Gordon and Breach Science Publishers: New York, 1967; Vol. 1. (13) The Chemsitry of the Actinide Elements; Katz, A. A., Seaborg, G. T., Morss, L. R., Eds.; Chapman and Hall: New York, 1986; Vols. 1, 2. (14) Usuda, S. J. Radional. Nucl. Chem. 1988, 123, 619-631. (15) Freeman, B. P.; Weiss, J. R.; Pietri, C. E. Rev. Sci. Instrum. 1981, 52, 12521254. (16) Sekine, T.; Hasegawa, Y. Solvent Extraction Chemistry. Fundamantals and Applications; Marcel Dekker: New York, 1977. (17) Extraction Chromatography; Braun, T., Gershini, G., Eds.; Elsevier: New York, 1975. (18) Barkley, D. J.; Blanchette, M.; Cassidy, R. M.; Elchuk, S. Anal. Chem. 1986, 58, 2222-2226. (19) Elchuk, S.; Burns, K. I.; Cassidy, R. M.; Lucy, C. A. J. Chromatogr. 1991, 558, 197-207. (20) Reboul, S. H.; Fjeld, R. A. Health. Phys. 1995, 68, 584-589. (21) Choppin, G. R. Sep. Sci. Technol. 1984-85, 19, 911-925. (22) Horwitz, E. P.; Dietz, M. L.; Diamond, H.; LaRosa, J. J.; Fairman, W. D. Anal. Chim. Acta 1990, 238, 263-271. S0003-2700(98)00411-9 CCC: $15.00

© 1998 American Chemical Society Published on Web 08/19/1998

Horwitz et al. described a sorbent extraction material comprised of a 0.75 M solution of bifunctional organophosphorus extractant octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) in tri-n-butyl phosphate (TBP) immobilized on inert porous polymeric resin.22,23 This sorbent material is available commercially as TRU-resin from EIChrom Industries, Inc. This separation chemistry can be used for a number of analytical purposes, including the separation of actinides as a group from the sample matrix (gross actinides);5,22,25 the separation of Am and Pu from each other and from other actinides; 5,23,25,26 and potentially the sequential elution of individual actinidessa possibility is shown in the seventh figure of ref 23. In some schemes for actinide separations by sorbent extraction, multiple columns are used.6,8,26 In a typical sorbent extraction separation, samples and various solutions for washing and eluting are added to the top of the open column, and fractions are collected for subsequent radiochemical analysis. These procedures represent a significant improvement over classical radiochemical separation methods. It is possible to automate a variety of sample handling and separation steps using flow injection (FI) analysis.27,28 Recently, there have been a few reports on the use of flow injection methods to separate and analyze radionuclides in environmental samples and radioactive waste. Dadfarnia and McLeod described the analysis of uranium in surface waters and seawater using a simple FI system with an alumina column for preconcentration and ICPMS for detection of the eluted species.29 Hollenbach et al. described an automated FI method to separate and preconcentrate Tc, Th, and U from soil samples and deliver the separated analytes to the ICPMS instrument.30 Aldstadt et al. described the use of FI to automate a separation on a TRU-resin column for U analysis with ICPMS detection.31 In a recent publication, we described a method using sequential injection (SI) to automate a sorbent extraction procedure for the separation and analysis of 90Sr.32 The 90Sr was then detected online with a flow-through liquid scintillation detector. The effectiveness of the new methodology was demonstrated in the analysis of the aged nuclear waste samples from the Hanford site. The development of the 90Sr analyzer with a flow-through liquid scintillation detector illustrated primarily the usefulness of FI/SI methodology to automate a radiochemical analysis procedure. However, this instrumentation can also be used to investigate separation materials and to design or optimize separation schemes. (23) Horwitz, E. P.; Chiarizia, R.; Dietz, M. L.; Diamond, H.; Nelson, D. M. Anal. Chim. Acta 1993, 281, 361-372. (24) Horwitz, E. P.; Dietz, M. L.; Chiarizia, R.; Diamond, H.; Essling, A. M.; Graczyk, D. Anal. Chim. Acta 1992, 266, 25-37. (25) Strebin, R. S. Separation of Am and Pu and Actinide Screen by Extraction Chromatography, PNL-ALO-417. In Analytical Chemistry Laboratory Procedure Compensdium, PNL-MA-599, Pacific Northwest National Laboratory: Richland, WA, 1993. (26) Americium, Putonium and Uranium in Water, Analytical Procedure ACW03; EIChrom Industries, Inc.: Darien, IL, 1995. (27) Fang, Z. Flow Injection Separation and Preconcentration; VCH: Weinheim, 1993. (28) Ruzicka, J.; Hansen, E. H. Flow Injection Analysis; 2nd ed.; Wiley-Interscience: New York, 1988; Vol. 62, p 498. (29) Dadfarnia, S.; McLeod, C. W. Appl. Spectrosc. 1994, 48, 1331-1336. (30) Hollenbach, M.; Grohs, J.; Mamich, S.; Kroft, M.; Denoyer, E. R. J. Anal. At. Spectrom. 1994, 9, 927-933. (31) Aldstadt, J. H.; Kuo, J. M.; Smith, L. L.; Erickson, M. D. Anal. Chim. Acta 1996, 319, 135-143. (32) Grate, J. W.; Strebin, R. S.; Janata, J.; Egorov, O.; Ruzicka, J. Anal. Chem. 1996, 68, 333-340.

Figure 1. Schematic diagram of the flow injection separation instrument. R, reagents; S, sample; W, waste; MPV, multiposition valve; A, B, C, transport lines; PP, peristaltic pump; DV, diverter valve; SC, sorbent column; CH, column heater, FD, flow-through detector; FC, fraction collector. Either the flow detector or the fraction collector can be connected to the column outlet.

