550
Anal. Chem. 1984,56,550-556
Interlaboratory Isotopic Ratio Measurement of Nanogram Quantities of Uranium and Plutonium on Resin Beads by Thermal Ionization Mass Spectrometry J. D. Fassett* a n d W. R. Kelly National Measurement Laboratory, Center for Analytical Chemistry, Inorganic Analytical Research Division, National Bureau of Standards, Washington, D.C. 20234
The use of high senskivlty thermal Ionization mass spectrometry for the accurate and preclse measurement of uranium and plutonlum lsotoplc ratios for safeguards accountablllty has been evaluated by means of an Interlaboratory analysls program (round robin). Nanogram amounts of lsotoplc Standard Reference Materials (SRM’s) and unknown samples were loaded onto anion exchange resin beads and transported to partklpatlnglaboratorles for measurement. U, Pu, and U plus Pu loaded beads were prepared and analyzed. The unknown samples were prepared from Isotopic SRM’s zssU,*”U, and and z44Puas well as assay SRM’s containing principally zsoPu. Typlcal accuracles and preclsions achleved In the measurement of major lsotoplc ratios were better than 0.30 %. Internal normalizationwhen applkable demonstrated Improvements In both preclslon and accuracy. I t Is concluded that Isotopic fractlonatlon Is a major source of lmpreclslon while the degree to whlch Isotopic fractlonatlon can be cailbrated limits the measurement accuracy.
The international growth of the nuclear industry combined with increased safety requirements has placed enormous demands on traditional measurement procedures used in nuclear accountability and material control. The Atomic Energy Commission and its successive organizations as well as the International Atomic Energy Agency (IAEA) have recognized the need for new measurement technology for safeguarding nuclear materials and standardization of measurement control techniques (I). The demonstration of the reliability of these techniques is a prerequisite for acceptance by various measurement and standards groups within the nuclear community. The resin bead sample loading technique in thermal ionization mass spectrometry has been developed and successfully applied by Oak Ridge National Laboratory (ORNL) to the isotopic analysis of U and Pu in safeguarded nuclear materials (2-5). The technique is innovative for routine nuclear materials analysis in both the sampling step and the use of high sensitivity pulse counting detection for the isotope ratio measurement. The resin bead provides the matrix for chemical separation, physical transportation, and thermal ionization. The determination of nanogram quantities, which are used to minimize radioactivity, requires high sensitivity detection. The U and P u can be determined sequentially from a single sample. Ion-exchangebead microstandards were first developed by Freeman et al. (6)and used in mass spectrometry (7, 8) in 1970. The first description of the use of beads in the measurement of U and Pu was by Walker et al. in 1974 (2). This work has been described in subsequent publications (3-5). Most recently the resin bead technique has been applied to the analysis of T h (9),Zr (IO),Ru (11),and Tc (12). The ionization mechanism of U from resin beads has been studied by use of secondary ion mass spectrometry (13).
The purpose of this exercise was the validation of the resin bead technique for the measurement of U and Pu isotopic ratios. By introducing the resin bead technique to mass spectrometric laboratories capable of making high sensitivity measurements, both a critical evaluation of the technique and a general indication of measurement “state-of-the art” in high-sensitivity U and Pu isotopic measurement were achieved. The unknowns provided in this exercise permitted evaluation of potential mass spectrometric measurement errors. These potential errors are contamination or blank, isobaric interferences, background determination, measurement system calibration, instrumental discrimination, and lack of isotopic fractionation control. Other round robins evaluatingthe use of thermal ionization mass spectrometry in the isotopic analysis of U and Pu have been conducted by the Safeguards Project at Karlsruhe (IDA-72, IDA-80) (14,15)and New Brunswick LaboratoryDOE (SafeguardsAnalytical Laboratory Evaluation-SALE) (16).These interlaboratory programs utilized conventional thermal ionization instrumentation and characterized the blind samples either by consensus (IDA-72)or by reference laboratories (IDA-80,SALE). The blind samples in this round robin were prepared gravimetrically from isotopic and assay SRM’s. The reference values and uncertainties for these unknowns were determined directly from certified values. A preliminary description of this interlaboratory experiment has been published (17).
