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Anal. Chem. 1986, 58, 1178-1181
(11) Draper, N. R.; Smkh, H. "Applied Reqesslon Analysis", 2nd ed.: Wiley: New York, 1981. (12) Ho, C. N.; Chrlstian, G. D.; DavMson, E. R . Anal. Chem. 1980, 5 2 , 1071. (13) Wokl, S. et al. "NATO Advanced Study InstRute on Chemometrics": Kowalski, B. R., Ed.; Reidel Publishing Co.: Dordrecht, Holland, 1984. (14) Culmo. R. F. Perkin-€/mer €/em. Anal. Appl. Study 2 . (15) Melin, E.; Ode'n, S. Sver. Geol. Unders., Ser. C 1918, C278. (16) Kaila, A. Mastaloust. Alkak. 1858. 2 8 , 18-30. (17) Keppeler, G. Angew. Chem. 1832, 2 9 , 473-476.
(18) German National Standard DIN 11 542, 1978. (19) Levesque, M.; Dinel, H. Can. J. Sol/ Scl. 1877, 5 7 , 187-195. (20) von Post, L. Sven. Mosskui?ud&en. TMskr. 1822, 1 , 1-27.
RECEIVED for review August 20, 1985. Accepted December 10, 1985. This work was supported by the Swedish Energy Administration (STEV).
Mass Spectrometric Determination of Lanthanum in Nuclear Fuels Nancy L. Elliot, Lawrence W. Green,* Bernadette M. Recoskie, and Richard M. Cassidy General Chemistry Branch, Chalk River Nuclear Laboratories, Atomic Energy of Canada Limited, Chalk River, Ontario, Canada KOJ 1JO
A sensitive, precise technique was developed for determina-
tkn of ianthanun in nudear fuel samples by thennai lonlzatlon mass spectrometry. Interference from barium was sup pressed by measurement of Lao' at b w ftlament temperatwe and by eilmination of filament loadlng reagents. Nanogram quantities of lanthanum were used for analysis; the overall precision for fuel samples was 0.2%. Good agreement (0.5 %) was observed between resuHs obtained by this technique and those obtained by HPLC on a series of sampies. Slgnificant change in isotopic fractionation was not observed during the fkst 3 h of analysls. The SenSnMty of the observed Isotope ratio to filament temperature was studied, and the optimal range for measurement was 1200-1230 'C.
The 139 isotope of lanthanum is a stable, high-yield fission product that has most of the properties required for a fission monitor in nuclear fuel studies (I, 2). Although it was used in this manner by some laboratories (2-6), the fact that it never gained widespread use was probably due to difficulties associated with its determination by isotope dilution mass spectrometry (IDMS). The most serious difficulty is interference from 138Ba;138Lais the only isotope of lanthanum available for spiking, and it is not available in high enrichment. Barium, which has an ionization potential of 5.21 eV, is an easily ionized ubiquitous contaminant, especially in the rhenium filaments used in thermal ionization mass spectrometry, and its most abundant isotope is 138 (71.7%). Some laboratories have used tantalum filaments to reduce this problem (7),but our experience showed that this technique yielded poorer sensitivity and did not eliminate barium interference. Others (8) have used an addition of borax to the rhenium filament to promote emission of Lao+ a t low temperatures. This technique reportedly suppressed barium interference, since the filament temperature was lower and barium ionized preferentially as the metal ion. Recently, (1,9), a high-performance liquid chromatographic (HPLC) method has been developed for determination of '%a in fuel samples; this method takes advantage of the fact that 139Lais the only stable or long-lived isotope of La produced in fBsion, and thus its content may be determined by chemical means. The method uses dynamically coated ion exchangers on high-performance reversed-phase columns to effect the separations and analyses, and has provided large cost and time 0003-2700/86/0358-1178$01.50/0
savings in burnup analyses. In order to cross-check the HPLC-La results and establish the accuracy of the method, analysis of some of the samples by IDMS was required. The borax addition technique described above was used initially, but contamination problems were sufficiently severe that it was abandoned. Consequently, a reagent-free loading procedure was developed, and this paper reports a study of the accuracy and precision of this new procedure and gives detaiIs of the loading and analysis parameters that were essential to obtain good signals and consistent results.
