Anal. Chem. lQ82, 5 4 , 2475-2477
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Determination of Uranium in Aqueous Samples by Laser-Induced Fluorescence Spectrometry Peter G. Whltkop
E. I . du Pont de
Nemours
CI Company, Savannah River Laboratory, Aiken, South Carolina 29808
A new pulsed laser method sultable for trace analysis of uranlum In plutonium process stream samples contalnlng large quantities of fluorescence quenchers hac3 been developed. This technlque Includes the extractlon of uranlum from an aqueous sample into trl-m-butyl phosphate, with subsequent strlpplng into dilute phosphoric acld. A standard addition method Is Incorporated into the analysls. Analytlcal data show that a detectlon ilmlt of 1 ppb for a particular matrix has been achieved. The prelclsion of the analysis Is In the range of f7 % reiatlve standard deviatlon. Variation of these results with sample matrix and analytical procedure is discussed.
For years, analysis of plutonium solutioins containing trace amounts of uranium has been carried out by fluorescence measurements on pellets made by fusing the uranium sample with salts such as sodium or lithium fluoride. These proce dures usually incorporate an extraction step, where uranium is partitioned between aqueous and organic phases (1-4). The uranium-bearing organic phase is pipettetl onto a salt pellet and fused. A fluorescence signal is measured when the fused pellet is exposed to ultr,sviolet radiation (5). More recently, several techniques involving laser irradiation of samples have been developed. Among these techniques are procedures which coprecipitate uranium with calcium fluoride. This is followed by calciniztion and pellet formation. Uranium fluorescence is monitored from the laser-irradiated pellet (6, 7). A pulsed nitrogen laser has also been used to directly irradiate solutions containing uranium. A standard addition method has enabled this technique to detect part per trillion quantities of uranium with good precision (8). The intensity of fluoyescence from sollutions containing uranium is extremely sensitive to temperature and quenching by various inorganic ions and organic matter (9-11). Acidity can also affect the luminescence of uranyl ions in solution (12). Each of the previously described analytical procedures minimizes these quenching problems by two widely used techniques. In the fusion method, the preliminary extraction separates the uranium from most of the intdering substances. Laser methods usually solve the quenching problem by first diluting the samples to a degree where quenching by impurities no longer interferes with the analysis. In some instances, analytical sensitivity may be lost if the dilution method is employed. When a large dilution of samples containing trace amounts of uranium is required to minimize quenching interferences, the resulting solution may be near or below the detection limit of a particular apparatus. A uranium solution, which is originally within the detection limits of an instrument, imay not produce a sufficient signal for accurate analysis after large dilutions. This report describes a promising technique for the analysis of uranium in plutonium process stream samples that contain large quantities of quenching species. The detection capabilities of a particular apparatus are maximized by eliminating the necessity of sample dilution. This is done by partitioning uranium between an aqueous and organic ]phaseto eliminate 0003-2700/82/0354-2475$01.25/0
fluorescence quenching by impurities. The organic phase is partitioned with dilute phosphoric acid to strip uranium from the organic phase and increase the fluorescence yield of the uranyl ion. The phosphoric acid phase is analyzed by laser fluorometry using a pulsed nitrogen laser (13). Although the technique described here has been tailored for a specific instrument, it should be applicable to other instruments with pulsed laser excitation sources.