EXPERIMENTAL SECTION Participation of Laboratories. A request to participate in this exercise was made to all known laboratories that make high sensitivity measurements which included the nuclear industry and academic and military laboratories. Of the 15 laboratories initially contacted, 11expressed interest and received samples. Eight laboratories submitted at least partial results; five laboratories submitted all results requested. The program was voluntary and did require a significant commitment of time and resources by the laboratories involved. The laboratories who restricted their participation stated that they could not make this commitment. The majority of laboratories that did participate possessed expertise in the measurement of U and Pu and in the manipulation of particulate samples. Samples. The exercise was divided into four phases. In the initial phase, participants analyzed beads of two different sizes that had been saturated with U. The larger beads of approximately 150 pm diameter contained 300 ng per bead and the smaller beads of approximately40 pm diameter contained 30 ng per bead. These samples were used to assess the performance of each laboratory in making accurate measurements under statisticalcontrol. These samples provided base line measurements and allowed instrument calibration. In the other three phases of the exercise, participants analyzed beads prepared following ORNL procedures containing nominally 3 ng per bead of U, Pu, or U and Pu together. The basis for the loading technique is the work of Kressin and Waterbury (18) who separated Pu by using anion exchange beads. Resin beads (Bio-RadAG1X2) of 100-200 mesh (150-75 pm) were used in the exercise, although 200-400 mesh beads also were prepared and
Thls article not subject to U S . Copyrlght. Published lQ84 by the American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
Table I. Round Robin Samples phase
I
samplea
amt, ng
Pu
300 300 300 300 30 30 3 3 3 3
947lU-500 s
Pu Pu U/Pu
3 3 313
Hu
UIPU
313
U-500 s u-100 s u-900 s
Au U-500 s
Bu I1
U-500 s
I11
Du 947 s
c u
Eu Gu IV
element U U U U U U U U U
Key: s = standard, u = unknown. >0.01.
major isotopesb 235, 238 235, 238 235, 238 235, 238 235, 238 235, 238 235, 238 233, 235, 238 235, 238 239, 240, 241, 24 2 239, 240, 244 239, 240, 241 235, 2381239, 240, 241, 24 2 233, 235, 2381 239, 240, 244
Atom fraction
used in the first phase. The beads were loaded in the chloride form in the first phase and in the nitrate form, as prescribed by the ORNL procedure,for all subsequent phases. Table I lists the samples analyzed in the exercise. Both standards and blind samples (unknowns)were included in each phase of the exercise. The NBS SRMs U-100, U-500, and U-900 were submitted for analysis in the first phase of the exercise to assess the linearity of the individual measurement systems, and SRMs U-500 and 947 were included in subsequent phases to allow external normalization of isotopic fractionation for U and Pu, respectively. The blind samples for the exercise were prepared gravimetrically from SRMs, both isotopic spike SRMs certified for isotopic content and assay SRM’s certified for elemental and isotopic content. The spike SRMs 993 (28sU) and 995 (zssU)and assay SRM 960 (natural U) were used to prepare the U unknowns. The spike SRM 996 (244Pu)and assay SRM 949e (239Pu)were used to prepare the Pu unknowns. By use of SRMs to prepare the blind samples, reference values could be assigned to the unknowns with the uncertainties calculable from the uncertainties in the individual certified SRMs. The dominant contributions to the uncertainties in the reference values of the unknowns were the uncertainties in the assays of the spikes. Both the weighing uncertainties and the spike isotopic composition uncertainties were negligible. Unknowns required measurement of =U/238U and 244Pu/2s9puratios with five mass unit differences which served to accentuate isotopic fractionation effects. Two of the U unknowns were prepared containing three major isotopes zssU,23sU,and 238U,which permitted internal normalization of the 23aU/238U ratio to the 2saU/238U ratio to eliminate isotopic fractionation as a source of systematic error. These unknowns also contained very low 234Uand 23eUabundances. In the measurement of these isotopes, isobaric interferences are potentially the major source of inaccuracy; ion counting statistics and ion scatter will limit precision. For each sample, 7 to 12 beads were delivered to the participating laboratories. For shipment, the beads were isolated between an optical well slide and a flat slide and were immobilized by placing a drop of collodion diluted 1:lO with ethanol on each bead. Participants were asked to analyze each sample in quadruplicate and to report the following: (1)observed isotopic ratios;