The great advantage of the new methodology is that it allows facile alteration of the experimental conditions under automated remote control, with immediate results via on-line detection. We now describe the use of a FI instrument with on-line radioactivity detection to investigate and optimize the separation of Pu and Am using TRU-resin. Prior use of FI and SI techniques to automate radiochemical analyses has been limited to separations involving load, wash, and elute steps; i.e., where the sample is loaded under conditions where the desired analyte or group of analytes is selectively retained, the unretained species are washed from the column, and finally the desired radionuclide(s) is eluted from the column.29-32 The single-column separation of Pu and Am on TRU-resin in the presence of other interfering actinides requires that on-column reactions are performed to adjust the speciation of Pu. Optimization of these on-column redox reactions in an automated format is the primary topic of the present study. EXPERIMENTAL SECTION Flow Injection System. A modular injection system MIS-1B (Alitea USA, Medina, WA), configured with a peristaltic pump, 10-position Cheminert selector valve (Valco Instruments, Houston, TX), and 8-port 2-position Cheminert valve (Valco) operated as a two-way diverter valve, was used to set up a flow injection instrument (Figure 1). All transport and reagent lines were made of 0.8-mm-i.d. PTFE tubing (Upchurch Scientific, Oak Harbor, WA). The length of transport lines A, B, and C were 25, 35, and 16 cm, respectively. The column outlet was connected to the flowthrough detector via a 76-cm transport line. The peristaltic pump tubing was Tygon of 1.52-mm i.d. and 15.5-cm length. When temperature-programming experiments were performed, transport line C was replaced with a 200-cm coil tightly wrapped around an aluminum cylinder; this unit and the sorbent column were placed inside a CH-150 column heater unit (Eldex Laboratories). The sorbent columns were constructed of parts from the Omegachrom column system (Upchurch Scientific) using disposable columnend frits (Isolab, Akron, OH). The column was always positioned Analytical Chemistry, Vol. 70, No. 18, September 15, 1998

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vertically, and the reagent streams were always delivered to the column inlet on the bottom. Three column tube sizes, 50 × 3 mm, 100 × 3 mm, and 100 × 4.6 mm (calculated volumes 0.35, 0.7, and 1.66 mL, respectively), were used. To minimize sorbent consumption, smaller size sorbent columns were used at the beginning stages of the separation development; later the separation was scaled to larger sorbent column sizes. Dispersion studies were performed using a Brinkman photocolorimeter equipped with a bifurcated optical cable interfaced with the sandwich flow cell (Alitea USA), which was positioned in place of a sorbent column. The nitrite retention and column crossover studies were conducted using SPD10AV UV-visible detector (Shimadzu) positioned in place of the radioactivity detector. The FI system was controlled using Atlantis software (Lakeshore Technologies, Chicago, IL) running on a 486 PC equipped with a Real Time Devices ADA 1100 general purpose interface board. On-Line Radioactivity Detection and Fraction Collection. A Radiomatic A515A (Packard Instrument Co., Meriden, CT) flowthrough liquid scintillation detector equipped with a 0.5-mL flow cell was operated as described previously,32 using detector update times of 12 or 6 s. A sample-to-cocktail ratio of 1:1 was utilized unless stated otherwise. For the off-line radioactivity measurements, the column outlet transport line was connected to a programmable FC205 (Gilson, Middleton, WI) fraction collector equipped with a two-way diverter valve. The time windows fraction collection cycle was initiated externally by a signal from the computer controlling the FI system. Off-Line Radioactivity Measurements. All liquid scintillation measurements were performed with a Tri Carb 2550 TR/AB liquid scintillation counter (Packard). R-Spectroscopy was performed using ULTRA (EG&G Ortec, Oak Ridge, TN) ion implanted detectors housed in model 576A spectrometers (EG&G Ortec). γ-Spectroscopy was performed using HPGe detectors (EG&G Ortec) equipped with Canberra electronics and data acquisition system. The samples for R-spectroscopy were prepared using the neodymium fluoride microprecipitation procedure, as described elsewhere.5 Sorbent Column Preparation. Columns were packed with 25-50 µm TRU-resin (Eichrom Industries, Inc., Darien, IL) slurried in deionized water. The experiments critical with respect to potential analyte or reagent cross-contamination (Pu oxidation and reduction studies) were performed using freshly prepared sorbent columns. The free column volume was found to be 0.67 mL/mL of bed and the bed density was found to be 0.43 g/mL of bed; both were measured gravimetrically. The ratio of the stationary to mobile phase volume was calculated to be 0.26 assuming 0.411 mL/g of sorbent extractant loading.23 Reagents. All chemicals used were of analytical grade. A TiCl3 reagent solution was prepared daily by appropriate dilution of a 20% stabilized solution (EM Science, Gibbstown, NJ). Ascorbic acid solutions in nitric acid were prepared fresh daily. A 0.5 M solution of ferrous sulfamate reagent was prepared weekly as described elsewhere.26 The ascorbic acid-ferrous sulfamate reagent was prepared fresh daily. Ammonium hydrogen oxalate (bioxalate) solution was prepared by combining equimolar quantities of ammonium oxalate and oxalic acid. Low-viscosity liquid scintillation cocktail Ultima Flo-M (Packard) was used for on-line radioactivity measurements. Ultima Gold (Packard) cocktail was 3922 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998