EXPERIMENTAL SECTION Reagents and Materials. Nitric acid solutions were prepared from subboiling distilled nitric acid (Seastar Chemicals, Sidney, B.C.) diluted with deionized water. A 2.8 m g / d borax solution was prepared from Anachemia (Anachemia Chemicals, Ltd., Montreal, Quebec) Na2B,0r10H,0 dissolved in deionized water. HPLC reagents, a-hydroxyisobutyric acid (HIBA) and n-octanesulfonate, were passed through a strong cation-exchange resin for purification (9). The lanthanum primary standard was prepared from Specpure La203(Johnson & Matthey & Co., Ltd., London, U.K.), which was ignited to constant weight at 900"C. The calculated concentration agreed with titrimetric standardization within 0.170. The lanthanum spike solution was prepared from ORNL (Oak Ridge National Laboratories, Oak Ridge, TN) ls8Laenriched (6.7%) La203 and was calibrated by IDMS using the above primary standard as a spike. The results of five replicate calibration determinations gave a '%La concentration of (2.579 f 0.013) X lO-'mol/kg. The primary standard was also used for all standard curves prepared for the HPLC analyses. Apparatus. Fuel solutions were sampled in a hot cellglovebox facility; the hot cell contained a Mettler PC 2000 (Mettler Instr., AG, Switzerland) electronic balance for precise weighing of sample and spike aliquots, and the glovebox contained an HPLC for separation of lanthanum from the fuel solution. Used with the HPLC was a 15 cm X 4.6 mm i.d. analytical column packed with 5Mm Supelcosil LC-18 reversed phase. Postcolumn reaction with Arsenazo I11 was used for detection, and fractions were collected in 1-mL polyethylene microvials. Full details of the HPLC are given elsewhere (9). The mass spectrometer was a Nuclide (Nuclide Corp., State College, PA) 90" magnetic sector instrument equipped with Cathodeon type 553 triple filament assemblies, a Vacumetrics 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
(Vacumetrics, Ventura, CA) ETP AEM 1000 electron multiplier, and an IBM PC based automation system (10). The original center filament was replaced with Rhenium Alloys (Rhenium Alloys, Inc., Elyria, OH) zone-refined Re ribbon, which was outgassed at 1850 "C and 2 X 10" torr for at least 3 h before use. Filament temperatures were measured with a Pyro Micro-Opticalpyrometer (The Pyrometer Instrument Co., Inc., Northvale, NJ) that had been calibrated against a tungsten ribbon filament lamp. Reported temperatures are observed temperatures, uncorrected for window effects or emissivities. Procedure. Fuel solutions were diluted with 0.5 M HN03 to 2.4 pg/mL La, and weighed aliquots (-1 g) of these were mixed with weighed aliquots of spike solution in acid-washed glass vials. Concentrated HN03 was added to each vial to adjust the final HNO, concentration to 4 M; then the vials were covered with watch glasses and gently heated to reflux conditions while stirring on a magnetic hot plate for 30 min. The equilibrated mixtures were gently evaporated to dryness, and redissolved in 250 ~ u L of 0.5 M "0,. Aliquota (100 pL) of the latter solutions were injected into the HPLC after the injection system was flushed with 0.5 M HNO, and an aliquot of the sample. The dynamic modifier was 0.01 M ammonium octanesulfonate at pH 3.8. Metal ions were eluted by HIBA, the concentration of which was programmed from 0.1 to 0.4 M over 10 min at a flow rate of 2 mL/min. La fractions were collected over 30-s intervals to yield 1-mL sample volumes. For samples to which borax was later added, sufficient 8 M HNO, was added to adjust the HNO, concentration of 5 M. Aliquots of -5 pL (2-3 ng of La) were deposited on Re center filaments that were heated resistively to dry the deposits. To avoid spreading, samples were deposited in 0.5-pL units that were dried in two steps, the first at 1.0 A and the second at 1.6 A, before deposition of the next. In initial work, the sample was covered with a 5-pL borax deposit and again heated to 1.6 A to dry. In later work, the borax deposit was omitted and the filament was heated overnight in air (fume hood) at 1.6 A. During this long heating period the filament assembly was covered with an inverted polypropylene beaker to reduce airborne contamination. The filament and beaker assembly was mounted 6 in. above the bench top to allow for continual exchange of air around the filament. The filament assembly was loaded into the mass spectrometer and after 10 min degassing at 2.0 A the center filament temperature was gradually increased to 1210 OC, as measured with an optical pyrometer, for analysis. Lanthanum oxide peaks were measured 30 times by peak stepping in an ABBA-type sequence with appropriate correction for base mass.
