Anal. Chem. 1993, 65, 3320-3325
3320
Determination of Trace Lanthanide Impurities in Nuclear Grade Uranium by Coupled-Column Liquid Chromatography Charles A. Lucy,l9+Lutfiye GureliJ and Steve Elchuk AECL Research, General Chemistry Branch, Chalk Riuer Laboratories, Chalk Riuer, Ontario, Canada KOJ 1JO
Impurities such as Sm, Gd, Eu, and Dy will degrade the neutron economy of a nuclear reactor when present even at sub-parts-per-millionlevels, as a result of their high neutron absorption cross sections. Conventional determinations of lanthanide impurities in uranium require 0.5-100 g of uranium. A coupled-column chromatographic procedure has been developed which dramatically reduces the quantity of uranium required. The first column, a semipreparative reversed-phase column, removes the uranium matrix, while the second column, an analytical-scale cation exchange column, concentrates and separates the lanthanides prior to their postcolumn reaction detection with arsenazo 111. The maximum loading of uranium onto the reversed-phasecolumn is determined by the volume overload of the lanthanides rather than the concentration overload of the uranium. Using 20 mg of uranium, a detection limit of 0.02 pglg of U is achieved for Sm, Gd, Eu, and Dy with no interference from transition or alkaline earth metals present in the uranium. INTRODUCTION Trace impurities in nuclear fuel can significantlyaffect its performance in a reactor due to their metallurgical and neutron absorption properties. Thus, it is important to determine and control the concentration of Gd, Sm, Eu, and Dy at sub-parts-per-million levels in uranium owing to their high neutron absorption cross sections.' The classical method for the determination of impurities in uranium is carrier distillation dc arc, but this method lacks sensitivity for those elements which form involatile or refractory species.2 Direct excitation and detection of impurities in uranium samples has been attempted using inductively coupled plasma atomic emission spectroscopy (ICP-AES), but the detection limits for many elements are severelyrestricted by the interferences from the uranium spectral lines.3 Use of inductively coupled plasma mass spectroscopy (ICPMS) eliminates the spectral interference problem, but it is in turn limited by ionization suppression of the analytes by the uranium matri~.~.s Currently, the most common method for determining trace impurities in uranium is to remove the uranium matrix using + Present address: Department of Chemistry, University of Calgary, 2500 University Drive N.W., Calgary, Alberta, Canada T2N 1N4. Present address: Cekmece Nukleer Arastirma ve Egitim Merkezi, P.K. 1 Havaalani, Istanbul, Turkey. (1) Friedlander, G.; Kennedy, J. W.; M a c h , E. S.; Miller, J. M. Nuclear and Radiochemistry, 3rd ed.; Wiley: New York, 1981; Appendix D. (2) Scribner, B. F.; Mullin, H. R. J . Res. Natl. Bur. Stand. 1946,37, 379-389. (3) Pepper, C. E. A Review of Spectrochemical Emission Methods and AssociatedProblemsfor theDeterminationofImpurities inNuclear Grade Uranium; Atomic Energy Commission NLCO-999, 1967. (4) Beauchemin,D.; McLaren, J. W.; Berman, S. S.Spectrochim. Acta 1987,42B, 467-490. 0003-2700/93/0365-3320$04.00/0
an extraction procedure and then to determine the impurities by ICP-AES. Liquid-liquid extraction has been performed with reagents such as tributyl phosphate (TBP)? tri-noctylphosphineoxide (TOPO)? tris(2-ethylhexyl)phosphate (TEHP),"" tri-n-octylamine,'2 triisooctylamine,'3 and N f l dihe~y1acetamide.l~Extraction chromatography using cellulose collectors16 and TEHP,'6J7 and fluoropolymers coated with di-n-hexyl Nfl-diethylcarbamoylmethylenephosphonate," has also been used. These extractionprocesses are time-consuming and labor-intensive and generate waste disposal problems, particularly when samples of enriched uranium are used. Furthermore, significant quantities of uranium (0.5-100 g) are required for these procedures. This work presents a coupled-column high-performance liquid chromatographic (HPLC) method, wherein the uranium matrix is removed on a reverse phase column and the individual impurities are separated subsequently on a cation exchange column prior to their detection by postcolumn reaction with a metallochromic reagent. The determination of lanthanides in uranium is the focus of this discussion, because of the difficulties in determining them by ICPAESlaZ1and because of their significanceas neutron poisons.