used for static liquid scintillation spectroscopy in Tri Carb 2550. Radionuclide Standards. Standard 2 M nitric acid solutions of 241Am, 239Pu, and 230Th were prepared by appropriate dilution of certified NIST traceable standards obtained from Isotope Products Laboratories (Burbank, CA). The 233U and 90Sr standard solutions were prepared using stock solutions obtained from the in-house standards laboratory. To ensure statistically reliable detection of the elution peaks, the radioactivity of the injected sample was maintained in the range of 2.5 × 104-5.0 × 104 dpm per radionuclide. The Pu(III) solution in nitric acid was prepared from 239Pu standard solution in 2 M HNO3 by making it 0.1 M in sulfamic acid and 0.05 M in Fe(NO3)3. Next, this solution was titrated with 0.8 M solution of ascorbic acid in 2 M HNO3 until no blue color formation was observed upon addition of the next portion of the ascorbic acid titrant.5,23 Liquid-Liquid Extraction Experiments and Nitrite Retention Studies. The degree of Ti(III) extraction from 4 M HCl was estimated spectrophotometricalyly in liquid-liquid extraction experiments using a 0.75 M solution of CMPO (Strem Chemicals, Newburyport, MA) in TBP (Aldrich, Milwauke, WI) as an organic phase. A 4-mL aliquot of freshly prepared stock solution of 0.05 M NaNO2 in 2 M HNO3 and in 4 M HCl and a 4-mL aliquot of 0.05 M TiCl3 stock solution in 4 M HCl were each equilibrated with 2 mL of CMPO-TBP phase for 15 min. After phase separation by centrifuging, absorbance of an aqueous phase was measured at 372 nm for nitrite and at 512 nm for Ti(III) measurements using an HP8451A diode array spectrophotometer (Hewlett Packcard). The distribution ratio (D) was calculated from the ratio of the aqueous phase absorbance after extraction (A) to the absorbance of the stock solution (A0) as

D)

(

)

A0 Vaq -1 A Vorg

where Vaq and Vorg are the volumes of the aqueous and organic phases used in the extraction experiment. The linear relationship between absorbance and concentration was verified by measuring the absorbance of the series of the stock solution dilutions. The capacity factors (k′) were obtained from the distribution ratios (D) as k′ ) 0.26D, where 0.26 is the stationary- to mobile-phase volume ratio for the packed columns used in this study. The nitrite retention was measured by injecting 100-µL aliquots of freshly prepared 0.05 M NaNO2 solution in nitric and hydrochloric acids and measuring the retention time of the nitrite band on TRUresin column using a UV-visible detector set at the wavelength of 380 nm. The capacity factor was determined by converting the retention volume to the number of free column volumes. Caution! High-activity actinide standards used in this work present severe radiological hazards and may be handled only in specialized facilities. Required safety measures associated with handling, storage, and disposal of these solutions and generated mixed wastes must be strictly observed. RESULTS AND DISCUSSION Flow Injection System. The automation of radionuclide separations on TRU-resin requires reproducible delivery of sample and multiple reagent streams to the sorbent column under welldefined dispersion and residence time conditions and transport

of the separated radionuclides to a detector or fraction collector. The FI system shown in Figure 1 was designed to facilitate selection and delivery of a sample and various reagents streams to the TRU-resin column. In typical operations, a reagent solution is selected by connecting it to the main line of the multiposition valve and then delivered to the sorbent column for a given period of time at a specified flow rate. Next, the pump is stopped, the multiposition valve is switched to the next reagent line, and the flow is resumed to deliver the selected reagent solution to the column. The eluent switch-delivery sequence is performed until the separation procedure is complete. The detection or fraction collection cycle is started by a signal from the PC controlling the FIA system. Dispersion and injected zone residence time were characterized by injecting varying volumes of 0.01% bromothymol blue dye in 0.02 M borate buffer and measuring the transient absorbance signal of the injected dye zone. The key dispersion parameter of the FI system is S1/2, which is the injected volume where the peak maximum of the dye zone is one-half the absorbance of the undiluted dye solution. This injected volume results in a maximum dilution of the carrier stream by a factor of 2.28 For this system, the value of S1/2 was found to be 131 µL by plotting the natural logarithm of 1 minus the ratio of the peak absorbance of the dispersed zone to the absorbance of the undiluted dye solution, as described previously.33 Despite noticeable back-pressure in the FI system with a packed column, reproducible and accurate time-based injections of even relatively small volumes could be achieved with appropriate adjustment of the peristaltic pump occlusion and regular flow rate calibrations. With a 100 × 4.6 mm column and a flow rate of 1 mL/min, the accuracy of five consecutive 250-µL injections was better than 2%, while the relative standard deviation was less than 0.6%. Separation Approach. Actinide elements in the III, IV, and VI oxidation states are strongly retained on TRU-resin in nitric acid media. At greater than 1 M nitric acid concentration, capacity factors, k′, range from ∼100 for the least-retained Am(III) to over 104 for the most strongly retained tetravalent actinides. Trivalent actinides, on the other hand, show practically no retention (k′ < 1) in hydrochloric acid solutions of up to approximately 5 M concentration, while tetravalent actinides and hexavalent U remain strongly retained under these conditions, with k′ > 103 in 4 M HCl.6,23 In addition, all of the retained actinides can be eluted from this sorbent material using complexant eluents such as oxalic acid or ammonium hydrogen oxalate (bioxalate).22,23,25 Sequential individual separation of Am and Pu can be carried out by loading the sample in nitric acid, washing the column with additional nitric acid to remove matrix and interferences (e.g., fission products), adjusting the Pu valence to the IV-state with an oxidizing agent, eluting the Am(III) with hydrochloric acid, and then selectively recovering Pu by reduction to Pu(III) and eluting in hydrochloric acid.5,23,25 Other tetra- and hexavalent actinides such as U and Th will remain on the column under these conditions and can be recovered using ammonium bioxalate complexing eluent.23,25 Iron is likely to be present in a wide range of actinide sample matrixes, and Fe(III)-Fe(II) reduction is necessary prior to the sample loading to eliminate the suppressing effect of Fe(III) ion (33) Pollema, C. H.; Ruzicka, J. Anal. Chem. 1994, 66, 1825-1831.