RESULTS AND DISCUSSION Mass Spectra. A typical mass spectrum of a spiked La sample with borax added showed strong, well-defined peaks 139La160+,13gLa170+, for the isotopes of Lao+: 138La160+, 13gLa1sO+ (Figure 1). There were trace peaks for Nd isotopes also present, note the lOOOX scale factor in Figure 1, but other fission products were not evident indicating satisfactory separation of La by the HPLC technique. Strong BaO+ peaks were also absent. Occasionally the Lao+ signal was too weak for precise measurement, likely due to incomplete oxidation of the sample, and the filament temperature was raised to 1550 "C for measurement of La+. Figure 2 shows a typical spectrum of isotopically natural La+; the peaks were well-defined with no evidence of major Ba interference. Chemical Blank. Comparison of initial La results for fuel samples with HPLC results consistently showed that the IDMS results were 8-9% higher. Analysis of the HPLC reagents and the borax solution showed significant amounts
I 166 162 158
1179
100 f A
154
MIZ
Flgure 1. Mass spectrum of La0 in spiked fuel sample with borax added to filament: 3.89 amu/min.
0 0 0 rl
x
t
I
loofA
dz Figure 2. Mass spectrum of La in natural lanthanum: 1.28 amu/min.
of Ba and La contaminants in each. Subsequently, the HPLC reagents were purified by cation exchange as described previously, and the associated blanks were reduced to picogram levels. In order to eliminate contaminants from the borax and other reagents, and to minimize handling of the filament assembly during loading, a reagent-free loading procedure was developed. After some experimentation,the most satisfactory procedure was that which involved deposition of microliter quantities of the HPLC-La fraction directly on the rhenium filament followed by heating overnight in air at -600 "C. The heating treatment was necessary to decompose the mobile phase and thus promote emission of Lao+ during analysis. Typical mass spectra obtained with this procedure were very similar to those obtained with the borax addition (Figure 1); the high sensitivity and good signal quality were retained for Lao+. Filament Blank. Part of the difficulty with La contamination was due to the high sensitivity of the method for La; a 1-ng sample yielded a strong ion beam (- 1 X A) that lasted for hours. Considerable care in filament loading and handling was required to avoid ambient contamination. Filament background La was unacceptably large unless the
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
Table I. 138/139 Ratio vs. Time for a Spiked Fuel Sample run no.
138/139 av at. ratio"
1 2 3 4 5 6
0.025 40 0.025 40 0.025 38 0.025 36 0.025 43 0.025 43
R S
0.025 40 0.000 06
7 8 9 10 11 12 13 14 15 16 17
0.025 43 0.025 44 0.025 40 0.025 38 0.025 40 0.025 36 0.025 38 0.025 40 0.025 38 0.025 41 0.025 40
overall R
0.025 40 0.000 02
S
138/139 at. ratio
analysis no.
R S
% RSD"
ORNL valueb
138/139 1175 1200 1230
0.07245 0.072 42 0.072 42 0.072 22 0.072 42
0.072 08 0.072 17 0.072 22 0.072 36
0.072 39 0.000 08 0.11 0.072 39
0.072 21 0.000 10 0.14 12.
"C. Reproducibility and Calibration, Data for replicate
filament temp, "C at. ratio" filament temp, "C at. ratiou 0.025 29 0.025 28 0.025 35 0.025 37
La+
decreases for lower temperatures. Samples in this work were analyzed at 1210 O C , with an estimated reading error of A10
Table 11. 138/139 Ratio vs. Temperature for a Spiked Fuel Sample
1080 1110 1130 1150
Lao+
" 70 Relative standard deviation. bSee ref
"Each value is the mean of three comdete ABBA data sets.