(2) corrected atom fractions of each isotope or corrected isotopic ratios; (3) an indication of internal (within a single run) and external (among runs) precision; and (4) corrections applied to data and how they were determined. The majority of laboratories analyzed all the beads directly. One laboratory (E) desorbed the sample material from the bead and dried it directly onto a carburized rhenium filament in its sample loading procedure. Sample Assay. The U content of the beads was determined by us in the first two phases of the exercise. Assay wm determined by isotope dilution of representative individual beads for the first phase samples,verifying the completeloading of these beads. The U content of the beads in the second phase of the exercise was determined by difference by placing a known number of beads in a solution of known U content. After absorption of material onto the beads, the beads were separated and rinsed. The resulting solution was assayed by isotope dilution and the U lost from the original solution was assumed to be homogeneously distributed on the sample beads. The results of this experiment are shown in Table 11. Although the loading efficiency of U is dependent upon the time in solution, concentrations,and mixing conditions, the 6 9 % loading efficiencies observed are in good agreementwith published results of ORNL (4). Instrumentation and Measurement. All mass spectrometers used in this exercise were noncommercial instruments and included single, double, and triple sector thermal ionization mass spectrometers. Translation of peaks was accomplished by magnetic field and electric field switching. Electron multiplier and “Daly” detectors were used in pulse counting operation for ion detection. All samples were loaded onto single rhenium filaments. The measurement strategies varied considerably among the participating laboratories; however, all laboratories maintained signal levels in the range of 106 to 106ions/s for the major isotopes of U and Pu during measurement. Although the integrated ion currents were not requested from the laboratories, their measurements should not be counting statistics limited since this level of ion current could be maintained for an extended period of time. No laboratory reported an inadequate signal for any of the samples.
RESULTS AND DISCUSSION A total of 14 samples, which included seven standards and seven unknowns, were analyzed in this exercise. Each sample was analyzed by five to seven laboratories in quadruplicate. Approximately 300 individual resin beads were analyzed. Results from each phase of the exercise are presented in Tables 111, IV, V, and VI for unknowns A, C, G, and H, respectively. The averages and standard deviations for replicate measurements of unknown beads are presented in these tables. The fractionation correction uncertainty that was included in the measurement uncertainties reported by two laboratories has been excluded in these tables to allow direct comparison of measurement precisions. The histogram in Figure 1displays the accuracy relative to reference values for the major isotopic ratios (0.01 C R C 100) of all unknowns. Figure 2 graphically represents the individual measurement results for these major isotopic ratios. Both the measurement uncertainty and confidence limits of the reference values are also included which provides a clearer indication of accuracy. Table VI1 summarizes the measurement precision and accuracy among laboratories for all major ratios of the unknown samples. Precision refers to the standard deviation (s) of replicate measurements of each unknown by each laboratory. The median absolute bias is presented in Table VI1 in order to discount compensating negative and positive deviations.
Table 11. Uranium Content of Beads sample U-500 C D
no. of beads 326 326 326
added 2.537 5.325 5.632
551
moles of 238U( x ~ O - ~ ) recovered on beads 2.298 5.007 5.192
0.239 0.318 0.440
% loading
ng of U per bead
9.4 6.0 7.8
3.5 2.7 3.3
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
Table 111. Results of Unknown A uranium ratios lab code
2341238 (x10-4)
N
4 2 4 4 4 4
A B D E F G
a
2351238
std dev
0.05 0.017
0.121 32 0.121 30 0.121 84 0.121 48 0.122 28 0.121 96 0.121 29 0.000 24
0.000 22 0.000 1 3 0.001 64 0.000 05 0.000 78 0.000 14
1.07 1.040 1.053 1.030 1.03 1.059 1.052 0.005
ref value +95%C.L.
2361238 (xio-5)
std deva
0.011
0.013 0.06 0.004
6.0 6.62 6.55 6.59 6.51 6.77 6.56 0.05
std dev 0.2 0.04 0.03 0.10
0.06 0.07
std dev = standard deviation, s.
Table 1V. Results of Unknown C uranium ratios labcode
N
A 4 B 4 C 4 D 4 E 4 F 4 ref value +95%C.L.