Table 1. Reagent Delivery Sequence Used in Am, Pu(III) Elution Experimentsa-c step 1 2 3 4 5

description column conditioning sample load column wash/ matrix removal trivalent actinide elution complexing wash

reagent 2 mL to 2 M HNO3 0.25 mL of 241Am or 239Pu(III) in 2 M HNO3 2 mL of 2 M HNO3 5 mL of 4 M HCl (Am experiment) 10 mL of 4 M HCl (Pu experiment) 5 mL of 0.1 M NH4HC2O4

a 50 × 3 mm sorbent column. b Flow rate, 1 mL/min. c Detection cycle was initiated in the beginning of step 4 and terminated at the end of step 5.

on Am retention in nitric acid during the sample load.23 The presence of ferrous ion in the sample determines that plutonium in the sample will be in a reduced state and will be retained on the column as Pu(III).11,12,34 Without the Pu(III)-Pu(IV) oxidation step described above, selective Am elution is not possible, because both Pu(III) and Am(III) will be removed from the column with 4 M HCl solution. After removal of Am(III), the reduction step mentioned above is used to elute Pu. The overall separation procedure is critically dependent on the effectiveness of these oncolumn redox reactions, and successful automation with reproducible recoveries requires that these reactions be well understood and reliable under the conditions of the automated protocol. On-Column Pu(III)-Pu(IV) Oxidation. Sodium nitrite is known to be an efficient and fast oxidizing reagent to convert Pu(III) to Pu(IV) in homogeneous acidic solutions, and it does not further oxidize Pu to higher valence states.12,34 In a manual opencolumn separation format, a solution of nitrite in 2 M nitric acid is used as the column wash after the sample load to perform the Pu(III)-Pu(IV) valence state adjustment.25 Because nitrite is not stable in acid, this solution must be prepared fresh before use (a 100-µL aliquot of 0.1 g/mL NaNO2 solution in water is added to 5 mL of 2 M HNO3). Using the reagent delivery sequence listed in Table 1, we first verified that Pu is present in the sample as Pu(III) and that it is not retained on the TRU-resin column in 4 M HCl. We then developed the following injection sequence to automatically prepare acidic nitrite solution from a stable aqueous solution of sodium nitrite and to immediately deliver this solution to the column for Pu (III)-Pu(IV) oxidation. After loading the Pu(III) sample and washing the column (Table 1, steps 1-3), a 125-µL aliquot of 0.5 M NaNO2 aqueous solution was injected, followed by 1 mL of 2 M nitric acid. An additional 125 µL of 0.5 M sodium nitrite was injected, followed by 4 mL of 2 M nitric acid solution. The hydrochloric acid wash was then performed to elute any remaining trivalent Pu(III), followed by a bioxalate column wash to remove Pu(IV) (Table 1, steps 4 and 5). Figure 2 illustrates Pu behavior without (trace A) and with (trace B) the on-column oxidation procedure. Without oxidation, Pu(III) elutes in the HCl eluent, as does Am(III). With on-column oxidation, no Pu activity was evident in the HCl eluent, and Pu was quantitatively eluted (34) Cleveland, J. M. The Chemistry of Plutonium; Gordon and Breach, Science Publishers: New York, 1970.

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Table 2. Reagent Delivery Sequence for Pu Reduction Experimentsa-c step

description

reagent

1 2 3 4

column conditioning sample load column wash oxidation procedure (performed in triplicate) column wash trivalent actinide elution Pu reduction-elution complexing wash

2 mL of 2 M HNO3 0.25 mL of 239Pu in 2M HNO3 2 mL of 2 M HNO3 125 µL of 0.5 M NaNO2-1 mL of 2 M HNO3 4 mL of 2 M HNO3 5 mL of 4 M HCl 15 mL of reductant in 4 M HCl 5 mL of 0.1 M NH4HC2O4

5 6 7 8

a 50 × 3 mm sorbent column. b Flow rate, 1 mL/min. c Detection cycle was initiated in the beginning of step 7 and terminated at the end of step 8.

Figure 2. Detector traces showing Pu(III) elution behavior using 4 M HCl eluent (trace A) and the behavior of Pu after the nitrite oxidation cycle has been employed to convert Pu(III) to Pu(IV) (trace B, vertical offset for clarity). Am(III) coelutes with Pu(III) as in trace A (not shown). Time zero corresponds to the beginning of the 4 M HCl wash. Peak areas in traces A and B are different due to lower detection efficiency in bioxalate eluent. Sorbent column, 50 × 3 mm (free column volume, 0.235 mL); separation flow rate, 1 mL/min.

with 0.1 M ammonium bioxalate solution. (The bioxalate solution will elute any actinides remaining on the column22 and is used in this and subsequent experiments to recover any actinides remaining on the column after the previous steps.) Therefore, the oncolumn oxidation achieves the required Pu valence state adjustment so that Am can be separated from Pu originally present in the sample in the trivalent state. The injection sequence of 125 µL of 0.5 M NaNO2-1 mL of 2 M HNO3 provides acidic oxidizing reagent solution prepared fresh on-line. Selection of the injected reagent volume of 125 µL (S1/2 ) 131 µL) sequenced with 1 mL of carrier solution ensures that the carrier stream is never diluted by the injected reagent more than by a factor of 2. This is important since Am(III), the least strongly retained actinide, is poorly retained on TRU-resin in less than 0.5 M nitric acid solutions. The contact time of the injected oxidant zone with the sorbent column is at least 1 min, assuming a 100-fold oxidizing reagent excess and no nitrite retention. Because nitrite is retained to some degree on the column (see below), the actual contact time is longer. To ensure reliability of the on-line Pu(III) oxidation, we used a 125 µL of 0.5 NaNO2-1 mL of nitric acid injection sequence performed in triplicate in all subsequent experiments. Selective Pu Elution via On-Column Pu(IV)-Pu(III) Reduction. After oxidizing the Pu(III) to Pu(IV) and eluting Am(III) with 4 M HCl, the next step is to reduce Pu(IV) back to Pu(III). The Pu(III) then elutes in the hydrochloric acid medium used for the reduction. Other tetravalent and hexavalent actinides remain on the column. Horwitz et al. described the use of 0.1 M hydroquinone solution in 4 M HCl to selectively remove Pu from TRU-resin,23 and a similar approach has been adopted in the manual separation procedure employed at PNNL.5,25 However, low and inconsistent Pu recoveries were often observed when using 0.1 M hydroquinone solution in 4 M HCl to elute Pu from the TRU-resin column.35 Pu reduction experiments were performed using the reagent delivery sequence given in Table 2. When we used 0.2 M (35) Fadeff, S. K., Personal Communication.