EXPERIMENTAL SECTION Apparatus. The coupled column chromatographic system used is shown schematicallyin Figure 1. The eluent was pumped using a gradient pump (Model LC 610, Shimadzu,Kyoto, Japan) (5) Crain, J. S.; Houk, R. S.;Smith, F. G. Spectrochim. Acta 1988,43B, 1355-1364. (6) Maney, J. P.; Luciano, V.; Ward, A. F. Jarrell-Ash Plasma Newsl. 1979, 2, 11. (7) Bear,B. R.;Edelson, M.C.; Gopalan,B.;Faesel,V.A. In Analytical Spectroscopy; Lyon, W. S., Ed.; Elsevier: Amsterdam, 1984; p 187. @)Floyd, M. A.; Morrow, R. W.; Farrar, R. B. Spectrochim. Acta 1983, 38B, 303-308. (9) Coleman, C. J. In Analytical Spectroscopy; Lyon, W. S.,Ed.; Elsevier: Amsterdam, 19&1;p 195. (10) Halouma, A. A.; Farrar, R. B.; Heater, E. A.; Morrow, R. W. In Analytical Spectroscopy; Lyon, W. S., Ed.; Elsevier: Amsterdam, 1984; p 201. (11) Short, B. W.; Spring, H. S.; Grant, R. L. Determinution of Trace Impurities in Uranium Hexafluoride by an Inductively Coupled Plasma Spectrometer; CAT-T-31&1;Goodyear Atomic Corp., Piketon, OH, 1983. (12) Argekar, A. A.; Thulasidas, S. K.; Kulkarni, M. J.; Bhide, M. K.; Sampathkumar, R.; Godbole, S. V.; Adya, V. C.; Dhawale, B. A.; Rajeshwari, B.; Goyal, N.; Purohit, P. J.; Page, A. G.; Dalvi, A. G. I.; Bangia, T. R.;Sastry, M. D.; Natarajan, P . R. Nucl. Technol. 1989,84, 196-204. (13) Seshagiri, J. K.; Babu, Y.; Jayanth Kumar, M. L.; Dalvi, A. G. I.; Sastry, M. D.; Joahi, B. D. Talanta 1984,31, 773-776. (14) Palmieri, M. D.; Fritz, J. S.; Thompson, J. J.; Houk, R. S.Anal. Chin. Acta 1986, 184, 187-196. (15) Burba, P.; Willmer, P. G. Freseniua 2.Anal. Chem. 1986,323, 811-817.
(16)Pan,F.;You,S.;He,Q.;Wang,X.;Ma,H.;Huang,Y.;Xu,Y.;Xu,
Y.; Wu, T. Spectrochim. Acta 1986,41B, 1211-1216. (17) Huff, E. A. Spectrochim. Acta 1987,42B, 275-283. (18) Boumans, P. W. J. M.; Tielrooy, J. A.; Maessen, F. J. M. J. Spectrochim. Acta 1988,43B, 173-199. (19) Boumans, P. W. J. M.; Vrakking, J. J. A. M.; Heijms, A. H. M. Spectrochim. Acta 1988,43B, 1365-1404. (20) Boumans, P. W. J. M.; ZhiZhuang, H.; Vrakking, J. J. A. M.; Tielrooy, J. A.; Maessen, F. J. M. J. Spectrochim. Acta 1989,44B, 31-93. (21) Kanicky, V.; Toman, J. ZCP Inf. Newl. 1990,15,444-478. Q 1993 American Chemical Society
, ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993
Reagent
HPLC Pump
Gradient Pump
-
Mixing
Tee
*
...