2331238
std dev
0.08050 0.08082 0.08024 0.08132 0.08057 0.08053 0.080 517 0.000 045
0.000 1 7 0.000 1 5 0.000 27 0.000 80 0.000 1 8 0.000 1 2
2341238 (XlO-') 1.06 1.10 1.100 1.095 1.096 1.131 1.104 0.004
std dev
2351238
std dev
0.02 0.02 0.015 0.010 0.031 0.004
0.099 26 0.099 49 0,09899 0.09974 0.099 20 0,09924 0.099 1 3 0.000 17
0.000 1 2 0.000 05 0.000 23 0.000 52 0.000 04 0.000 04
2361238 (X10-5) std dev 5.7 5.4 5.50 5.34 5.34 5.38 5.38 0.04
0.3 0.2 0.20 0.11 0.16 0.05
Table V. Results of Unknown G
lab code A
B C
D E
N
2381239 (X10m4)
4 4 3 4 4
9.8 9.7 5 9.72 9.75
std dev
2401239
0.1 0.10
0.217 38 0.217 46 0.217 37 0.216 92 0.217 38 0.217 56 0.000 26
0.02 0.13
n.r.
ref value *95% C.L.
10.3 0.7
plutonium ratios 2411239 std dev (X10-2) 0.000 1 5 0.000 21 0.000 1 2 0.000 28 0.000 26
2.36 1.929 2.030 2.002 1.914 1.927 0.005
std dev 0.29 0.004 0.082 0.080 0.003
2421239 (X10-3) 5.63 5.64 5.63 5.56 5.63 5.62 0.07
std dev 0.01
0.02 0.02 0.05 0.04
Table VI. Results of Unknown H lab code
N
A
B C D E ref value *95% C.L.
lab code A B C D E
ref value i95% C.L.
2381239 2401239 (X10-4) std dev ( x ~ O - ~ std ) dev
4 4 3 3 3
3.5 3.1 7.3 0.2 0.50 0.6 0.1
N
4 4 3 3 3
0.9 1.7
2.4 0.6 0.13
2331238 (X~O-~) 3.456 3.438 3.444 3.378 3.442 3.4493 0.0019
2.954 2.955 2.962 2.957 2.961 2.956 0.011
std dev 0.010
0.016 0.004 0.063 0.004
0.011
0.006 0.002 0.009 0.003
plutonium ratios 2411239 2421239 ( x ~ O - ~std ) dev (X10-3) std dev 7.85 0.03 10.2 0.5 0.05 7.89 0.4 9.6 7.89 0.03 0.10 9.44 0.03 7.86 2.5 11.6 0.012 7.860 0.04 9.37 9.5 7.83 0.3 0.05
2341238 (x10e5) 5.4 6.3 6.67 5.7 6.42 6.56 0.02
uranium ratios 2351238 std dev (XIO-Z) 1.733 1.1 1.729 0.2 1.7353 0.05 0.8 1.737 1.7345 0.14 1.7309 0.0018
std dev 0.004 0.004 0.0015 0.005 0.0024
2441239 0.576 23 0.582 25 0.580 76 0.577 83 0.57957 0.576 68 0.000 37 2361238 (X10-6) 3 10
11.2 8 9.3 5.8 0.1
std dev 0.001 58 0.000 64 0.000 51 0.002 59 0.00039
std dev 11 1 1.1
2 0.4
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
553
Table VII. Summary of Results precision, % unknown
N
element
A B C
6 6 6
D E
6 5
G
5
H
5
U U U U U U Pu Pu Pu Pu Pu Pu Pu Pu U
ratio 2351238 235/23Bb 2331238 2351238 ( 235/238)nC 2351238 2441239 240 1239 (240/239)nd 240 1239 241 I23 9 2441239 2401239 ( 240/239)nd 233/23Se 2351238 ( 235/238)nC
U U
accuracy, %
mean
median
0.41 0.21 0.34 0.15 0.12 0.41 0.23 0.25 0.27 0.10 4.7 0.20 0.21 0.24 0.25 0.20 0.18
0.41 0.24 0.22 0.08 0.11 0.27 0.18 0.20 0.21 0.10 4.2 0.11 0.20 0.19 0.20 0.23 0.18
'
mean
mediana
t 0.32
0.30 0.10 0.20 0.14
t 0.18
+0.19 +0.19 t 0.08 t 0.32 t 0.43 t 0.11 t 0.02 -0.12 t 6.2 t 0.46 t 0.06 -0.02 -0.12 t 0.26 t 0.18
0.08
0.22 0.47 0.10 0.09 0.09 3.9 0.50 0.07 0.06 0.20 0.21 0.18
~/[ Accuracy of one measurement not included (-0.94%). Normalized to [ " 3 U / " s U ] ~ ~233U/ Absolute value. Normalized to [ 2 4 4 P ~ / 2 3 9 P 244P~/239P~]mead. ~]m~/[ e Precision and accuracy of one measurement not included UImed. ( i 1 . 8 6 and -2.07%). a
238
RBDEFG RBCEFG RBCDEF RBCDEF RBCDEF RBCDE
15
ljy
;
u)
A T A
10
a > !A
0
0
z
5
R-U
235,238
I
1
.B
.6
.4
(-1
.2
0
BIRS
.2 2
.4
.6
.8
1
. 12129
I
(+)
Figure 1. Measurement accuracy relative to reference values for maor isotopic ratlos, 0.01 < R < 100 0, 240Pu/23ePu;1, 2"Pu/238Pu; 3, i33U/238U; 4, 2uPu/23ePu; 5, 235UP38U.