3924 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998

Figure 3. Detector traces showing selective Pu(IV) reductionelution using 0.2 M solution of hydroquinone in 4 M HCl with (a) and without (b) prior nitrite oxidation procedure. Time zero corresponds to the beginning of the Pu elution step. Sorbent column, 50 × 3 mm; separation flow rate, 1 mL/min.

hydroquinone solution in 4 M HCl to reduce and elute Pu from a TRU-resin column, we obtained the results shown in Figure 3a. The plutonium elution proceeded slowly, giving a tailed elution profile. The reduction-elution process was incomplete in 15 min or with over 65 free column volumes of reducing eluent, as indicated by by substantial amount of Pu radioactivity eluted with bioxalate at the end of the procedure. The slow kinetics of the on-column reaction with hydroquinone (revealed using the FIA system with on-line detection) could be the cause for low and irreproducible plutonium recoveries observed in manual batch separations. We set out to find a more efficient and reliable means of performing on-column Pu(IV) reduction. Reduction with hydroquinone could be improved by performing the reaction at higher temperatures, as shown in Figure 4. Pu elution improves noticeably as the temperature is increased from 23 (Figure 3a) to 55 (Figure 4a) and 65 °C (Figure 4b). The Pu(III) peaks are sharper, and there is less unreacted Pu(IV) left on the column (as indicated by the reduced size of the peak eluted by bioxalate at the higher temperatures). Nevertheless, raising the temperature may not be the optimal method of improving the reduction, because the negative enthalpy of Am extraction with

Figure 4. Detector traces (update time, 12 s) showing Pu(IV) reduction-elution using 0.2 M hydroquinone-4 M HCl eluent at 55 (a) and 65 °C (b). (See Figure 3a for comparison at room temperature.) Sorbent column, 50 × 3 mm; separation flow rate, 1 mL/min.

CMPO results in decreased extraction of Am from the sample at elevated temperatures.36 The equilibrium distribution coefficient for Am, the least-retained actinide on TRU-resin, is reduced by a factor of 2 when the temperature is increased from 24 to 50 °C.6 We also observed that Pu reduction and elution using mild reducing agents (ascorbic acid and hydroquinone were tested) could be improved by using a slow flow rate through the column or even stopping the flow, providing longer reaction times (data not shown). Nevertheless, long reaction times are disadvantageous, since they lead to increased separation time. Therefore, the best solution appeared to be finding an alternative reducing agent. We required a reducing agent that would rapidly convert Pu(IV) to Pu(III) under conditions where Pu(IV) is strongly complexed in the stationary organic phase of TRUresin. In addition, we have found that prior on-column Pu oxidation with nitrite affects the rate and effectiveness of the Pu(IV) reduction-elution process (see Nitrite Effects section below). Using our FI system with on-line detection, we empirically evaluated several widely used reducing reagents known to provide “instantaneous”, “very fast”, and “fast” Pu(IV)-Pu(III) reduction in aqueous acidic solutions.12,13,34,37 Detector traces corresponding to Pu elution experiments using selected reducing reagents are shown in Figure 5. Just as in the case of hydroquinone (Figure 3a), Pu(IV) reduction with 0.2 M solutions of semicarbazide (Figure 5a) and ascorbic acid (Figure 5b) proceeded slowly and resulted in wide and severely tailed elution profiles. The reduction-elution process was not complete in 15 min or with over 65 free column volumes of these reducing eluents. We also tested 0.05 M SnCl2 and 0.2 M NaI solutions in 4 M HCl as potential Pu(IV) reducing reagents, but unacceptably slow Pu elution with these reagents was observed in both cases (data not shown). On the other hand, reduction with 0.05 M (36) Horwitz, E. P.; Diamond, H.; Martin, K. A.; Chiarizia, R. Solv. Extr. Ion Exch. 1987, 5, 419-446. (37) Newton, T. W. The Kinetics of the Oxidation-Reduction Reactions of Uranium, Neptunium, Plutonium, and Americium in Aqueous Solutions; ERDA Technical Information Center: Oak Ridge, TN, 1975.

