Anal. Chem. 2001, 73, 1112-1119
On-Line Speciation of Uranyl Chelates in Supercritical CO2 by Time-Resolved Laser-Induced Fluorescence Spectroscopy R. Shane Addleman, Mike Carrott, and Chien M. Wai*
Department of Chemistry, University of Idaho, Moscow, Idaho 83844 Tom E. Carleson
Department of Chemical Engineering, University of Idaho, Moscow, Idaho 83844 B. W. Wenclawiak
Department of Chemistry, Universitat-GH Siegen, D-57068 Siegen, Germany
The dynamic emission characteristics of selected uranyl complexes in supercritical (ScF) CO2 have been quantified with time-resolved laser-induced fluorescence (TRLIF). A variety of factors were found to affect the emission from uranyl complexes in ScF including ligand, adduct, pressure, and temperature. TRLIF was shown to allow on-line speciation and quantitative analysis of uranyl complexes with different primary ligands in ScF CO2. The method was applied to real-time analysis of multiligand uranium extractions from aqueous solutions with ScF CO2. Time-resolved laser-induced fluorescence (TRLIF) has been shown to be effective at determination of uranyl complexes in aqueous environments.1-17 With the ability to select excitation wavelength, emission wavelength, and temporal region, TRLIF (1) Kenney-Wallace, G. A.; Wilson, J. P.; Farrell, J. F.; Gupta, B. K. Talanta 1981, 28, 107-113. (2) Fujimori, H.; Matsui, T.; Suzuki, K.; Shotaro, H.; Wada, Y. J. Nucl. Sci. Technol. 1986, 23, 1069-74. (3) Fujimori, H.; Matsui, T.; Suzuki, K. J. Nucl. Sci. Technol. 1988, 25, 798804. (4) Hong, K. B.; Jung, K. W.; Jung, K. H. Talanta 1989, 36, 1095-1099. (5) Moulin, C.; Beaucaire, C.; Decambox, P.; Mauchien, P. Anal. Chim. Acta 1990, 238, 291-296. (6) Moulin, C.; Rougeault, S.; Hamon, D.; Mauchien, P. Appl. Spectrosc. 1993, 47, 2007-2012. (7) Moulin, C.; Decambox, P.; Couston, L.; Pouyat, D. A. J. Nuc. Sci. Technol. 1994, 31, 691-699. (8) Moulin, C.; Decambox, P.; Mauchien, P.; Pouyat, D.; Couston, L. Anal. Chem. 1996, 68, 3204-3209. (9) Brina, R.; Miller, A. G. Anal. Chem. 1992, 64, 1413-1418. (10) Moriyasu, M.; Yokoyama, Y.; Ikeda, S. J. Inorg. Nucl. Chem. 1977, 39, 21992203. (11) Miguel, M.; Formosinho, S.; Cardoso, A.; Burrows, H. J. Chem. Soc., Faraday Trans.1 1984, 80, 1735-1744. (12) Meinrath, G.; Kato, Y.; Yoshida, Z. J. Radioanal. Nucl. Chem. 1993, 174, 299-314. (13) Eliet, V.; Bidoglio, G.; Omeneto, N.; Parma, L.; Grenthe, I. J. Chem. Soc., Faraday Trans. 1995, 91, 2275-2285. (14) Kato, Y.; Meinrath, G.; Kimura, T.; Yoshida, Z. Radiochim. Acta 1994, 64, 107-111. (15) Moulin, C.; Decambox, P.; Moulin, V.; Decaillon, J. G. Anal. Chem. 1995, 67, 348-353.