138/139
Table 111. Replicate Analysis of ls*La Spike as Laot and La+
0.025 44 0.025 32 0.025 40
"Each ratio is an average of two or three separate analyses on seDarate samde loadings. at the given temDerature. filaments were outgassed a t 1850O C for at least 3 h. This outgassing procedure removed other filament contaminants as well: Ce, Pr, Nd, and most of the Ba. A trace of BaF+ was evident in most of the spectra; the source of this was attributed to residual impurities in the filaments. The 135Ba19F+ peak caused a minor interference, -0.2%, on the 138La160+peak and was corrected for by measurement of the 138Ba19F+ peak with correction for the 139La180+ signal and by use of Ba isotopic composition data (11). It was unnecessary to correct for 136Ba19F+ interference on 139La160+.Interference of Ba+ on La+ was more problematical than that of BaF+ on Lao+ because the degree of interference varied from 0.1 to 2% and correction for 138Ba+ interference by measurement of the weak 13'Ba+ peak was not very accurate. Thus analysis of the Lao+ species was preferred. Fractionation and Temperature Effects. In order to estimate the rate of fractionation of the Lao+ species, samples were analyzed for a prolonged period, -3 h. The results for a typical sample showed (Table I) that there was not a significant degree of fractionation over this time period at the level of precision obtained with these measurements. The overall mean after 17 runs was identical to that after the first 6, which was the normal analysis period. Note that the value listed for each run is the mean of three complete ABBA sets of ratio measurements. The sensitivity of observed isotope ratios to the temperature of the rhenium filament was also investigated; 138/139 ratios in a spiked fuel sample were measured at temperatures from 1080 to 1230 "C. The results (Table 11) showed insignificant changes in the ratio between 1130 and 1230 OC, but slight
analyses of the 13SLaspike material using either the Lao+ or La+ signal are shown in Table 111; a fresh aliquot of sample was used for each analysis. Both sets of data show comparable precision, but the mean of the Lao+ data is in exact agreement with the given value for the spike (12),whereas the mean of the La+ data is 0.2% lower. The cause of the lower ratio for La+ is unknown, but may have been a greater fractionation rate for the La+ species. Absolute calibration of lanthanum isotope ratio measurements was not possible due to the lack of lanthanum isotopic standards. The comparison shown in Table I11 indicates only a consistency between the two laboratories for the Lao+ species. However, for lanthanum, lack of absolute calibration does not mean bias is introduced into results obtained by isotope dilution analysis. Examination of the isotope dilution equations, expressed in the forms shown below, readily shows that any bias incurred in sample analysis is exactly cancelled by the same bias incurred in spike calibration. Spike calibration:
and sample determination:
where m,, m,, mp = mass (9) of aliquot of spike (s), sample (x),and primary standard (p), respectively; [13sLa],[139La]= concentration (mol/g) of 13sLaand 13gLa,respectively, subscripts as above; Ralg, Rg18 = atomic ratio of 1381139 and 139/ 138, respectively; subscripts are as above except that subscript m denotes sample-spike mixture. Also, because natural La is essentially monoisotopic, a primary standard can be prepared with a high degree of certainty. This is not true for multi-isotopic elements, e.g., Nd and Ce, because unless their isotope ratio measurements can be absolutely calibrated, unknown biases exist in their isotopic compositions and hence in their primary standards. These biases are usually considered to be small; nevertheless, elimination of them is a particular advantage of using 139La as a fission monitor. To establish the reproducibility of the overall method for fuel samples, a sample was analyzed repeatedly through the entire procedure, beginning with aliquoting of the sample and spike. The results, shown in Table IV, showed a relative standard deviation of 0.19%, which was satisfactory for nu-
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Anal. Chem. 1986, 58, 1181-1186
Table IV. Precision of Determination of lasLain Fuel analysis no. 1 2 3 4
106[139La], analysis no. mol/kg fuel 6.681 6.704 6.678 6.688
lo6[laeLa], mol/kg fuel
5 6
6.703 6.709
z
6.693 0.013 0.19%
S
RSDa
" Relative standard deviation. Table V. Comparison of HPLC and IDMS Determination of lasLain UOz Fuel
sample
[lagLa] HPLC/ IDMS
1 2 3 4
1.009 1.005 0.985 0.981
sample
[laeLa] HPLC/ IDMS
5 6
1.001 0.988
z
0.995 0.012 1.2%
S
RSD"
" Relative standard deviation. clear fuel studies. Comparisons of IDMS results, obtained with the reagent-free loading procedure, with HPLC results for a series of fuel samples showed good agreement (Table V), with only 0.5% average bias between them. The 1% standard deviation between the two sets of data is a measure of the overall uncertainty of the methods with respect to each other.