Figure 5. Detector traces showing selective Pu(IV) reductionelution using 4 M HCl solutions of various reducing reagents: (a) 0.2 M semicarbazide, (b) 0.2 M ascorbic acid, (c) 0.02 M titanium trichloride, (d) 0.1 M ascorbic acid-0.05 M ferrous sulfamate. Time zero corresponds to the beginning of the Pu elution step. Sorbent column, 50 × 3 mm; separation flow rate, 1 mL/min.

ferrous sulfamate-0.1 M ascorbic acid (Figure 5c) and 4 M HCl solutions of 0.02 M Ti(III) (prepared from TiCl3) (Figure 5d) gave sharp and complete Pu elution without significant tailing. The TiCl3-based eluent is easier to prepare (dilution of commercially available solution), and it was found to be directly compatible with R-spectroscopy sample preparation using Nd fluoride microprecipitation procedure. In addition, we observed that TiCl3-based eluent performs more reliably as compared to ferrous sulfamateascorbic acid eluent when the separation is carried out on larger columns. Visual observations in the manual format suggested that Ti(III) is retained on TRU-resin in hydrochloric acid media; the column turns violet upon the passage of 0.02 M Ti(III)-4 M HCl solution, and the color is only gradually removed with 4 M HCl wash. We estimated the distribution ratio for Ti(III) extraction from 4 M HCl with 0.75 M CMPO solution in TBP to be 21 (see Experimental Section). This corresponds to the capacity factor k′ ) 5 for Ti(III) retention on TRU-resin columns used in this study. In separate experiments, we established that excess Ti(III) does not displace trace quantities of U or Th from the column during Pu elution. Scaling the Size of a Sorbent Column. A small column size (50 × 3 mm, 0.35 mL volume) was convenient for the initial Analytical Chemistry, Vol. 70, No. 18, September 15, 1998

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Figure 7. Detector traces showing effects of column crossover processes on Am elution using 4 M HCl (column size, 100 × 4.6 mm; free column volume, 1.1 mL). Trace A (UV-visible, λ ) 300 nm) corresponds to nitrate concentration profile in the eluent (left y-axis). Radioactivity detector trace B (right y-axis) shows the corresponding Am elution profile. Time zero corresponds to the beginning of the 4 M HCl wash. Flow rate, 1 mL/min.

Figure 6. Detector traces showing (a) Am(III) elution with 4 M HCl and (b) Pu(IV) reduction-elution using 0.02 M TiCl3-4 M HCl eluent. Traces A, B, and C on both plots correspond to elution experiments performed using 50 × 3 mm, 100 × 3 mm column; and 100 × 4.6 mm sorbent column, respectively. Traces B and C are offset for clarity on both plots. Time zero corresponds to the beginning of the Am (a) and Pu (b) elution steps, respectively. Separation flow rate, 1 mL/ min.

investigation of the on-column reaction procedures. In analytical applications, however, it may sometimes be desirable to use a larger column (e.g., to preconcentrate actinides from a large sample volume). The effect of the column size on Am and Pu elution behavior was investigated using 50 × 3 mm, 100 × 3 mm, and 100 × 4.6 mm columns, giving column bed volumes of 0.35 (0.15 g of sorbent), 0.70 (0.30 g of sorbent), and 1.66 mL (0.71 g of sorbent), respectively. Note that bed volume of a conventional open column TRU-resin cartridge is 2 mL (0.74 g of TRU-resin). Detector traces in Figure 6a demonstrate the effect of column size in Am elution experiments using 4 M HCl. Elution time increases as the column size increases, as might be expected. However, there is no significant increase in the Am peak width at the baseline in terms of time or volume (1 mL/min flow rate in each trace). The retention volumes observed for Am are greater than the free column volumes. These would be expected to be the same when k′ < 1 if the eluent does not interact with the sorbent and the column crossover to the next eluent solution occurs rapidly. However, both nitric acid and hydrochloric acid are known to be extracted by the CMPO-TBP organic phase,36 so additional 4 M HCl is required to complete the column crossover from nitric to hydrochloric acid, resulting in the delay in Am elution. This effect is illustrated in Figure 7 using a 100 × 4.6 mm TRU-resin column. Trace A (UV-visible) corresponds to the nitrate concentration profile as the eluent is switched from 3926 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998

2 M nitric acid to 4 M hydrochloric acid. At first, the nitrate concentration promptly drops to approximately 0.4 M, reflecting the physical dispersion in the FI system (trace A, 1). However, beginning at approximately the 3 min mark, the nitrate concentration in the eluent decreases slowly (trace A,2) as retained nitric acid is back-extracted and eluted from the column. The Am elution (trace B) does not begin until the nitrate concentration in the eluent has been reduced below ∼0.1 M. In the column size scaling experiments for Pu elution, Ti(III) was used as reducing reagent (Figure 6b). Column conditioning, sample load, column wash, and on-line oxidation were performed as described above. The column was washed with 5 mL of 4 M HCl for the 50 × 3 mm column and 10 mL of 4 M hydrochloric acid for larger columns to simulate Am elution. The Pu peak positions shifted to longer elution times with increased column size. Pu elution also required more eluent volume than free column volumes would dictate (e.g., four additional free column volumes of reductant was required before Pu elution begins on the 100 × 4.6 mm column (Figure 6b, trace C). Delay in Pu elution reflects column crossover from oxidizing to reducing conditions (see the following section). There was essentially no change in Pu peak width at baseline in terms of time or volume as the column size was increased by a factor of 5. Therefore, Am-Pu separation can be effectively performed on larger columns without a significant degree of elution peak broadening. Nitrite Effects. A number of observations prompted us to examine the fate of nitrite injected onto the column and its effect on subsequent reduction and elution of Pu. We found that (1) nitrite partitions into the organic phase on the column, (2) nitrite interacts to a certain degree with polymer support of the TRUresin, and (3) after removal of the elutable nitrite, the column still exhibits oxidative properties as a result of a nitrite-polymer support interaction. Batch liquid-liquid extraction experiments revealed that nitrite is extracted by 0.75 M CMPO-TBP phase from 2 M HNO3 (D ) 19) and is strongly extracted from 4 M HCl (D ) 171). These