\
-
UVNis Detector
3321
*
:*@***
6
for details.
to the injection valve (Model 7125, Rheodyne, Berkeley, CA) fitted with a 0.10-, 5.0-, or 14.5-mL loop. From the injection valve the eluent flows to the two six-port switchingvalves (Model CV6-UNPa-N60,Valco Instruments, Houston, TX) which enable selection of either one or both of the columns to be in the flow path. The metal ions eluted from the columns were monitored by complexation with the metallochromic reagent arsenazo I11 [3,6-bis[(arsenophenyl)azo]-4,5-dihydroxy-2,7-naphthalenedisulfonic acid]. The postcolumn reagent was added to the eluate at 0.5 mL/min with a syringe pump (IscoM314, Lincoln, NE) via a low dead volume screen-mixing tee.22 The resultant complex was monitored by a variable-wavelength absorbance detector (Model SPD-GAV, Shimadzu or Model 490, Waters Associates, Milford, MA) set at 658 nm, the signal from the detector being fed to a computing integrator (Model CR501 Chromatopac, Shimadzu). An isocratic HPLC pump (Model3000,Waters) was used to back-flush the uranium from the reversed-phase column. For studies of the capacity of the reversed-phase column, the cation exchange column was removed from the flow path using six-port valve B. The mixing tee was also removed from the flow path, and the detection wavelength was set at 414 nm. Reagents and Materials. Freshly distilled water purified through a Milli-Q deionizing unit (Millipore, Bedford, MA) was used for all solutions and eluents. The a-hydroxyisobutyric acid (HIBA)was purified by passagethrough a cation-exchangeresin in the H+-form. All other chemicals were reagent grade. The uranium standard used was NBL No. 98-6 (US. Atomic Energy Commission, New Brunswick Laboratories, New Brunswick, NJ). Lanthanide standards were obtained from Spex Industries (Edison, NJ). The uranium was added to an equal molar quantity of HIBA, and the solution was then adjusted to contain an excess of 0.025 M HIBA. The sample solutions were adjusted to pH 3.8 with NhOH. Uranium concentrations were maintained below 5 mg/mL, were protected from light, and were used within 1 day of preparation to avoid losses due to precipitation. The analytical-scale columns were 4.6 X 150 mm 5-pm SupelcosilLC-18and the semipreparative-scale column was 21.2 X 100 mm lo-" Supelcosil LC-18 (Supelco, Bellefonte, PA). The cation exchange column for lanthanide separation was prepared by equilibrating an analytical 5-pm Supelcosil LC-18 column at 30 OC with 1500 mL of 2.5 X 1W M C d O r in 25% acetonitrile-water. Procedures. For the determination of lanthanide impurities in uranium, the apparatus was configured as shown in Figure 1, (22) Cassidy, R.M.; Elchuk, S.;Daegupta, P. K. Awl. Chem. 1987,59,
85-90.
with both columns positioned in the flow path. The sample loop was thoroughly flushed with uranium sample and injected into the 0.025 M HIBA (pH 3.8) flowing at 1.5 mL/min. After the lanthanides were quantitatively eluted from the reversed-phase column, it was switched out of the flow path using valve A. The eluent gradient, ranging from 0.025 to 0.25 M HIBA (pH 3.8) over 20 min, was initiated after the reversed-phase column had been removed from the flow path. The metal ions were detected at 658 nm after reaction of the eluate with the postcolumnreagent (1.5 X 1W M arsenazo 111, 0.02 M urea, and 0.24 M HNOs).