Applications. This reagent-free mass spectrometric method for La has been very useful for validation of new methods for determination of burnup in nuclear fuels and should be applicable to many other types of materials. Its high sensitivity is a definite advantage in nuclear fuel work, since sample quantities and radiation exposures to the analysts can be reduced. Registry No. La, 7439-91-0. LITERATURE CITED (1) Knight, C. H.; Cassidy, R. M.; Recoskie, B. M.; Green, L. W. Anal. Chem. 1884, 56, 474. (2) Larson, R. P.; Laug, M. T.; McCouin, J. J.; Ebersole, E. R. Eighth Conference on Analytical Chemistry in Nuclear Technology, Gatllnburg, TN, 1964, Paper 14. (3) Ebersole, E. R.; Laug, M. T.; Villarreal, R. Libby/CockcroR Exchange Meeting on Burnup, 1969, U. S. Atomic Energy Commission Report LA-4430-MS, 1970; pp 140-144. (4) Meneghetti, D.; Ebersole, E. R.; Walker, P. Trans. Am. Nucl. SOC. 1873, 17, 533-534. (5) Meneghetti, D.; Ebersole, E. R.; Walker, P. Nucl. Techno/. 1975, 25, 406-415. (6) Kucera. D. A.; Meneghetti, D.; Ebersole, E. R. Trans. Am. Nucl. SOC. 1876, 23, 573-574. (7) Tromp, R. L.; Delmore, J. E.; Nielsen, R. A.; Chapman, T. C. Exxon Nuclear Idaho, Co., Idaho Falls, I D Report ENICO 1094, 1981. (8) Laug, M. T. Argonne National Laboratory, Idaho Falls, ID, private communication. (9) Cassidy, R. M.; Eichuk, S.; Elliot, N. L.; Green, L. W.; Knight, C. H.; Recoskie, B. M. Anal. Chem. 1886, 58, 1181-1186. (10) Green, L. W.; Barsczewski, J. S.; Elliot, N. L. Int. J. Mass Spectrom. Ion Processes, in press. (11) Holden, N. E.; Martin, R. L.; Barnes, I.L. Int. Union Pure Appi. Chem. 1984, 56, 675-694. (12) Walker, R. L. Oak Ridge National Laboratory, Oak Ridge, TN, private communication.
RECEIVED for review October 17,1985. Accepted December 3, 1985.
Dynamic Ion Exchange Chromatography for the Determination of Number of Fissions in Uranium Dioxide Fuels R. M. Cassidy,* S. Elchuk, N. L. Elliot, L. W. Green, C. H. Knight, and B. M. Recoskie General Chemistry Branch, Chalk River Nuclear Laboratories, Atomic Energy of Canada Limited, Chalk River, Ontario, Canada KOJ 1JO
The llquld chromatographic determlnatlon of the fission monitor '''La has been examined for the determlnatlon of the number of flssions and burnup in UO, fuels. Lanthanum was separated from the fuel and other flssion products on a reversed phase dynamically modified wlth 1-octanesulfonate, and eluted metal Ions were detected after an "on-line" postcoiumn reactlon with Arsenaro 111. The relatlve standard devlatlon for lanthanum determinations done over a 2-week period was 0.6%. The agreement between the chromatographic procedure and standard mass spectrometrlc procedures using 145+14eNd as a flssion monitor was wlthln -2%; the majority of this 2 % dlfference was attributed to uncertainties in the accepted values for fisslon yleids. The agreement between chromatographic and mass spectrometric procedures for the determination of La In fuel solutions was within 1 %.
-
Previous studies (I) have shown that dynamic ion exchange techniques can provide rapid and high-resolution separation 0003-2700/86/0358-1181$01.50/0
of fission products in irradiated thorium-uranium dioxide fuels. When coupled with a postcolumn detection system this chromatographic system gave low detection limits and good precision ( e l % relative standard deviation) for the determination of the number of fissions in these fuels; a comparison between high-performance liquid chromatography (HPLC) results and standard mass spectrometric techniques established the accuracy of the HPLC technique. These highperformance separations gave savings of up to 10-fold in analysis times and cost, and this HPLC technique is now used on a routine basis for atom percent fission determinations in thorium-uranium fuels. Atom percent fission, or burnup (2),is an important parameter required for the study of nuclear fuels, and because of a continual need for its determination in uranium dioxide fuels, a similar type of analysis method was investigated for these fuels. The fission monitor chosen for this work, 139La, was the same as that used for thorium-uranium dioxide fuels. Because La is essentially monoisotopic (139La)in the fuel, chemical analysis techniques can be used for ita determination. The suitability of I3%a as a fission monitor has been discussed elsewhere (1). 0 1986 American Chemical Society