distribution ratios correspond to nitrite capacity factors k′ ) 5 and k′ ) 44 for nitrite retention on a TRU-resin column in 2 M HNO3 and 4 M HCl, respectively. In column studies with UV-visible detection using a 50 × 3 mm TRU-resin column, we measured k′ for nitrite to be 18, 12, 9, and 30 for 0.5 M HNO3, 1 M HNO3, 2 M HNO3, and 4 M HCl, respectively. Only about 70% of the injected nitrite was recovered in the elution band from nitric acid solutions, while nitrite elution with 4 M HCl (k′ ) 30) was severely tailed and incomplete with over 65 free column volumes. We have also observed that the TRU-resin column turns blue after treatment with nitrite.38 We also established that acidic nitrite solution interacts with the polymer support alone (acrylic ester polymer particles identical to TRU-resin but with no immobilized extractant, obtained from EIChrom.). In batch experiments, white polymer support particles turned light blue when exposed to nitrite solutions in nitric acid, and the color persisted after the polymer support particles were extensively washed with water. In column injection experiments using a 100 × 4.6 mm column packed with polymer support particles, nitrite was retained in 2 M HNO3 with k′ ) 3. Only 40% of the injected nitrite was recovered in the elution band. Therefore, the interaction of nitrite ion with TRU-resin involves multiple processes, including extraction by the CMPO-TBP organic phase and a certain degree of irreversible interaction with the polymer support material (speciation of reaction products is unknown). On the small column (50 × 3 mm TRU-resin column) used for the majority of experiments described above, the 2 M HNO3 column wash after the nitrite oxidation (Table 2) is sufficient to remove the bulk of the elutable nitrite prior to the Pu reduction step (Table 2, step 5). This was confirmed in experiments with UV-visible detection to observe the nitrite elution. However, the nitrite-treated column still exhibits oxidative properties that affect subsequent Pu reduction-elution when using mild reductants such as ascorbic acid or hydroquinone. This was demonstrated by examining the elution of Pu from the small column using hydroquinone as the reductant but omitting the prior oxidation step. These results are shown in Figure 3b. The column was loaded with a solution containing Pu(IV), and any residual Pu(III) present was removed with the 4 M HCl eluent (Table 2, step 6). In the absence of oxidation procedure, the Pu(IV)-Pu(III) reduction-elution proceeds noticeably faster and with less tailing, as compared to the Pu elution with oxidation procedure in place (Figure 3a). We further observed that incorporation of nitrite scavengers12 (both sulfamic acid and hydrazine were tested) in the reducing eluent in experiments with the prior oxidation procedure in place did not result in improved Pu elution using hydroquinone or ascorbic acid. We speculate that the observed effects are due to a nitrite-polymer support interaction product with oxidative properties, but which is not efficiently scavenged by sulfamic acid, hydrazine, or mild reductants. Based on the measured nitrite capacity factors, the volumes of nitric acid and hydrochloric acid employed after the oxidation procedure (Table 2) are insufficient to elute the bulk of the retained nitrite prior to the Pu reduction-elution step on a 100 × 4.6 mm column. Using Ti(III) as reductant, we compared the Pu (38) One of the known decomposition products of nitrous acid, dinitrogen trioxide, is blue.

Table 3. Reagent Delivery Sequence for Am-Pu-U/Th Separation experimentsa-c step

description

1 2

column conditioning sample load

3

column wash/matrix removal Pu oxidation (performed in triplicate) Am elution Pu elution

4 5 6 7 8 9

U(VI), Th(IV), Np(IV) elution column cleanup wash column wash

reagent 5 mL of 1 M HNO3 1 mL of mixed actinide standard in 2 M HNO3 12 mL of 1 M HNO3 125 µL of 0.5 M NaNO2-1 mL of 1 M HNO3 10 mL of 4 M HCl 15 mL of 0.02 M TiCl3 in 4 M HCl 10 mL of 0.1 M NH4HC2O4 10 mL of 0.5 M NH4HC2O4 5 mL of water

a 100 × 4.6 mm sorbent column. b Detection or fraction collection cycle was initiated in the beginning of step 5 and terminated at the end of step 7. c Steps 3 and 4 provide greater than 15 mL of column wash to remove fission products.

elution behavior on a 100 × 4.6 mm column with and without a prior oxidation step. When the oxidation procedure was not used, Pu elution began approximately 3 min earlier, and the elution peak was slightly narrower (data not shown), as compared to elution after nitrite. Using nitrite scavengers sulfamic acid and hydrazine in combination with the Ti(III) reductant also resulted in an approximately 3-min shift toward shorter elution time in experiments where the nitrite oxidation step was included. (The peaks were also noticeably tailed, results not shown.) These results confirm the presence of retained nitrite on the large column when Pu reduction is initiated, which can influence the effectiveness and elution time in the Pu reduction elution process. As shown in Figure 6b, trace C, the Ti(III) reductant provides effective Pu elution even when nitrite is still present on the column. Flow Rate Scaling. Since our ultimate goal is rapid analytical separations, we examined the influence of flow rate on the success of these separations involving on-column reactions. A mixed actinide standard solution in 2 M nitric acid containing 5.03 × 104 dpm/mL of 241Am, 5.02 × 104 dpm/mL of 239Pu, 2.49 × 104 dpm/mL of 230Th, and 2.47 × 104 dpm/mL of 233U was prepared for the sample, which was used in 1-mL aliquots. We used the procedure described in Table 3 designed to load sample, wash the column, oxidize all Pu to Pu(IV), elute Am, reduce and elute Pu, and finally elute remaining actinides with complexant eluent using a 100 × 4.6 mm sorbent column. Samples were analyzed at various flow rates, but to ensure consistent dispersion of the injected nitrite zones in the Pu oxidation cycle (Table 3, step 4), nitrite injection was always performed at a 1 mL/min flow rate. Separation experiments (all steps in Table 3, except for step 4) were performed at 1, 2, and 3 mL/min volumetric flow rates, which correspond to 7.9, 15.8, and 23.7 mL cm2 min-1 linear flow velocity through a column bed (void volume ratio 0.67). To assess the reproducibility of the analytical procedure and separation recovery, the experiments at all flow rates were performed in triplicate. Detector traces at the three different flow rates are compared in Figure 8. These are all plotted on the same scale in terms of eluent volume; time is shown in the top x-axis on each plot. The results show that the separation speed can be increased by at least a factor of 3 without significantly increasing the elution peak Analytical Chemistry, Vol. 70, No. 18, September 15, 1998