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provides triple selectivity. Direct speciation of uranyl complexes is possible with TRLIF due to the characteristic changes in emission spectra and fluorescence decay time.10-17 The long fluorescence decay time of uranyl compounds, in some environments over 200 µs, allows easy separation from the emissions of organic compounds such as polyaromatic hydrocarbons, which have fluorescence decay times typically less than 100 ns. TRLIF provides a large dynamic range and is a sensitive technique with detection limits reported near 1 ng/L for uranium.5,9 The sensitivity, selectivity, and large dynamic range are clearly advantageous and TRLIF has been the basis for commercial uranium analyzers.9 Determination of organometallic uranium complexes in supercritical fluids (ScFs) is of interest because of analytical applications and the potential development of a supercritical fluid extraction (SFE) process for nuclear materials. It has been shown that uranium in water or nitric acid can be extracted by ScF CO2containing complexing agents such as tributyl phosphate or β-diketones with efficiencies comparable to the conventional solvent extraction processes.18-22 Due to the high diffusivity and low viscosity of supercritical fluids, uranium in solid materials such as soil, sediments, and mine tailings can also be extracted by ScF CO2-containing suitable ligands.21,22 SFE technology may provide a waste-free method of removing uranium and other metals from solid materials for environmental monitoring and remediation purposes. The potential for industrial-scale waste-free uranium (16) Geipel, G.; Brachmann, A.; Brendler, V.; Bernhard, G.; Nitsche, H. Radiochim. Acta 1996, 75, 199-204. (17) Meinrath, G. J. Radioanal. Nucl. Chem. 1997, 224, 119-126. (18) Lin, Y.; Smart, N. G.; Wai, C. M. Environ. Sci. Technol. 1995, 29, 27062708. (19) Laintz, K. E.; Tachikawa, E. Anal. Chem. 1994, 66, 2190-2193. (20) Meguro, Y.; Iso, S.; Takeishi, H.; Yoshida, Z. Radiochim. Acta 1996, 75, 185-191. (21) Toews, K. L.; Smart, N. G.; Wai, C. M. Radiochim. Acta 1996, 75, 179184. (22) Phleps, C. L. Extraction of Uranium form Uranium Oxides (Uo(X)) Using Beta-Diketones and Alkyl Phosphates Dissolved in Supercritical Carbon Dioxide. Doctoral dissertation, Department of Chemistry, University of Idaho, 1997. 10.1021/ac001039p CCC: $20.00
© 2001 American Chemical Society Published on Web 02/17/2001
extraction with chelate-modified ScF CO2 motivates the development of an on-line measurement technique for uranyl complexes in ScF CO2. TRLIF’s ability to do measurements via optical fibers is particularly advantageous for the nuclear processes where radiation fields can be significant and personnel radiation exposure must be minimized. On-line speciation of uranium complexes in ScF CO2 also has analytical applications. Supercritical fluid chromatography (SFC) is a separation and analysis method typically used for compounds that are not amenable to HPLC or GC,23 and recent work has shown SFC to be useful for the separation of metal complexes.24,25 Detectors traditionally used for SFC, the flame ionization detector (FID) and UV-visible absorbance spectroscopy, are not well suited for measurement of metal complexes in supercritical fluids. For uranyl chelates, these detectors do not provide any significant selectivity and have limited sensitivity. The FID also suffers from metal deposition on detector surfaces due to the decomposition of the organometallic complexes in the flame, which can result in detector instability and blockage of the flame jet. TRLIF should provide a very sensitive on-line SFC detector for uranyl complexes and other luminescent analytes. Recently, we reported the on-line measurement of UO2(NO3)2‚ 2TBP in ScF CO2 and the issues involved in analytical application of TRLIF to a ScF.26-29 Herein we report fundamental TRLIF measurements of selected uranyl chelates in ScF CO2, demonstrate the feasibility of on-line speciation, and discuss TRLIF applications to industrial and analytical on-line ScF measurements. EXPERIMENTAL SECTION Instrumentation. The high-pressure and TRLIF experimental apparatus were previously described in detail.26,27,29 However, modifications to the optical data system were required for on-line speciation studies with TRLIF. The optical detector was a blueenhanced intensified charge-coupled device (ICCD) from Princeton Instruments with 256 × 1024 pixels. The ICCD was read out with a 16-bit Princeton Instruments ST-130 controller at 50 kHz and analyzed with a 486 PC computer. The image intensifier, controlled with a Princeton Instruments 200 pulse generator, was synchronized with the laser trigger pulse, allowing the