RESULTS AND DISCUSSION HPLC has been used to separate the lanthanides as a group23 and individually from geological standards, prior to ICP-AES24 and postcolumn reaction26J6 detection. The postcolumn reaction detection system involves complexation of the metal ions eluting from the HPLC column with a colorimetric reagent, with subsequent detection using a UVvisible spectrometer.n An average detection limit of -2.5 ng for the lanthanides is achieved using arsenazo I11 as the postcolumn reagent; the exact value of the detection limit depends on the lanthanide used.% In this work, the lanthanides are determined in nuclear grade uranium using a coupled-column HPLC separation method with postcolumn reaction detection. The first column selectively retains the bulk uranium matrix on its hydrophobic surface as the uranyl-a-hydroxyisobutyric acid complex. The second column is a cation-exchange column which initially preconcentrates the lanthanides eluting from the reversedphase column and then separates the individual lanthanides when a HIBA gradient is applied. The individual steps in this method are discussed in detail below. Reversed-Phase Retention of Uranium. Uranium, and to a lesser extent other actinides, are retained on reversed(23) Aulis, R.;Bolton, A.; Doherty, W.; Vander Voet, A.; Wong, P. Spectrochim. Acta 1985,40B, 377-387. (24) Yoehida, K.; Haraguchi, H. A d . Chem. 1984,56,2580-2585. (25) Cassidy, R.M. Chem. Geol. 1988,67, 185-195. (26) Barkley, D. J.; Blanchette, M.; Cassidy, R. M.; Elchuk, S.Anal. Chem. 1986,58, 2222-2226. (27) Caseidy, R.M.; Karcher, B. D. In Reaction Detection in Liquid Chromatow"; Krull. I. S., Ed.:Marcel Dekker: New York, 1986: Chapter 31 (28) Knight, C. H.; Cassidy, R. M.;Recoskie, B. M.;Green, L. W. Anal. Chem. 1984,56,474-478.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993
phase supports from HIBA eluents.26*2931Lanthanides, Cu(II), Zn(II), Fe(I1) Fe(III), Mn(II), Co(II), P b W , Ni(IhZ6 Na(I), K(I),Mg(II), Ca(II), Sr(II),29and Al(I1I)M do not show significant retention. The selective retention of uranium and the actinides has been exploited in the determination of U and Th in samples from refining processes,26in the preconcentration and determination of trace uranium in natural ground waters,m in the determination of uranium in irradiated nuclear fue1,30 and in the determination of thorium in plutonium.31 Mandelic acid, an aromatic homolog of HIBA, has also been used for the separation of the actinides.32 The retention of uranium decreases with decreasing pH of the HIBA eluent and increasing HIBA concentration,% and sharpening of the chromatographic peaks has been observed on the addition of methanol26 and ammonium chl0ride.3~ In this work, a reversed-phase column is used to separate the uranium matrix from its impurities, and thus the capacity of the reversed-phase column is of key importance. It is possible to obtain an estimate of the column capacity from the distortion of the chromatographic peak arising from concentration overload. Expressions relating the position of the peak front with the column capacity have been derived33 and verified,34 using the idealized assumptions of infinite column efficiencyand of a Langmuir isotherm. The efficiency of the separation of uranium under the present conditions (0.025 M a-HIBA, pH 3.8) has been measured to be N = 2090. This value is sufficient to satisfy the infinite efficiency assumption for moderate to large overload conditions.33 The expression relating the degree of overload and the peak front of a tailing peak is33934 (1) Lf= 11 - [(tf- t, - t,)/(tR,O - ~~)1”212 where the loading factor, Lf, is the ratio of the sample amount
injected to the amount of sample required to saturate the column, tf is the retention time of the shock front of the peak, to is the dead time of the column, t, is the duration of the injection, and ~ R , Ois the retention time of the sample under analytical conditions. The column capacity is then simply capacity = , C V~IL, (2) where Cd and Vhj are the concentration and volume of sample injected. Figure 2 shows an overlay of the peaks observed for the injection of increasing concentrations of uranium into an 0.25 M HIBA (pH 3.8) eluent. The general peak shape, consisting of a sharp front and long tail, is indicative of a convexisotherm such as the Langmuir isotherm. Furthermore, the tails of all of the peaks converge, indicating that the same isotherm is obeyed under all loading conditions. The cause of the discontinuity on the tail of peaks in Figure 2 is not known, but it does not affect the conclusionsdrawn from these studies. The column capacities derived from the peaks shown in Figure 2 using eqs 1 and 2 are shown in Table Ia. For the lowest loadings (0.05-0.2 mg), the ideal model of column overload is not valid because the intrinsic band broadening caused by the column is greater in magnitude than the broadening caused by the overload of the adsorption isotherm.% At intermediate (29) Kerr, A.; Kupferschmidt, W.; Attas, M. Anal. Chem. 1988, 60, 2729-2733. (30) Cassidy, R. M.; Elchuk, S.; Green, L. W.; Knight, C. H.; Miller, F. C.; Recoskie, B. M. J.Radioanol. Nucl. Chem., Art. 1990,139,55-64. (31) Hamilton, V. T.; Spall, W. D.; Smith, B. F.; Peterson, E. J. J. Chromatogr. 1989,469, 369-377. (32) Elchuk, S.;Burns,K.I.;Casaidy,R.M.; Lucy, C. A. J . Chromatogr. 1991,558, 197-207. (33) Golshan-Shirazi, S.; Guiochon, G . Anal. Chem. 1988, 60, 23642374. (34) Golshan-Shirazi,S.;Guiochon, G. Anal. Chem. 1989,61,462-467. (35) Lucy, C. A.; Carr, P. W. J. Chromatogr. 1991,556, 159-168.