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Table 4. Radiochemical Analysis of the Separated Actinide Fractionsa,b separation flow rate 1 mL/min 241Am

recovery, % other detected activity, % carryover activity, %

239Pu

recovery, % other detected activity, % carryover activity, %

230Th

recovery, % recovery, % other detected activity, %

233U

2 mL/min

Am Fraction 100 ( 4 100 ( 4 none detected non detected

102 ( 4 non detected

0.06

0.09

0.07

Pu Fraction 98 ( 3 98 ( 3 241Am, 0.03 0.15

101 ( 3 241Am, 0.13

0.16

0.40

241Am,

0.13

Th-U Fraction 80 ( 3 86 ( 3 82 ( 3 88 ( 3 239Pu, 0.73 239Pu, 1.70 241Am,

carryover activity, %

3 mL/min

1.2

1.1

0.16

93 ( 4 73. ( 3 239Pu, 1.1 241Am,

0.06

1.2

a Recovery errors are given as (2σ (n ) 3) and calculated from the on-line detection data. b Carryover activity is given as total activity detected in the blank run immediately following the sample run.

Figure 8. Detector traces (update time, 6 s) showing Am-Pu-U/ Th separation on 100- × 4.6-mm column performed at 1 (trace A), 2 (trace B), and 3 mL/min flow rate (trace C). Sorbent column, 100 × 4.6 mm. Scintillation cocktail:column eluate ratio was 1:1, except for the separation run at 3 mL/min, where a 3:2 ratio was used to avoid sporadic chemoluminescence events observed with the 1:1 ratio. Time zero corresponds to the beginning of the Am elution step.

volume. The Pu elution peak at the highest flow rate was slightly shifted toward longer retention. This shift likely reflects the overall kinetics of the redox column crossover and Pu reduction process, which is different from the simpler column crossover process governing the Am and U-Th elution. As expected for the extraction-based separations,39,40 some degree of peak broadening is observed at higher flow rates (Figure 8b,c). However, this peak broadening is insignificant, and actinide elution peaks remain well separated. Using the same reagent delivery sequence (Table 3), additional experiments at the same three flow rates were performed, and the separated actinide fractions were collected for subsequent (39) Sarakitbanharn, Y.; Muralidharan, S.; Freiser, H. Anal. Chem. 1991, 63, 2642-2645. (40) Horwitz, E. P.; Bloomquist, C. A. A. J. Inorg. Nucl. Chem. 1972, 34, 35813871.

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radiochemical analysis. To evaluate carryover, each separation run was followed by a blank run performed under identical conditions, except that the sample load step was omitted. Onemilliliter aliquots of the collected fractions were analyzed by liquid scintillation spectrometry to estimate the total amount of R-radioactivity present in each fraction. To obtain actinide speciation data, 100-µL aliquots of the fractions collected during the separation runs were analyzed by R-spectrometry. Plutonium fractions obtained during the separation run were also analyzed by γ-spectrometry to estimate the amount of 241Am activity coeluted with Pu. The results of radiochemical analysis are summarized in Table 4. The recoveries for Am and Pu were quantitative, and separation factors of at least 2 orders of magnitude were obtained for Am and Pu at all flow rates. The results obtained with on-line and off-line detection indicate that Am-Pu separation can be performed rapidly (Am-Pu elution time under 10 min) with linear flow velocities that are at least 30 times higher than those typically used in manual gravity flow separations. In addition, our results also indicate that the columns may be successfully reused, as suggested previously,23,30,31 provided that the observed carryover levels are acceptable for a given application. Discussion. We have demonstrated that a FI instrument with on-line radioactivity detection is useful for investigating and automating sorbent extraction-based radiochemical separations and that automated procedures involving on-column adjustments of speciation through redox reactions can be developed. Nitrite oxidation of retained Pu(III) to Pu(IV) was automated with the advantage of on-line preparation of an unstable reagent. The procedure required detailed knowledge of the dispersion so that the nitric acid carrier would not be excessively diluted. We optimized a selective Pu elution procedure via on-column reduction. Mild reducing reagents such as hydroquinone and ascorbic acid were found to be unacceptable for selective Pu reduction/ elution. A replacement, Ti(III), was found that provided the rapid

and quantitative reaction needed in an automated system. In addition, we found the on-column redox chemistries and elution processes to be far more complex than previously known, with multiple effects related to the nitrite behavior on the column. Capacity factors for the retention of nitrite and Ti(III) were determined. Given both a qualitative and quantitative understanding of the processes on the column, including the capacity factors for oxidants and reductants in addition to the capacity factors for the radionuclides, it is possible to design an automated Pu-Am separation. In addition, the flow rate through the column could be greatly increased relative to gravity flow manual procedures, providing faster separation times. Our ultimate goal is to develop automated systems to support the analysis of radioactive samples from DOE sites, which will require sufficient decontamination (41) Egorov, O.; O’Hara, M. J.; Ruzicka, J.; Grate, J. W. Anal. Chem. 1998, 70, 977-984.

factors for the removal of fission products prior to the elution of the actinides of interest. We will address these issues and describe the analysis of real samples using an improved sequential injection instrument41 for automated actinide separations in a future publication. ACKNOWLEDGMENT The authors acknowledge Professor Jaromir Ruzicka for many stimulating discussions on applications of flow injection analysis. 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 April 24, 1998. 1998.

Accepted July 6,

AC980411M

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