EXPERIMENTAL SECTION Apparatus. All fluorescence measurements were taken with the commercially available UA-3 uranium analyzer manufactured by Scintrex Ltd., Concord, Ontario, Canada. This instrument uses a pulsed nitrogen laser source and has been previously described in detail (13). Normally, the UA-3 analyzer is equipped with quartz cells holding approximately 7 mL of sample. For the present study, samples were contained in quartz cells of approximately 4 mL volume. This enabled smaller sample sizes to be analyzed. A Sartorius 2434 balance was used to weigh sample and standard addition solutions. Reagents. Reagent grade chemicals and deionized distilled water were used throughout the procedure. Synthetic uranium solutions containing species were prepared according to the following procedure: 3.7 g of A1(N03)3.9H20(Fisher), 1.3 g of NaN02 (Fisher),and 22.4 g of Fe2(SO4),.9H20(Fisher)were added to 250 mL of water. When the salts were completely dissolved, 66.7 mL of concentrated HN03 (Mallinckrodt), 30 mL of 2 M ferrous sulfamate,and enough water to dilute the solution to lo00 mL were added. This mixture was used to dilute various amounts of uranium standard stock solutions to the appropriate concentrations. The volume of diluent was always much greater than the volume of uranium stock solution. An appropriate standard uranium stock solution was obtained by dissolving U308 (NBS standard 950a) in 6 M HN03 and diluting to the appropriate volume with water. Ferrous sulfamate (2 M) was prepared by dissolving appropriate amounts of iron powder and sulfamic acid in water and diluting to 100 mL. Solutions containing 30% (by volume) of tri-n-butyl phosphate (TBP)(Eastman) in n-hexane and 20% (by weight) of phosphoric acid (Baker) in water were also prepared. Procedure. To a small vial, 3 mL of sample, 2.5 mL of 3 M HN03, 0.5 mL of 2 M ferrous sulfamate, and 3 mL of 30% TBP were added. The vial was sealed with Teflon tape, capped, and shaken for 3 min. After the phases separated, 2.0 mL of the TBP layer were withdrawn and pipetted into another vial containing 3 mL of 20% phosphoric acid. The vial was also sealed with Teflon tape, capped, and shaken for 5 min. If an emulsion of TBP in phosphoric acid formed at this point, the solution was centrifuged for approximately 1min. This procedure dispersed any emulsion that formed. From this solution,2.5 mL of the aqueous phase was removed and pipetted into a preweighed quartz cuvette containing a magnetic stirring bar. The cuvette was weighed again. The uranium analyzer zero adjust was used to zero the instrument with a blank consisting of pure 20% phosphoric acid. The gain of the photomultiplier tube was adjusted so that the fluorescence measurement did not deflect the meter by more than one-third full scale. The instrument was rezeroed with 20% H3P04and a fluorescencemeasurement of the sample was taken. An appropriate amount (usually 10 or 25 WL)of standard uranium solution, in the range of 10 ppm, was added to the cuvette and a total weight obtained. A final fluorescence measurement was made after the solution in the cuvette was stirred with a magnetic stirrer for approximately 20 s. 0 1982 American Chenilcai Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982 Aoueous Pnase
Organic Phase
Sanple
3M
tNOj
+ F.S.'
+
Uraniun
TBP
30,
+ Uranium Uranium C o n c e n t r a t i o n (ppm)
Figure 2. Fluorescence intensity of uranyl ion in 20% H,PO, as a function of uranium content. 20"
H,PC.
+ Uranium
Indicates f e r r o u s sulfamate (2M)
Flgure 1. Schematic flow diagram of sample preparation.
RESULTS AND DISCUSSION In the procedure described above and shown schematically in Figure 1,the 3 M HNOBis a salting agent which supplies the common ion necessary for efficient extraction of uranium from the aqueous phase into the organic phase, The addition of 2 M ferrous sulfamate has been incorporated into the procedure to facilitate the analysis of uranium in solutions containing plutonium. The purpose of this reagent is to reduce plutonium(1V) in solution to plutonium(III), which is not extractable into the organic phase. Although actual uranium samples containing plutonium could not be analyzed because of the lack of a contained instrument, several experiments were performed to determine the amount of plutonium in the phosphoric acid phase. In all cases, a counting techniques showed that a decontamination factor of approximately 1000 can be achieved. Dilute phosphoric acid serves a dual purpose in that it efficiently strips the uranium from the TBP layer and at the same time acts as a fluorescence enhancer for the uranium. Direct determination of uranium in the organic phase could not be done because the TBP/hexane solution totally suppressed the uranium fluorescence. During the stripping process, emulsions usually formed, becoming more severe at greater concentrations of TBP in hexane. A centrifugation step is necessary to break up these emulsions. A series of experiments were also carried out to determine the optimum phosphoric acid concentration for efficient stripping and fluorescence enhancement. These studies involved H3P04 between 15 and 30% and indicated that this large variation in phosphoric acid concentration could be tolerated without any adverse effects on either extraction efficiencies or fluorescence enhancement, No change in the zero point of the instrument could be detected when an extracted blank was substituted for pure 20% phosphoric acid. The calculations required to experimentally determine the uranium concentrationafter extraction and stripping are given by the formula