Accuracy is calculated relative to the reference values. See Table I for identification of these samples. The compilation of all measurements made in the round robin is available from the authors. The samples in the first phase of the resin bead experiment differed considerably from the subsequent phases, where ORNL procedures were used to prepare the samples. The beads had roughly 1OX and lOOX more U than the second phase U samples, yet the precision in measurement of the 23sU/238U ratio of unknowns A and B was worse for all laboratories when compared to measurement of the 236U/238u ratio of unknown C. Measurement accuracy was not significantly different. The beads were prepared in the chloride form in the first phase to guarantee their long term storage stability. Since this practice is inconsonant with safeguards chemical procedures, the subsequent samples were prepared in the nitrate form. Fears in long term stability were unfounded as beads loaded in the nitrate form have been stored for 3 years with no apparent degradation. There was no noticeable difference in results between the large and small beads of phase I, with half the laboratories making more precise and accurate measurements on the large beads and half making the better measurements on the small beads. Significantly different mass spectrometric behavior for the differing types of beads was not apparent. One goal of the resin bead exercise was the evaluation of systematic errors and the elucidation of the residual mea-
'E-U
C-U
235/238 10150
235/238 .E9913
.
C-U 233/238 .E8052
D-U 235/238 .E2022
E-Pu 24W239 .E2979
RBCDE RBCDE RBCDE RBCDE RBCDE RBCDE RBCDE 1 .E
1
0
1
+
(b)
I
1.1I
G-PU G-PU H-PU E-PU H-PU H-U 244/239 L40/239 241/239 240/239 244/239 2 3 4 / 2 3 8 233/238 ,61145 .E1756 .01927 ,02956 . 5 7 6 6 8 ,81731 .E3449 H-U
Figure 2. Results for major Isotope ratio measurements. Laboratories and unknowns are identlfled by code. Ratios and reference values are indlcated. Error bars represent f 1s of measured values. Shaded areas represent 95 96 confidence limits of reference values. An asterisk signifies an outlier.
surement error. The results of phases 11-IV of the exercise are examined in this light in the succeeding sections. The handling and determination of nanogram quantities of any material requires that contamination from the environment be scrupulously controlled. The problem is critical since the environment could contain the material of interest, as in the analytical laboratory of a fuel recycling facility. The U contamination from the handling of the bead, loading of the bead into the mass spectrometer, or from the filament substrate could give rise to a significant blank. In our laboratory the U loading blank has been measured and is typically 50 fg of 238U(19).