ICCD to be gated relative to the laser pulse. Speciation with an ICCD requires adjustment of the delay time and gate time on the intensifier. The delay time is the interval between the laser pulse and the beginning of data acquisition, and the gate width is the interval the image intensifier is activated and spectral signal is applied to the CCD. The Princeton Instruments 200 allows adjustment of timing parameters manually or automatically with rapid preprogram values for continuous measurements of different temporal regions. Rapid data collection for on-line speciation was achieved by programming the Princeton Instrument 200 to cycle through the selected temporal regions of interest in coordination (23) Supercritical Fluid Extraction and Chromatography: Techniques and Applications; Charpentier, B. A., Sevenants, M. R., Eds.; ACS Symposium Series 366; American Chemical Society; Washington, DC, 1988. (24) Lin, Y.; Wu, Y.; Smart, N. G.; Wai, C. M. J. Chromatogr., A 1998, 793, 107113. (25) Wenclawiak, B.; Bickman, F. Fresenius Z. Anal. Chem. 1984, 319, 305. (26) Addleman, R. S.; Wai, C. M. Phys. Chem. Chem. Phys. 1999, 1, 783-790. (27) Addleman, R. S.; Wai, C. M. Anal. Chem. 2000, 72, 2109-2116. (28) Addleman, R. S.; Wai, C. M. Radiochim. Acta, in press. (29) Addleman, R. S.; Hills, J. W.; Wai, C. M. Rev. Sci. Instrum. 1998, 69, 31273131.
with the detector array readout. Intensity measurements were found to have a standard deviation of less than 3%, and decay times typically had a standard deviation of less than 2%. A detailed methodology for determination of fluorescence lifetimes in ScF with an ICCD system was previously reported.27 Fluorescence lifetime measurements were typically conducted so that correlation coefficients (R2) were greater than 0.99 over a minimum of 3 τ. High-precision τ measurements, for analysis of complexes with differing adducts, were conducted so that the correlation coefficients (R2) for τ determinations were greater that 0.996, with a minimum of 4 τ. Scanning fluorometry was done with a Hitachi F-4500 at 60 nm/min scan rate with a slit width of 10 µm. Scanning fluorometry data were collected in n-hexane, a liquid with solvent properties similar to ScF CO2. Absorption spectra were collected by a Cary 1E UV-visible spectrometer (Varian Instruments, Sugarland, TX) with a fiberoptic interface. The optical fibers were connected to a previously reported fiber-optic cell (1-cm path length) built for ScF work.30 All absorption spectra were collected under identical conditions: scanning from 200 to 800 nm, at a rate of 900 nm/min, with a data interval of 1 nm. The spectrometer was operated in doublebeam mode for all experiments. Calibration. Calibration curves were created by injecting a known amount of chelate dissolved in 0.5 mL of hexane into the pump head during filling to produce a solution of a known concentration. Subsequent dilution of the solution by addition of CO2 to the pump allowed a calibration curve to be constructed for a given set of conditions. The presence of a low concentration of hexane, less than 0.2% hexane in CO2 and typically closer to 0.002%, does not significantly altered the ScF CO2 properties. Extraction Equipment and Methods. Liquid extraction studies were performed by placing a liquid extraction cell between the pump and the optical cell. The liquid extraction cell was modified from a commercial SFE vessel (Dionex, 1.0 cm i.d. × 13 cm) with an internal volume of 10 mL. Tubing (1/16 in.) was extended to the length of the vessel, forcing the ScF CO2 to flow from the bottom of the vessel up through the aqueous phase and out the top of the vessel through outlet tubing (1/16 in.). The extraction cell was placed in the water bath with the optical cell for temperature control. Extractions were performed by placing aqueous samples, typically 7 mL of 0.02 M UO2(NO3)2 in 1 N HNO3, in the extraction vessel and allowing ligand-modified ScF CO2 to flow through at 1.0 mL/min. Ligand mixtures were premixed, and known quantities were injected into the pump head as it was filling with liquid CO2 (Oxarc, SFE grade). Reagents. The primary uranyl complexes investigated using the ligands tributyl phosphate (TBP), thenoyltrifluoroacetone (TTA), trioctylphosphine oxide (TOPO), and tributylphosphine oxide (TBPO). Other ligands used include benzoylacetone (BZA), acetylacetone (ACAC), hexafluoroacetylacetone (HFA), 2,2,6,6tetramethylheptane-3,5-dione (THD), and trifluoroacetylacetone (HTFA). UO2(NO3)2‚2TBP was prepared by quantitatively dissolving uranyl nitrate hexahydrate (Aldrich, 99%) in a known volume of TBP (Aldrich, 99%). The uranyl thenoyltrifluoroacetone complex, (30) Carrott, M. J.; Wai, C. M. Anal. Chem. 1998, 70, 2421-2425.
