Anal. Chem. 1992, 64, 2330-2343
2599
High-Resolution Determination of 147Pmin Urine Using Dynamic Ion-Exchange Chromatography Steve Elchuk, Charles A. Lucy:*+ and Kerry I. Burns
AECL Research, General Chemistry Branch, Chalk River Laboratories, Chalk River, Ontario, Canada KOJ 1JO
A procedure has beon developed for measuring 147PmIn bloasmy sampks, baud on the wparation and preconcentration of lnPm from the urine m a t h by adrorptlon onto a conventional catlon-.xchange column with final Separation and purltlcatbn by HPLC wing dynamk ionaxchange chromatography. The concentration of 147Pmh determined by colhctlng the appropriate HPLC fraction and measuring the lr7Pmby liquid aclntiiiation counting. Tho limit of detection b 0.1 Bq (3 fg) 147Pmb a d on a SOOmL sample of urine and a counting tkne of 30 mln wtth a background of 100 cpm. Ten samples can be procesd in 1.5-2 days.
INTRODUCTION Within CANDU reactors, 3Handvolatile, long-lived fission produde such as 137Csand g0Sr/wY are the radionuclides which are most closely monitored to ensure the health and safety of personnel. However, in the production of separated radionuclidesfor radiopharmaceuticalapplications or during the maintenance of reactor components, rare-earth nuclides such as 147Pmand W e can be of concern as well. If ingested or inhaled, only a small fraction of the rare-earth inventory is absorbed, however that portion is translocated to the liver and skeleton where it is retained with a biological half-life of 3500 days.1 These long biological residence times can lead to significant radiation doses, thus making it necessary to regularly monitor peraonnel who come in contact with these radionuclides. Monitoringof personnel for internal exposure can be based on the following approaches: direct in vivo monitoring of the radionuclides, measurement of radionuclides in excreta, or estimation of intake based on a personal air sampler. The choice of measurement approach is dictated by the mode of decay of the radionuclide (Le. a,8, or y-ray), ita metabolic behavior, and the sensitivity, availability, and convenience of the appropriate meaeurement facilities.2 Direct monitoring of the whole body or specific organs provides a reliable means to measure the retained activity within the body following an intake. "Whole-body scanning" is usually performed using y spectrometers3and is the method of choice for y-emitting radionuclidessuch as 'We and 137Cs.2 Whole-body scanning, however, ie of little value in assessing retained activity of pure @-emitterssuch as 147Pmand Y3r/mY, except when there is high internal contamination and when the body burden of other radionuclides is very low.= At levels appropriate to routine internal dose monitoring, internal contamination by pure @-emittingradionuclides is t Present address: Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4. (1)ICRP. Limitefor Intakesof Radionuclidesby Workers. Publication 30; Pergamon Press: Oxford, U.K., 1981. (2) ICRP. Individual Monitoring for Intakes of Radionuclides by Workers: Design and Interpretation. Publication 54; Pergamon Press, Oxford, U.K., 1988. (3) IAEA. Directory of Whole-Body Radioactivity Monitors; STI/ PUB/213; IAEA: Vienna, 1970. (4) Johnson, J. R.;Lamothe,E.S.; Kramer,G. H.Radiat. h o t . Dosim. 1987,20,267-269.
determined by measuring their concentration in urine. The DosimetricResearch Branch at the Chalk River Laboratories (CRL) routinely screens personnel for radiologically significant @ contaminants by coprecipitating the radionuclides with calcium oxalate from buffered urine (pH 3.8). The dried precipitate is measured with a low background gas flow proportionalcounter.6 Although this technique is satisfactory for detecting high-energy @ emitters such as gOSr/BOY,it has poor counting efficiencies for low-energy fl emitters due to attenuation in the precipitate. Thus while this procedure may be useful as a screening test, it cannot be readily used to identify the contaminant, determine the level of intake, or establish the committed dose equivalent.2 To determine the committeddose equivalent, it is necessary to know the identity and concentration of each radionuclide contributing to the total body burden. Previously, if the screening indicated possible contamination, a solvent extraction procedurebased on bis(2-ethylhexyl)phosphoric acid (HDEHP) in n-heptane was used to separate %r, wY, 147Pm, and ' W e fromurine.7 However, a number of difficultieswere encountered when this procedure was used to measure 147Pm: formation of precipitate in the liquid scintillation cocktail; poor precision in the results because of variability in the recovery; interference due to the presence of