C is the concentration of uranium after extraction and
stripping, R1 and Rz are the fluorescence intensity measurements before and after standard addition spikes are added. Cz is the concentration of the standard addition spike, while M I and M z are the weights of the sample solution in the cuvette before standard addition, and the weight of the standard addition spike, respectively. MT is the sum of M I and Mz, and df is a dilution factor to express the fact that only a 2.0-mL aliquot of TBP was removed after partitioning with the aqueous sample. The dilution factor for this procedure is 3/2. The uranium concentrationcalculated by eq 1w ill be biased low and must be corrected to take into account the fact that the extraction processes are not quantitative. A calibration curve can be determined to correct for the effect of extraction inefficiency. By use of a series of standard solutions, a plot of the uranium concentration, as determined from eq 1,vs. the actual uranium concentration is constructed. This plot produces a straight line whose slope is equivalent to the overall extraction efficiency of the procedure. The true uranium concentration is obtained by dividing eq 1 by the slope of the calibration curve. Each point of the calibration curve was determined by at least three separate analyses and had a relative standard deviation (RSD) less than 10%. A linear least-squares analysis showed tha the slope was 0.83 f 0.02. The overall efficiency of the extraction and stripping process is, therefore, estimated to be 83%. A 10 ppm standard uranium solution was extracted with 30% TBP n-hexane. HN03 (2.5 mLN3 M) was added to the sample as the salting agent. By use of the dilution method outlined in ref 8, the amount of residual uranium in the aqueous phase could be determined. From these data, an extraction efficiency of 88% was estimated. A partition coefficient, expressed as the ratio of the uranium concentration in the organic phase to that in the aqueous phase, was estimated to be 14.0. This agrees with results for similar extraction conditions listed in ref 14. With this value and that of the overall efficiency, an approximation of the stripping efficiency of the dilute phosphoric acid can be made and has been calculated to be 94%. This less than quantitative stripping may be caused by the presence of small amounts of mono- and dibutyl phosphate in fresh TBP. These impurities can form strong uranium complexes which prevent quantitative stripping. In order to specify the uranium concentration region in which the single point standard addition method is valid, a linear response range has been determined. For these experiments, the laser intensity was reduced by placing a neutral density filter in the optical path of the instrument. Variations of fluorescence intensity vs. uranium concentration over ranges including 5-60 ppm uranium and 0.25-2.5 ppm uranium were
Anal. Chem. 1902, 5 4 , 2477-2487
noted. Figure 2 shows marked deviation from linearity in the 5-60 ppm range. Uranium concentrationsgreater than 70 ppm produced decreasing flluorescence intensity readings. Instrument response was linear for uranium concentrations less than 2.5 ppm. Values greater than 2.5 pprn showed deviations from linearity, indicating that 2.5 ppm uranium is an upper limit to linear response, Solutions with uranium concentrations in the upper ranges of this study would normally require dilution into a linear range to remain on-scale. Seven synthetic samples, ranging in concentration from 7.3 to 73.9 ppb uranium were analyzed four [separate times with an average RSD of f796. The RSD values ranged from *3 to *11%. Samples lower than 1 ppb uranium gave nonreproducible results, indicating that the lower detection limit for this particular matrix is approximately 1 ppb, although UA-3 uranium analyzer specifications show that lower quantities can be detected. Inductively coupled plasma (ICP) analyses of an extracted synthetic solution indicate that the phosphoric acid not only contains uranium but up to 13 ppm iron. This compares wiith a value of less than 0.8 ppm iron in pure 20% H3P04. '[t appears that some iron from the original solution is being carried through the extractions into the final solution. Reference 13 indicates that this concentration of iron would quench fluorescence about 78%. The fact that other cationic and anionic quenclhers may be present in the 20% H3P04(ICP analysis also showed aluminum in the phosphoric acid) may explain why the detection limits are greater than the instrument specificationis. These detection
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limits would also be matrix dependent, changing with both type and quantity of interfering species.
ACKNOWLEDGMENT The author thanks John Young for helpful discussions and for performing the a counting experiments.