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Figure 1. Absorption spectra of uranyl complexes in ScF CO2 (50 °C, 200 atm) with different primary ligands and TBP as the adduct. The spectral structure was found to be invariant with ScF pressure, temperature, and adduct.
UO2(TTA)2, was synthesized using a previously reported method.31 TBP, TBPO, and TOPO adducts of UO2(TTA)2 were prepared by addition of a slight molar excess of UO2(TTA)2 to a solution of the organophosphorus reagent in hexane and stirring for several hours at room temperature. Upon completion of the reaction, the hexane-soluble UO2(TTA)2‚X adduct (X ) TBP, TBPO, or TOPO) was separated from the residual, hexane-insoluble UO2(TTA)2 starting material, by filtration. The solvent was evaporated to yield bright yellow crystals of UO2(TTA)2‚X. The complexes UO2(BZA)2‚TBP, UO2(HFA)2‚TBP, and UO2(ACAC)2 ‚TBP were synthesized using the same method yielding yellow to orange crystals. RESULTS AND DISCUSSION Absorption Spectra. Absorption spectra of selected uranyl complexes in ScF CO2 with different ligands and TBP as the adduct are shown in Figure 1. Unlike some reported species of polyaromatic hydrocarbons the absorption bands of the uranyl chelate complexes were found not to shift in position or strength with changes in ScF CO2 temperature or pressure for the conditions investigated.32 This spectral stability supports the intuitive view that the fluorescent UO22+ ion is shielded from the ScF by the organic chelates, unlike the polyaromatic hydrocarbons with their π orbitals exposed to the solvation sphere. It can be seen in Figure 1 that the absorbance spectra of these uranyl complexes depend on the primary ligand. The electronic absorption spectrum of the uranyl ion is probably one of the most extensively studied, but understanding remains far from complete.17 Only recently has the electronic structure of UO22+ been (31) Comyns, A. E.; Gatehouse, B. M.; Wait, E. J. Chem. Soc. 1958, 4655-4665. (32) Rice, J. K.; Niemeyer, E. D.; Bright, F. V. Anal. Chem. 1995, 67, 43544357.
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finally resolved. The exact cause of the ligand effects upon the uranyl absorption spectra have been interpreted as a function of complex symmetry changes, ligand field effects upon transitions within the uranyl ion, and charge transfer from a ligand to an empty 5f orbital on the U atom.17,33-39 The similarity between the spectra of β-diketone complexes with a coordination number (CN) of 7 contrasted with the different spectra of UO2(NO3)2‚2TBP (CN 8) strongly suggests that complex symmetry is a very significant factor, but our limited data set cannot narrow the number of proposed mechanisms. We have observed that changing the complex adduct from TBP to TOPO or TBPO does not affect the absorption spectra of the uranyl β-diketone complexes. Absorption spectra independence of adduct is not surprising since changing the adduct does not alter the complex symmetry and the adduct is coordinated weakly in the equatorial position. It has been reported that the equatorial ligand fields have little impact on the potential around the central uranium atom.17,33-35 However, it should be pointed out that the adduct is critical to the complex’s chelation sphere, as the solubility of uranyl β-diketone complexes in ScF CO2 is highly dependent upon the identity of the adduct.30,40 The uranyl β-diketone complexes have broad overlapping absorption spectra that lack any specific signature. Free