-k
I
l
0.6 mg U
a
OI C
m
e
81
2
I
‘
I
I
I
10
0
1
1
20
30
I
Retention Time (minutes) Figure 2. Uranium peaks eluting from the reversed-phase column for various column loadings. Experimental conditions: column, 4.6 X 150 mm 5-pm Supelcosll LC18 with 4.6 X 20 mm guard column; eluent, 0.25 M HIBA (pH 3.8): flow rate, 1.5 mL/mln; column temperature, room temperature: injection volume, 100 pL; detector wavelength, 414 nm; b, 1.53 mln; i&,o(uranlum),24.42 min (k’= 15.0). Table I. Capacity of the Analytical-Scale Reversed-Phase Column for Uranium from an a-Hydroxyisobutyric Acid Eluent. column capacity (mg) uranium injected (me)
a. Eluent: 0.25 M HIBA pH 3.8b 0.05 0.10 0.20 0.30 0.40 0.50 0.60 0.70
130 26 23 I8 17 17 18 22
b. Eluent: 0.025 M HIBA pH 3.8‘ Sample: contains HIBA of equivalent molarity as U 0.10 6.7 0.20 7.4 0.40 12.1 2.5d 20 5.od 34 c. Eluent: 0.025 M HIBA pH 3.8 plua 0.2 M N&CP Sample: contains HIBA of equivalent molarity as U 0.10 18 0.40 24
I, Experimental conditions: flow rate, 1.5 mL/min; column temperature, room temperature;injection, 100 pL; detectorwavelength, 414 nm. Column: 4.6 X 150 mm 5-pm Supelcosil LC-18 with 4.6 X 20 mm guard column; to, 1.53 min; t&o(uranium), 24.42 min (k’ = 15.0). Column: 4.6 X 150 mm 5-pm Supelcosil LC-18; to, 1.35 min; tRo(uranium), 27.32 min (k’ = 19.2). d A s in (c) except injection volume, 5.0 mL. e As in ( c ) except for eluent change noted; tR&.“), 32.22 min (k’ = 22.9). loadings (0.3-0.6mg), the assumptions within the ideal model of column overload are obeyed and a column capacity of 17.5 f 0.6 is observed. At loadings greater than 0.6 mg, peak fronting becomes noticeable. This peak fronting results from the overall isotherm becoming concave at these loadings because of the finite concentration of HIBA present in the eluent.% Thus, for the 0.25 M HIBA (pH 3.8) eluent, the adsorption isotherm for uranium is “S”-~haped,9~ with convex character below 0.6-mg loading onto the analytical column as a result of the finite surface area available on the support and (36) Lucy, C. A.; Luong, T.-L.;Elchuk, S. J. Chromatogr. 1991,546, 27-36. (37) Guiochon, G.; Ghodbane, 5.; Golshan-Shirazi, S.; Huang, J.-X.; Katti, A.; Lin, B.-C.;Ma, 2. Talanta 1989, 36, 14-33.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993
concave character at high loadings (e.g., 0.7 mg) due to the finite concentration of HIBA available in the eluent for complexation of the uranium. As discussed below, it is desirable to minimize the concentration of HIBA in the eluent so that the lanthanides will be strongly retained and reconcentrated on the head of the cation exchange column. However, reduction of the HIBA concentrationwill exacerbatethe peak fronting of the uranium peak on the reversed-phase column by even further limiting the concentration of the complexing agent available. To circumvent this problem, an equivalent molar quantity of HIBA was added to the uranium samples. When such uranium-HIBA solutions were injected into 0.025 M HIBA (pH 3.8), the capacity (Table Ib) and the retention of the uranium on the analytical-scale column were observed to increase. Formation of uranium-HIBA precipitates limits the sample solutions to uranium concentrations of less than 5 mg/mL. However, the uranium is strongly retained on the reversed-phase column (k’> 20), and injections of volumes of up to 5 mL do not result in significant volume overloading of the uranium peak. The increased injection volume allowed injection of up to 5 mg of