554
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
Isobaric interferences from elemental and polyatomic ions could arise since thermal ionization mass spectrometers are operated at low mass resolution and these ions would not be resolved from U and Pu isobars. No hydrocarbon background was reported by the participants, and the general accuracy of the measurements of minor isotopes indicates that isobaric interferences were not significant. There exist two cases of isobaric interference inherent in the determination of U and Pu from resin bead samples which were apparent in this study: %lAm interference of %lPu and the mutual 238Pu,238Uinterference. The 241Puhas a half-life of 14.34 years and decays at a rate of about 0.4% per month forming %lAm. Since the beads in the exercise were prepared up to a year before analysis, the 241Amcontent of the beads was significant. Thus, interference from 241Amseverely compromised the accuracy and precision in 241Pumeasurements in the round robin as evidenced in Tables V and VI and Figure 2b. It should be noted that the problem was addressed by several of the laboratories, e.g., laboratories B and D for unknown G. Since Am vaporizes at a lower temperature than Pu, the 241Amcould be eliminated by delaying measurement of 241Pu/239Pu.It should be noted that this problem only occurs when there is a delay between sampling and measurement, since Am is not efficiently loaded onto beads and would not exist in the expeditious analysis of nuclear materials as required by safeguards. One of the advantages of the resin bead technique is the ability to measure both U and P u from a single resin bead because the optimum emission temperatures for the two elements differ by approximately 200 OC (Pu 1450 "C; U 1650 "C). The problem of mutual 238Uand 238Puinterferences has been discussed by ORNL (2). A correction can be determined by monitoring the U signal while measuring Pu, and vice versa. The mixed Pu/U sample, unknown H, of this study was not a good example to test adequately the necessary correction procedures because of the extremely small atom fraction of 23aPuand large atom fraction 23aU. The correction of 23aPu due to was large and difficult to determine. Conversely, the 238Pucorrection of 238Uwas small and insignificant. The results, which are displayed in Table VI, illustrate the difficulties in the 238Pumeasurement: laboratories B, D, and E made corrections, but laboratories A and C did not. The results of these measurements are more appropriately compared to unknown E which had approximately the same isotopic composition of Pu as unknown H, but contained no U. For unknown E, all 2s8Puresults agreed with the reference value. The U and Pu isotopic measurements, which were made from a single bead, were compared with the isotopic measurements made from beads containingU and Pu alone. With the exception of 2s8Puas discussed above, neither the Pu nor the U isotopic measurements noticeably were degraded. Two corrections for detector electronics were made to the data by all laboratories: a background (dark current) correction and a pulse circuit dead time correction. The dark current corrections are insignificant except for the very minor isotopes and are easy to determine and implement. The dead time corrections are signifcant and in general were determined empirically from U isotopic standards. Although the actual value of the dead time affects the magnitude of this correction, it is the uncertainty in the absolute value which will be reflected in the uncertainty and accuracy of the measured data (20).The isotopic SRMs U-100 and U-900 were included in the study which allowed laboratories to assess the accuracies of individual dead time corrections. Although this assessment is limited by the precision in the reported U measurements, no gross errors due to improperly calibrated dead time were detected.
Table VIII. Discrimination/Fractionation Corrections (% per Atomic Mass Difference) lab A
B-1 -2a
A
B
0.057 0.33
0.004 0.33 0.255
C D E F G
0.37 0.200 0.37 0.1 5
0.154 0.37
Uranium C,D CnC 0.057 0.344 0.380 0.255 0.540 0.171 0.180
0.157 0.436 0.416 0.187 0.734 0.154 0.182
H
lab
E,G
Plutonium EnC
A
0.178 0.41 0.255 0.737 0.187
0.122 0.312 0.113 0.643 0.147
B C D E
0.182 0.33 0.255 0.737 0.158
H
Hn
0.159 0.33
0.199 0.270
0.255 0.540 0.166
b
0.220 0.095
Hn
0.201 0.137 0.158 0.697 0.079
Two instruments. Out-of-control measurements. Determined in normalization procedure described in
a
text.