ligands, always present in excess for any extraction process, absorb in the UV where the uranyl chelate absorbance is strong enough to provide the method with some sensitivity. Consequently, uranyl chelate speciation by UV-visible absorption spectroscopy was not pursued as a ScF on-line speciation technique. Primary Ligand Effects upon Emission. Figure 2 shows normalized TRLIF spectra of uranyl chelates with identical adducts but different primary ligands in ScF CO2, at 50 °C and 200 atm. Table 1 organizes the various TRLIF parameters measured for uranyl complexes with different primary ligands. The primary ligand clearly has a large effect upon the emission spectral structure and decay time of the uranyl complexes. Compared to the uranyl β-diketone complexes (CN 7), UO2(NO3)2‚2TBP (CN 8) has a fundamentally different spectrum and a significantly longer fluorescence decay time (τ). Similar to the absorption band structure, it would seem that the symmetry of the complex strongly affects emission characteristics of the uranyl chelates. Some uranyl chelates were found to be nonluminescent for λex ) 355 nm. For excitation at 355 nm, the nonluminescent uranyl complexes included those with the ligands BZA, ACAC, THD, and HTFA. Examination of the uranyl complexes by scanning fluorometry found that excitation at wavelengths other than 355 nm would result in luminescence. Table 2 shows the excitation and emission maximums of the uranyl complexes in n-hexane, a (33) Denning, R. G. Struct. Bonding 1992, 79, 215. (34) Goerrler-Walrand, CH.; Jaegere, S. D. Spectrochim. Acta Part A 1972, A28, 257-268. (35) Goerrler-Walrand, CH.; Vanquickenborne, L. G. J. Chem. Phys. 1971, 54, 4178-4286. (36) Rabinowitch, E.; Belford, R. L. Spectroscopy and Photochemistry of Uranyl Compounds; Pergamon Press: Macmillan Co.: New York, 1964. (37) Burrows, H. D.; Kemp, T. J. Chem. Soc. Rev. 1974, 3, 139-165. (38) Frimmel, F. H.; Gremm, T. Fresenius J. Anal. Chem. 1994, 350, 7-13. (39) Collman, J. P.; Hededus, L. S. Principles and Applications of Organotransition Metal Chemistry; University Science Books; Mill Valley: CA, 1980; Chapter 3. (40) Addleman, R. S.; Carrott, M. J.; Wai, C. M. Anal. Chem. 2000, 72, 40154021.
Table 3. Effects of Uranyl Chelate Adducts on τ in ScF CO2 at 50 °C τ (ns)
Figure 2. Normalized TRLIF spectra of uranyl chelates in ScF CO2 (50 °C, 200 atm, 10 µM). The primary coordinating ligand clearly has a significant effect upon the emission spectra. The adduct was found to have no effect upon the emission spectra. Table 1. Primary Ligand Effects upon Uranyl Chelates τ in ScF CO2 at 50 °C coorτ (ns) dination compound peaks (nm) 100 atm 200 atm no. UO2(NO3)2‚ 490, 511, 534, 558, 2480 ( 50 2200 ( 50 2TBP 584 UO2TTA2‚TBP 507, 522, 544, 569 50.7 ( 1.0 42.2 ( 0.4 UO2HFA2‚TBP 506, 524, 545, 572 1075 ( 20 830 ( 18
8 7 7
Table 2. Excitation and Emission Maximums of Uranyl Chelates in n-Hexane compound
λex (nm)
λem nm
UO2(NO3)2‚2TBP UO2TTA2‚TBP UO2HFA‚TBP UO2BZA2‚TBP UO2ACAC2‚TBP UO2HTFA2‚TBP UO2THD2‚TBP
452 428 441 454 452 451 452
511 522 524 526 520 520 521
solvent very similar to ScF CO2 in terms of polarity and solubility parameters. The scanning fluorometery data show that the excitation maximums do not vary much. Those complexes that were nonluminescent for λex ) 355 nm were also observed to have significantly lower fluorescence intensities at their respective λex
chelate
adduct formula
100 atm
200 atm
UO2TTA2‚TBP UO2TTA2‚TBPO UO2TTA2‚TOPO
-OP(OC4H9)3 -OP(C4H9)3 -OP(C8H17)3