LITERATURE CITED (1) Nletzel, 0. A,; DeSesa, M. A. Anal. Chem. 1957, 29, 756. (2) Maeck, W. J.; Booman, G. L.; Elliot, M. C.; Rein, J. E. Anal. Chem. 1958, 30, 1902. (3) Eberle, A. R.; Lerner, M. W. Anal. Chem. 1957, 2 9 , 1134. (4) Palge, B. E.; Elliot, M. C.; Rein, J. E. Anal. Chem. 1957, 29, 1029. (5) Centanni, F. A.; Ross, A. M.; DeSesa, M. A. Anal. Chem. 1958, 28, 1651. (6) Johnston, M. V.; Wright, J. C. Anal. Chem. 1981, 53, 1050 (7) Perry, D. L.; Klalner, S. M.; Bowman, H. R.; Mllanovich, F. P.; Hlrshfeld, T.; Miller, S. Anal. Chem. 1981, 53, 1048. (8) Zook, A. C.; Collins, L. H.; Pletrl, C. E. Mikrochima Acta 1981, 2 , 457. (9) Morlyasu, M.; Yokoyama, Y.; Ikeda, S. J. Inorg. Nucl. Chem. 1977, 39,2211. ( I O ) Moriyasu, M.; Yokoyama, Y.; Ikeda, S. J . Inorg. Nucl. Chem. 1977, 39, 2199. (11) Morlyasu, M.; Yokoyama, Y.; Ikeda, S. J. Inorg. Nucl. Chem. 1977, 39, 2205. (12) Marcantonatos, M. D. Inorg. Chlm Acta 1977, 25, L101. (13) Robblns, J. C. CIM Bull. 1978, 793, 61 (14) Rodden, C. J. "Analysis of Essential Nuclear Reactor Materials"; U S . Atomic Energy Commlsslon, 1964; Chapter 1
RECEIVED for review June 15,1982. Accepted August 18,1982. The information contained in this article was developed during the course of work under Contract No. DE-AC09-76SR00001 with the U.S. Department of Energy.
Matrix and Solvent Effects on the Room-Temperature Phosphorescence of Nitrogen Heterocycles S. M. Ramasamy andl R. J. Hurtubiso" Department of Chemistry, The Unlversl?v of Wyoming, Laramie, Wyoming 82071
Several experimental p,arameters were studied to enhance the room-temperature phosphorescence of nitrogen heterocycles adsorbed on polyacryilc acid-salt mixtures and filter paper. The room-temperature phosphorescence was very sensitive to the salt content of the solid surface and the solvent used to adsorb the phosphor onto the surface. Sodium chloride and methlanoi were partlclularly important for inducing strong room-temperature phosphorescence. The results obtalned partially explain some of the Interactions responsible for room-temperature phosphorescence from nitrogen heterocycles.
The experimental conditions used to induce room-temperature phosphorescence (RTP) from compounds adsorbed on solid surfaces are imlportant in improving the sensitivity and selectivity of RTP. In addition, impoi.tant insights about the interactions responsible for RTP can be obtained by studying chemical and physical interactions of the phosphors. Little has been reported on variation of conditions for inducing RTP. Variables such i3s the solvents used to adsorb the phosphor onto the matrix and salts mixed with the solid matrix have not been studied in detail. Parker et al. ( 1 , Z ) have discussed the physical aspects of RTP and have reviewed 0003-270O/82/0354-2477$0 1.25/0
a variety of conditions needed for RTP. Hurtubise (3) considered interactions responsible for RTP and several experimental conditions for inducing RTP. Schulman and Parker ( 4 ) studied the effects of moisture and oxygen on RTP, and McAleese et al. (5) considered the elimination of moisture and oxygen quenching in RTP. Jakovljevic (6) investigated the effect of a variety of thallium and lead salts adsorbed on filter paper. Cinoxacin was used as a model compound, and Jakovljevic found that thallous fluoride and lead tetraacetate induced strong RTP from cinoxacin. Selective heavy-atom perturbation for analysis of complex mixtures by RTP has been discussed by Vo-Dinh and Hooyman (7). De Lima and de M. Nicola (8) considered the effect of sodium hydroxide concentration, source irradiation time, and temperature during RTP measurements for 1,8-naphthyridinederivatives. Parker et al. (9) investigated the effects of moisture and support treatment on the RTP of pteridines. Sodium acetate treated filter paper was found to enhance the RTP of adsorbed pteridines. The effect of HC1 and HBr concentrations on the RTP of nitrogen heterocycles was studied (IO),and several brands of silica gel chromatoplates were tested for inducing RTP from nitrogen heterocycles (11). The comparison of conditions for RTP of nitrogen heterocycles and aromatic amines adsorbed under a variety of conditions on filter paper, a silica gel chromatoplate, and 0.5% polyacrylic acid 0 1982 American Chemical Society