uranium onto the analytical-scale reversed-phase column and resulted in column capacities comparable to and larger than that achieved using the more concentrated eluent. The enhanced capacity resulting from increased loadings under these conditions is not well understood, but it may mirror the behavior reported for the extraction of uranyl ion from nitrate solutions.38 Increasingthe ionic strength of the eluent has been reported to increase the retention of uranium on reversed-phase columns.31 Table ICshows the increased column capacity which resulted from the addition of 0.2 M NH&l to the eluent. However, since simply increasing the uranium loading resulted in even greater enhancement of the column capacity, further investigations of the effect of ionic strength were not undertaken. Regenerating the Reversed-Phase Column. Prior to repeating the analysis sequence, the uranium must be quantitatively removed from the reversed-phasecolumn. Any uranium remaining on this column at the beginning of the next analysis would be eluted onto the cation exchanger, and its intensely colored complex with arsenazo I11would obscure the lanthanide peaks. In order to maximize the sample throughput, the regeneration of the reversed-phase column should ideally be performed while the lanthanides are being separated on the cation exchange column. Figure 3A shows that the bulk of the uranium is eluted from the reversed-phasecolumn within 40 min using the 0.025 M HIBA eluent. Unfortunately, the remaining uranium displays severe tailing, such that excessively long regeneration times are required before the uranium is reduced to trace levels. The regeneration process can be accelerated by backflushing the reversed-phase column, and no difficulties were experiencedwith column integrity when commercial columns were back-flushed. However, the most effective means of regenerating the reversed-phase column was to add 25% methanol to 0.1 M HIBA solution. Methanol has previously been observed to reduce the uranyl-HIBA retention* and to sharpen the peaks.30 Using this solution, the reversed-phase column could be regenerated and reequilibrated with the aqueous loading eluent within the 40 min required to accomplish the lanthanide separation gradient. Determination of Lanthanides in Uranium. Reversedphase supports can be made into high-efficiency ion exchangers by uniformly coating a hydrophobic surfactant, CzoS04,onto the reversed-phase support from an aqueous (38) Sekine, T.; Haeegawa, Y. Solvent Ertraction Chemistry: Fundamentals and Applications; Marcel Dekker: New York, 1977; p 271.
9323
Time (minutes)
0
5
10
15
20
25
30
35
40
Time (minutes) Flguro 3. Separatlon of lmpuritles from uranlum: (A, top) reversedphase column; (B, bottom) cation exchange column uslng ahydroxyIsobutyric acid as eluent. Experimental conditions: reversed-phase column, 4.6 X 150 mm 5-pm Supelcosil LC-18 wlth 4.6 X 20 mm guardcolumn:catlon-exchangecolumn. 4.6 X 150 mm 5-pm Supelcosl LC-18 coated with 2.5 X lo4 M CmSO, In 25% acetonitrile-water; flow rate, 1.5 mUmln; column temperature, roomtemperature: Injection, 5.0 mL of 0.5 mg/mL of U contalnlng 0.006 pg/mL of each lanthanide; reversebphase separatlon eluent, 0.025 M HIBA (pH 3.8); cationexchange eluent, 0.025 M HIBA (pH 3.8) for 9 mln followed by linear gradient from 0.025 to 0.25 M HIBA (pH 3.8) over 20 min; switchlng time to remove reversed-phase column from the flow path, 9 min; detection, 658 nm after postcolumn reaction wlth arsenazo 111.