50.7 ( 1.0 52.9 ( 1.0 55.3 ( 1.0
42.2 ( 0.4 44.0 ( 0.6 45.9 ( 0.2
maximums. Further, it is worth noting that only the poorly fluorescing complexes have methyl groups available upon the complexing ligands. Collectively, these observations suggest that the uranyl complexes with acetylacetone derivates are possibly being quenched by hydrogen abstraction from the ligands’ methyl groups. If needed, TRLIF of the complexes nonluminescent at 355nm excitation might be accomplished with alternative sources such as a dye or OPO laser system. However, these complexes have substantially lower quantum yields and consequently there will be a large reduction in analytical sensitivity. It can be seen in Table 1 that all complexes show a decrease in τ at higher ScF pressures. This is believed to be due to the increase in collisional interactions with the CO2 at higher fluid densities.27 Table 1 shows that the relative quenching due to increased pressure is more for the CN 7 β-diketone uranyl chelates than the CN 8 UO2(NO3)2‚2TBP. This difference might be due to the fact that the CN 8 complex isolates the central uranyl ion more than the CN 7 complexes, thereby reducing solvent interactions and making it less susceptible to ScF pressure quenching. Adduct Effects upon Emission. Changing the adduct of the uranyl chelate was found to have no affect upon the structure of the emission spectra. As previously mentioned, equatorial coordinated ligands have little impact on the potential around the central uranium atom and consequently are not expected to significantly change the emission spectra.17,33-35 However, as shown in Table 3, changing the adduct of a complex did result in small changes in the measured fluorescence decay time. The data in Table 3 suggest the effect of the adduct upon the complex’s τ may be both chemical and steric in nature. Comparison of TBPO with the TOPO, which have nearly chemically identical bonding with the uranyl ion, show the larger adduct (TOPO) has a slightly longer lifetime. This is consistent with an increase in steric shielding and subsequent reduction in solvent quenching. Comparison of the TBP and TBPO complexes indicates a possible chemical effect upon τ. The larger TBP adduct has a slightly smaller τ than TBPO, in contrast to that expected on the basis of steric shielding. However, the phosphate bond of TBP is a stronger electron-withdrawing group (Lewis acid) than the phosphine in TBPO, slightly changing the chemical environment of the uranyl atom and possibly having an effect upon the emission process.42,43 Complexation with other inorganic electron-withdrawing groups has been reported to change τ for the uranyl ion.36,42 (41) Orth, D. A.; Wallace, R. M.; Karraker, D. G. In Science and Technology of Tributyl Phosphate; Schulz, W. W., Navratil, J. D., Kertes, A. S., Eds.; CRC Press: 1984; Vol. 1. (42) Yokoyama, Y.; Moriyasu, M.; Ikeda, S. J. Inorg. Nucl. Chem. 1976, 38, 13291333. (43) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1986; Chapter 3.
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ns delay time. The contribution of UO2(NO3)2‚2TBP to the emission during the 0-250-ns region can be subtracted using the function
1 b FA ) Fexp(∆λ, ∆t) - FB a a
Figure 3. Normalized emission, integrated over the wavelength range 450-650 nm, as a function of time for UO2(NO3)2‚2TBP (τ ) 2200 ns) and UO2TTA2‚TBP (τ ) 42 ns) in ScF CO2 (50 °C, 200 atm, 1.0 mL/min). The primary coordinating ligand clearly has a significant effect upon the emission decay time.