Background corrections due to ion scatter were made by the one laboratorypossessing a single-stageinstrument. These corrections were made at half mass positions on each side of measured peaks and were typically 1-2 counts/s. Multiple stage instruments did not require this correction. Isotopic Fractionation. The most significant correction made to the data by all the laboratories is a mass dependent correction ascribed to discrimination or fractionation. Although these terms describe processes that are fundamentally different, it is difficult to differentiate empirically their relative effects. For this discussion, discrimination is defined as the difference in relative measurement efficiency of one isotope vs. another while fractionation is defined as the preferential evaporationof lighter isotopes and is a physicochemical effect. Discrimination can occur in the extraction, transmission, and measurement of ions formed. For example, if voltage switching is used,the relative isotopic extraction efficiencies in the source can differ. Similarly, relative isotopic pulse-counting detection efficiencies can differ if ion-to-electron conversion efficiencies are low in the first stage of current amplification. In contrast, isotopic fractionation causes the measured isotopic ratios to change with time and the extent of fractionation is dependent upon the temperature, sample size, surface-sample interactions, and matrix effects and is proportional to the mass difference between isotopes. Thus, to reproduce fractionation from one sample to another requires careful control of all measurement parameters. For the purposes of this discussion, both discrimination and fractionation will be referred to as fractionation. In this exercise, SRM U-500, an equal atom 23sU/238ustandard, and SRM 947, a commonly used Pu isotopic standard, were supplied to permit fractionation corrections to be determined from closely matched samples. These samples were used to determine the measurement corrections in two cases. In all other cases, empirical corrections were derived from other standard sample measurements and their applicability was justified by the accuracy in measurement of the standards supplied. The corrections used by each laboratory for each phase of the exercise are summarized in Table VIII. The uncertainty and variability in the fractionation correction is typically ignored in uncertainty statements for corrected, reported values. The use of resin beads precludes the absolute control of sample size and sample matrix which is required for the best precision. For this reason, the external calibration for mass fractionation is potentially more difficult.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
I 233 238111
235 238m
c
F ure 3. Resub of internal normalizationfor sample (a) measured 23 U/238U;(b) measured 23sU/238U; (c) 236U/238U ratios normalized to
9
233U/238U reference and measured values. Error bars represent f 1s of measured values. Shaded areas represent 95% confklence limits of reference values.
Calibration of fractionation for Pu suffers an additional difficulty. No Pu absolute isotopic standards exist but were originally based on U standards (21). The unit mass difference between the major isotopes of these standards also results in an increased uncertainty in corrections that are applied to 242Pu/239Pu or 2uPu/23gPumeasurements as made in this round robin and in isotope dilution measurements of Pu. In the case of U, control of isotopic fractionation was evaluated in the exercise by internal normalization: the fractionation correction determined from the ratio of reference/reported value for one ratio was used to correct the second ratio for each determination. This procedure was used to ascertain the improvement on both measurement precision and accuracy. The results of this procedure are shown for unknown C in Figure 3 where the 236U/298umeasurement was internally normalized to the 233U/238Ureference/measured ratio. A comparison of Figure 3b and Figure 3c shows that internal normalization significantly improves the precision and accuracy of the data set: interbead variability is reduced, most notably for laboratory D, indicating that their analytical procedures result in the least reproducible fractionation of the samples; it is also seen that the accuracy is improved,most notably for laboratories B, C, and D,indicating that inappropriate fractionation corrections were used. This procedure was also used for the 233U/238U:23sU/238U ratios of unknown H, and the 244Pu/239Pu:240Pu/239pu ratios of unknowns G and H where the results were much less dramatic and conclusive. One of the reasons was the increased uncertainty caused by the measurement of the smaller ratios of these unknowns. Another reason is that for Pu the uncertainty and extent of the correction are decreased when the five mass unit difference is extrapolated to the one mass unit difference. The fractionation corrections as determined by internal normalization are tabulated in Table VIII. The average per mass unit change in fractionation correction for U as determined by internal normalization was 0.06%;for Pu, 0.09%. Although it has been known for some time that isotopic fractionation of U can be normalized internally by double spiking using naU and 233U(22),only recently has the power of the technique been demonstrated (23, 24). Chen and Wasserburg have double spiked nanogram samples of natural U and achieved measurement precisions of 0.15% (1s) (23). One of the major obstacles to the use of the double spiking technique in the past has been the availability of the isotopically pure spike materials. High sensitivity mass spectrometry will reduce the quantity of spike material required by at least a factor of lo00 relative to conventional procedures and thus