acetonitrile solution.39 This colutnn yielded excellent separation efficiencies with a HIBA gradient, as can be seen in Figure 3B. Figure 3B shows the lanthanide separation associated with the injection of 5 mL of 0.5 mg/mL of U containing 12 pg of Ln/g of U of eachlanthanide. Peak height was used for quantification. Some baseline shift was observed under the heavy lanthanides due to the Fe3+present. Subsequent studies, discussed below, determined that the interference due to iron could easily be eliminated. Addition of 20 pg of Th/g of U had no effect on the lanthanide determination. In a blind test of the procedure, a uranium standard (NBL 98-6 No. 2) was spiked with a number of lanthanides. All of lanthanides added were successfully identified and quantified with a precision of 2-3 96 (lu)with the exception of ytterbium for which the precision degraded to 9 % due to the interference from iron. As shown in Table I1 the recoveries were quantitative for the lighter lanthanides, but some losses were experienced with the heavier lanthanides, presumably due to the iron interference making quantification difficult. Nevertheless, the accuracy and precision achieved for the lanthanide fission poisons is sufficient for analysis of nuclear grade uranium. (39) Cassidy, R. M.; Elchuk, S. Anal. Chem. 1982,54,166&1663.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993
Table 11. Recovery of Lanthanide Ions Added to NBL 98-6 No. 2 Uranium Standard in a Blind Test of the Coupled-Column Chromatographic Procedure. concn (pg of Ln/g of U) element
expec
obsd
obsdtexpec
Yb Tm Er DY Tb Gd
12.22 11.83 11.43 11.50 11.81 11.51 11.37 11.80 11.15 11.55
10.9 11.2 11.8 10.7 11.4 11.6 11.5 11.8 11.5 12.1
0.89 0.95 1.03 0.93 0.97 1.01 1.01 1.00 1.03 1.05
Eu Sm Nd Ce
Experimental conditions: reverse phase column, 4.6 X 150 mm 5-pm Supelcosil LC-18 with 4.6 X 20 mm guard column; cationexchange column, 4.6 X 150 mm 5-pm Supelcosil LC-18 coated with 2.5 X lo-' M C&O, in 25% acetonitrile-water; injection volume, 5.0 mL; initial eluent, 0.025 M HIBA (pH 3.8); gradient, 0.025-0.25 M HIBA (pH 3.8) over 20 min; flow rate, 1.5 mL/min; postcolumn reagent, 1.5 X lo-' M arsenazo I11 in 0.24 M HNOs and 0.02 M urea; reagent flow rate, 0.5 mL/min; detection wavelength, 658 nm; Sensitivity, 0.02 AUFS. ~~
~
However, in order to be useful for monitoring the fission poison lanthanides (Dy,Gd, Eu, Sm) in nuclear grade uranium, detection limits of 0.1 pg of Ln/g of U are required. For the determinations shown in Figure 3, the detection limit is approximately 0.5 pg of Ln/g of U for injection of 5.0 mL of 0.25 mg/mL of U. The detection limit can be improved by increasing the uranium sample concentration. Injection of 5.0 mL of a series of 4 mg/mL of U standards containing 0.25-1.45pg of Ln/g of U of each of Dy, Eu, Sm, Tm, and Gd resulted in linear calibration curves (r > 0.9998) with zero intercepts (within the 95% confidence interval). Detection limits for Dy, Gd, Eu, and Sm are 0.02 pg/g of U for these 20-mg U injections. Due to the limited solubility of U in the sample solution further increases in sensitivity require increased injection volumes. Unfortunately, since the lanthanides are only weakly retained on the reversed-phase column, they will experiencevolume overload when the volume injected is large relative to the column volume, resulting in incomplete separation of the lanthanides from the uranium. Use of a semipreparativereversed-phase column eliminates this problem. Figure 4A shows the determination of lanthanides in 72.5 mg of nuclear grade uranium, and Figure 4B shows the same sample spiked with 7.25 ng each of Dy, Gd, and Sm (0.1 pg of Ln/g of U) using the semipreparative reversed-phase column and the analytical-scale cation exchange column. The