TRLIF Speciation of Uranyl Chelates. When measurement conditions are consistent and the analyte spectra are known, spectral deconvolution allows speciation and quantitative multispecies analysis.15,43 TRLIF has been used to examine speciation of uranium in a number of aqueous solutions.10-17 The temporal capability of TRLIF provides an orthogonal parameter to the optical spectrum increasing the dimensionality of the data. In some cases, analytes can be directly isolated on the spectral-temporal map, eliminating the need for deconvolution. We will show that TRLIF allows on-line ScF speciation and quantitative analysis of uranyl chelates with different primary ligands with judicious selection of timing parameters. The chelates UO2TTA2‚TBP and UO2(NO3)2‚ 2TBP were selected for speciation studies because they are the principal complexes being considered for an industrial uranium SFE, and the process may result in an extraction of a mixture of these complexes.44 When used together, TTA and TBP have been reported to have a synergistic effect upon metal extraction efficiency.19,44-46 The TTA and TBP ligands also allow for selective uranium complexation, and both of the resulting complexes have high ScF solubilities.18-22,24,30,40,41,47 Figure 3 shows the temporal variation of the normalized emission (450-650 nm, 25-ns gate width) of UO2TTA2‚TBP and UO2(NO3)2‚2TBP in ScF CO2 at 50 °C, 200 atm, and a flow rate of 1.0 mL/min. Concentrations of UO2TTA2‚TBP and UO2(NO3)2‚ 2TBP can be determined by measuring the emission intensity in two time regions, 0-250 ns and 250 ns-10 µs (as measured from the trailing edge of the laser pulse). With a decay time 50 times longer than that of UO2TTA2‚TBP, all emission beyond 250 ns can be attributed solely to UO2(NO3)2‚2TBP. Due to the long decay time of UO2(NO3)2‚2TBP, not collecting its emission signal before 250 ns reduces the total signal by only 10% resulting in a negligible loss of sensitivity. Using a calibration curve, with matched ScF and ICCD timing parameters, the concentration of UO2(NO3)2‚2TBP can be directly determined, on-line, from emission (450-650 nm) beyond the 250(44) Smart, N. G.; Wai, C. M.; Phelps, C. L. Chem. Br. 1998, 34, 34-36. (45) Irving, H. M.; Edgington, D. N. J. Inorg. Nucl. Chem. 1960, 15, 158-170. (46) Sagar, V.; Chetty, K. V. Radiochim. Acta 1994, 68, 69-73. (47) Carrott, M. J.; Waller, B. E.; Smart, N. G.; Wai, C. M. Chem. Commun. 1998, 373-374.
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(1)
where FA and FB are the emission (peak areas) of the two species, Fexp(∆λ, ∆t) is the measured intensity for a selected spectral region and time interval, and a and b are experimentally determined coefficients for species A and B and depend on the species quantum efficiency, τ, and the selected detector gating parameters. The coefficients a and b are simply the fraction of the fluorescence emission in the spectral and temporal window being used for analysis. For a specific experimental configuration, these coefficients can be determined from the fluorescence decay profiles of the individual species. For this system and the window selected for analysis (450-650 nm, 0-250 ns), the coefficient a (for UO2TTA2‚TBP) was determined to be 1 while b (for UO2(NO3)2‚2TBP) was ∼0.1. With the true emission signals from UO2TTA2‚TBP derived, the concentration can be found using calibration curves. The calibration curves for UO2TTA2‚TBP (0-250 ns) and UO2(NO3)2‚ 2TBP (250 ns-10 µs) in ScF CO2 are similar to previously reported TRLIF calibration plots of uranyl species in ScF and aqueous solutions.7,27 The emission response of both species (∆intensity/ ∆concn) decreases with increasing concentration. This nonlinear response is believed to be due to a combination of inner filter effects and photodecomposition.26,27 However, for concentration ranges spanning less than two decades, emission response becomes highly linear. The difference in τ for UO2TTA2‚TBP and UO2(NO3)2‚2TBP clearly allows these uranyl chelate species to be directly resolved and analyzed on-line. Since these chelates can be temporally separated, the entire spectral peak area can be integrated, resulting in better signal-to-noise ratios and limits of detection. On-line quantitative TRLIF speciation of a more complicated mix of uranyl chelates is possible if there is sufficient uniqueness to the species emission spectra and decay times. Equation 1 can be generalized for more species and solved sequentially (largest τ to smallest τ) if the τ values are sufficiently different. Speciation with TRLIF of uranyl chelates with the same primary ligand but different adducts was not possible even with highprecision data. Uranyl chelates that differ only by adduct have identical emission spectra and, as shown in Table 3, very similar fluorescence decay times. There is no region on the spectraltemporal map that is unique to the adduct differentiated chelates and consequently these uranyl species cannot be separated and analyzed with TRLIF. The TRLIF speciation method used herein depends on adjustable time gates to determine species concentration and does not require the determination of the decay time(s). Determination of decay times with ICCDs requires acquisition of many spectra, substantially increasing the measurement time and decreasing the ability to measure dynamic changes in the sample stream. Measurement time is an important factor for on-line analysis in a dynamic system since it limits the ability to detect changes in the sample flow stream. In addition, accurate on-line determination of the decay time may not be possible in sample streams that