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make routine double spiking both attractive and cost effective. Measurement Accuracy, When safeguards measurements are performed, the ability to demonstrate and document the accuracy of the measurement system is of paramount importance (25). To put the accuracy achieved in this experiment in the proper perspective requires a comparison with mass spectrometric measurements currently being performed in the safeguards field. Preliminary results of the IDA-80 measurement program are now available (15). This experiment was composed of more than 25 laboratories making routine safeguardsmeasurements. As part of this exercise the isotopic composition of two materials containing U and PU were measured. These materials contained the major isotope ratios 23sU/238U and 240Pu/23ePu in one (RU), and 240Pu/239Pu and 242Pu/239Pu in the second (BU). Although the compositions are reported in terms of isotopic abundances,the errors in measurement will not differ significantly from errors in measurement of ratios. For the 235Uabundance, 13 of 28 laboratories were within f0.25% of the reference value; 20 of 28 were within f 0.50%;6 laboratories showed greater than 1.0% error. For the 240Puabundance in the two materials, 49 of 57 laboratories were within f0.25% of the reference value; 56 of 57 were within f 0.50%. For the 242Puabundance, only 8 of 29 laboratories were within f 0.25%;15 of 29 were within f0.50%; 7 laboratories showed better than 1.0% error. These data are comparable to the data presented here: 19 of 29 235U/238U measurements were within f0.3% and 23 of 25 were within f 0 . 5 % ; 15 of 15 240Pu/239Pu measurements were within f0.30%; and 4 of 10 244Pu/239Pu measurements were within f 0 . 3 % and 7 of 10 f 0 . 5 % . For none of these ratios in this experiment were there errors greater than 1.0%. The IDA-80 report does not present the precision of measurement in the preliminary results (15). The comparison between these interlaboratory experiments is made with the caveat that, in general, the missions of the laboratories involved and probably the participatory attitudes were distinctly different. In summary the median deviation from the reference values for major isotopic ratio measurement was +0.10%; 90% of these measurements were within -0.35 to +0.85% of the reference values. There were a total of 260 individual major isotopic ratios determined from 153 beads analyzed, representing 70 replicate measurements of the 13 major isotopic ratios in 7 unknowns by the participating laboratories. Of the 70 measurements of major isotopic ratios, two were measurement blunders, a blunder being defiied as a deviant value unexplainable by any known source of systematic error. Of the 153 unknown beads analyzed, two analyses were rejected by the participating laboratories because of rapid signal decay. Finally, less than 5% of the samples resulted in questionable measurements by the participating laboratories and greater than 96% of the measurements of the major isotopic ratios were made under reasonable measurement control.
CONCLUSIONS Other sample loading techniques used in conjunction with high sensitivity measurement have recently been reported (23, 26) for U measurement. The possible conjunction of the resin bead technique to sample and ship U and Pu combined with an alternative mass spectrometric sample loading technique may be necessary to gain the utmost precision and accuracy. The full potential of and possible improvement in the resin bead mass spectrometric loading technique have yet to be explored. The accurate isotopic measurement of Pu will require the placement of the Pu isotopic system on an absolute basis. This procedure is analogous to an absolute atomic weight determination and requires a combination of a highly accurate assay and precise mass spectrometric measurements of separated
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3, MARCH 1984
and mixed isotopes (27). This procedure has not been used in the past because of the lack of the separated Pu isotopes. However, these isotopes are now available and an experimental program has been initiated to put the Pu isotopic system on an absolute basis. The result of this program will be a 242Pu/239Pu equal atom isotopic standard analogous to the 23sU/238U uranium equal atom standard (SRM U-500). It can be concluded from this interlaboratory study that high sensitivity mass spectrometry can be used to make accurate and precise isotope ratio measurements of nanogram quantities of U and Pu. Furthermore, the resin bead has been shown to have no significant loading blank or mass spectral interferences for U and Pu. The resin bead has been demonstrated to be a convenient vehicle for sample transport, distribution, and analysis.
ACKNOWLEDGMENT This project would have been impossible without the generous support of David H. Smith and his colleagues a t Oak Ridge National Laboratory, Bob Lagergren and his colleagues a t Pacific Northwest Laboratories, Joe Goleb at the Department of Energy, and the participants in this exercise whose generous cooperation and enduring patience are deeply appreciated. The assistance of our colleague L. A. Machlan in preparing the Pu samples is also greatly appreciated. Registry No. 233U, 13968-55-3;234U, 13966-29-5;23sU, 15117-96-1; ‘W, 13982-70-2; =Pu, 13981-16-3; 24OPu, 14119-33-6; 241P~, 14119-32-5; 2 4 2 P ~13982-10-0; , 2 4 4 P ~14119-34-7; , 23gP~, 15117-48-3.
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RECEIVED for review July 19,1983. Accepted December 12, 1983. Preliminary results of this work were presented at the 25th ORNL Conference on Analytical Chemistry in Energy Technology, Gatlinburg, TN, 1981. This work was supported by the Office of Safeguards and Security, U.S.Department of Energy. Certain commercial equipment, instruments, or materials are identified in this paper to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