cation column does not need to be increased in size since only nanogram quantities of lanthanides are loaded onto it. The detection limit for this procedure is 0.005 pg of Ln/g of U. Duplicate spikes at the 0.3 pg of Ln/g of U yielded a relative standard deviation of less than 1% for Dy, Gd, and Sm. One drawback of this procedure is that Dy may be biased high due to the presence of Y, which will coelute with Dy with the HIBA gradient separation. Interferencee. A number of other metal ions are also present as impurities in nuclear grade uranium. Owing to their smaller neutron absorption cross sections, these metals can be present in much greater concentrations that the lanthanides without adversely affecting the neutron economy. The selectivity of the arsenazo I11 postcolumn reagent eliminates most of the potential interferences,but nonetheless, iron can pose a significant interference, as is illustrated by the baseline shift from 10to 13min in Figure 3B. Fortunately, Fe3+ is very weakly retained on the cation exchange column
A
i-
Sm
I
I
I
I
I
0
5
10
15
20
Time (minutes) Figure 4. Determination of lanthanide lmpuritles In nuclear grade uranium: (A) chromatogram from the InJection of 14.5 mL of 5 mg/mL of U (sample C64AR); (B) chromatogram from the InJectkm of 14.5 mL of 5 mg/mL of U (sample C64AR) spiked with 5 ng/mL Dy, W, and Sm. Experimental conditions as In Figure 3 with the following exceptions: revefsed-phase column, 21.2 X 100 mm 5-pm Supelcosll LC-18; Injectlon volume, 14.5 mL. Eu Gd.1
0
I
I
I
1
I
8
16
24
32
40
Retentlon Tlme (mlnutes)
Fe
B
!i
8
3
0
8
16
24
32
Retentlon Tlme (mlnutes)
40
Figure 5. Effect of maximum permlsslble concentrationsof transitkn and ekelineearth metalIonsonthe determinatbnof lanthanidelmpurltk In uranium: (A) Injection of 5.0 mL of 0.006 pg/mL of 14 lanthankles; (6)sample In (A) spiked with the maximum concentrations of other metals Ions that could be present In nuclear grade uranlum. Experimental condltlons as In Figure 3.
and can easily be removed by flushing the column with 0.025 M HIBA (pH 3.8) prior to eluting the lanthanides using the HIBA gradient. Of the remaining metal ions, only Cu2+,Ni2+, and Ca2+ pose any potential inference problem as can be seen in Figure 5. Figure 5A shows the chromatogram obtained from the injection of 5.0 mL of sample containing 0.006pg/mL of each lanthanide, and Figure 5B shows the same sample spiked
ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993
with the maximum permissible quantity of each transition and alkaline earth metal that can be in the uranium. As can be seen in Figure 5B, the gradient elution separates the Ni2+ and from all Of the lanthanides* Only cu2+ coelutes with one of the lanthanides. The interference of Cu2+ on the Tm*+is not of concern, since thulium does not possess a large neutron absorption capture cross section' and is not of great importance as an impurity in nuclear grade uranium. If the interference due to the transition and alkaline earth metal ions was of greater concern, their influence could be further reduced by acidifying the arsenazo 111 postcolumn reagent. For example, use of 0.34 M HNOa and 0.12 M urea with the 1.5 X lo-' M arsenazo I11 reduces the Ni2+and Ca2+ peak heights -2-fold and the Cu2+over 17-fold.
882S
ACKNOWLEDGMENT We are grateful to M. Hurteau for preparing the sample for the blind and to Dr. K. R. Betty for his helpful discuseions during this work. L.G. acknowledges the receipt of an Internatonal Atomic Energy Agency Visiting Fellowship to the Chalk River Laboratories. This work was presented by S.E. at the 1992 Ion Chromatography Symposium at Linz, Austria, October 1992.
RECEIVED for review May 18, 1993. Accepted August 18, 1993.6 *Abstract published in Advance ACS Abstracts, October 1,1993.