have rapidly changing concentration or composition. This would be particularly true for lower analyte concentrations that necessitate longer measurement times. Difficulty arises in using TRLIF for on-line work when uranyl chelate concentrations are low and the feature under observation change on time scales near or shorter than the time required for analysis. For example, for a 5-min measurement time, the limit of detection (10σ) for UO2(NO3)2‚2TBP is ∼10-8 M. If ScF flow measurements are desired every 15 s, and two species are being measured, the limit of detection (10σ) for UO2(NO3)2‚2TBP increases to ∼10-6 M. Quenching. The fluorescence decay time and intensity of uranyl chelates has been shown to change as a function of ScF temperature, pressure, and solvent composition.26,27 When matrix conditions shift, resulting in variable quenching, quantitative multispecies analysis has been shown to be possible in aqueous systems with TRLIF using deconvolution and extrapolation back to the initial intensity (I0) which is free of dynamic quenching and proportional to the analyte concentration.8 This method is well suited for instruments utilizing photomultipliers that give real-time emission decay profiles and has been successfully applied to uranium analysis in nitric acid solutions. Unfortunately, fluorescence lifetime determinations with an ICCD requires many measurements, significantly increasing TRLIF analysis times as well as compounding measurement error.27 In addition, accurate on-line determination of the decay time with an ICCD system may not be possible in sample streams that are changing concentration or composition rapidly. This would be particularly true for lower analyte concentrations, which necessitate longer integration times upon the CCD. Consequently, dynamic quenching correction methods based upon extrapolation back to Io are poorly suited for TRLIF instrumentation based upon ICCD detectors. An alternative method to compensate for quenching is to use an empirical correction function to adjust the emission intensity for the quenching. For on-line measurement of UO2(NO3)2‚2TBP in ScF CO2 with an ICCD-based TRLIF system, an intensity correction function has been shown to be accurate to within several percent for changes in ScF CO2 temperature and pressure.27 An intensity correction function is a faster method than extrapolation back to the initial intensity (I0) since its does not require the determination of the decay time. However, a significant drawback to using an intensity correction function is that it requires accurate determination of the quenching factors. If all quenching parameters are not accounted for, the analysis will be inaccurate. An ideal method for on-line TRLIF quenching correction may be a combination of the two systems described above. Using an ICCD TRLIF system while concurrently employing a monochromator with a PMT would allow the simultaneous measurement of both the fluorescence lifetime and spectrum. The spectral intensity could then be corrected back to I0 allowing for sensitive, rapid TRLIF analysis free of errors due to dynamic quenching. Multiligand Extraction. Recent studies have shown that uranium can be efficiently extracted from nitric acid solutions with TBP-modified ScF CO2.18,20,27 β-diketones have been widely employed, and the combination of TTA and TBP has been shown to have a synergistic effect for both conventional and ScF extractions of uranium.41,44-46 Figure 4 shows normalized spectra from on-line TRLIF monitoring of a mixed-ligand extraction of 7.0
Figure 4. Normalized TRLIF spectra from on-line monitoring of the extraction of 0.1 M uranium in 1 N nitric acid with ScF CO2 (150 atm, 50 °C) modified with 0.15 M TBP and 0.05 M TTA. Judicious selection of TRLIF time parameters clearly allows the spectra from the component species (lower two spectra), UO2TTA2‚TBP and UO2(NO3)2‚2TBP, to be resolved on-line.
mL of 0.01 M UO22+ in 1 N nitric acid with ScF CO2, at 150 atm and 50 °C, modified with 0.15 M TBP and 0.05 M TTA. Detector gate parameters clearly allow the spectra from the complexes UO2TTA2‚TBP (0-250 ns) and UO2(NO3)2‚2TBP (0.25-10 µs) to be resolved on-line. Spectral shape and decay times did not show any features that could not be attributed to either UO2TTA2‚TBP and UO2(NO3)2‚2TBP. However, the presence of other uranyl species cannot be entirely ruled out since species that are nonluminescent or have very short decay times (