other radionuclides in the 147Pmfraction; long sample preparation times. In this work, chromatographic procedures have been developed for the determination of rare-earth radionuclides, most notably 147Pm, in urine. Urine is a complex solution containing an array of inorganic salts and organic constituents, and its composition varies considerably depending on the dietary habits of the individual. In this matrix, the concentration of the radionuclides of interest will be at femtogram levels, because of their long biological half-life. Therefore a cation-exchange sample preparationstep was used to minimize the interference from the inorganic salts in the urine matrix and to preconcentrate the rare earths. The resultant rareearth sample was then analyzed by dynamic ion-exchange chromatography. Previous studies have shown that dynamic ion-exchange chromatography, when coupled with postcolumn reaction detection, enables precise and accurate determinations of nanogram amounts of lanthanides in complicated matrices.Gl0 It has also been used as a rapid and convenient procedure for separation and determination of lanthanides for nuclear fuel burnup measurementsllJ2 and in the separation of actinides prior to their measurement by a spec(5) Johnson, J. R. Report AECL-5854;Atomic Energy of Canada Ltd.: Chalk River, Ontario, 1977. (6) Mawson, C. A.; Fischer, I. Report CRM-456; National Research Council, Research Division: Chalk River, Ontario, 1950. (7) Kramer, G. H.; Davies, J. M. Anal. Chem. 1982, 54, 1428-1431. (8) Knight, C. H.; Casaidy, R. M.; Recoskie, B. M.; Green, L. W. Anol. Chem. 1984,56,474-418. (9) Cassidy, R. M.; Elchuk, S.; Elliot, N. L.; Green, L. W.; Knight, C. H.; Recoskie, B. M. Anol. Chem. 1986,58,1181-1186. (10) Barkley, D. J.; Blanchette, M.; Cassidy, R. M.; Elchuk, S.Anol. Chem. 1986,58, 2222-2226. (11)Elliot, N. L.; Green, L. W.; Recoskie, B. M.; Caesidy, R. M. Anal. Chem. 1986,58, 1178-1181.
0003-2700/92/0364-2339$03.00/0 0 1992 American Chemical Society
2940
ANALYTICAL CHEMISTRY, VOL. 64, NO. 20, OCTOBER 15, 1992
trometry.12J3 In this work, 147Pm is separated from other rare-earth elements by dynamic ion-exchange chromatography and determined off-line by liquid scintillation counting of the appropriate HPLC fraction. EXPERIMENTAL SECTION HPLC System. The HPLC system consisted of a Spectra Physics pump (Model 8700, Spectra Physics, Santa Clara, CAI, a Rheodyne sampling valve (Model 7125, Rheodyne, Berkeley, CA) fitted with a 1.0-mL sample loop, a 150-mm- X 4.6-mm4.d. Supelco column packed with 5-pm C1e reversed-phase particles, a Rheodyne switching valve (Model 7000), a fixed-wavelength (658-nm)detector (Model441, Waters Associates, Milford, MA), and a Spectra Physics Computing Integrator (SP4100). The eluted metal ions, except for promethium, were measured colorimetrically by a postcolumn reaction with 3,6-bis[ (oarsenophenyl)azo]-4,5-dihydroxy-2,7-naphthalenedisulfonic acid (Arsenazo111). The postcolumn reagent was added to the eluate downstream of the fraction collection valve via a screen-tee reactor,14 using a syringe pump (ISCO M314, Lincoln, NE). Reagents and Standards. a-Hydroxyisobutyricacid (HIBA) (Aldrich Chemical Co., Milwaukee, WI) was purified by passage through a 3- X 200-cm AG 50W-X8cation-exchange column and filtered through a 0.8-pm/0.45-pm cartridge filter (Nalge Co.). The modifier used for the reversed-phase column was l-octanesulfonate (Regis Chemical Co., Martin Grove, TN). All other reagents were reagent grade, and all aqueous solutions were prepared from quartz double-distilled and deionized water (Milli-Q system). All eluents were filtered through 0.45-pm Millipore filters before use to remove any particulate and to degas the solvent. Dilute solutions of lanthanide standards were prepared from Spex Industries standard solutions. The 14Tm standard was from Amersham Canada (Oakville, ON, Canada) and had an activityof 651.3Bq mgl at reference time 87-Nov-23@I 12hEST, based on calibration by 4 4 counting with 'Wo as an efficiency tracer. A half-life of 958.18 f 0.15 days was used to calculate the concentration of l47Pm at the time of use. Sample Preparation. To aid in the development of this procedure, a number of rare-earth tracers ("La, 147Nd,14eJa1Pm, and IWe) were prepared by irradiating the appropriate lanthanides (10 pg of La, 2.0 mg of Nd, and 1mg of Ce) in the NRU reactor (Chalk River Laboratories, Chalk River, Ontario) for a period of 19 h at a flux of 2 X 10" n cm-2 5-1. In initial studies, a 100-pL aliquot of each tracer containing 8 ng of Nd, 4 ng of Ce, 40 pg of La, and -20 pg of Pm was added to 500 mL of urine, while in subsequent recovery studies 100ng of U, 11Bq of 14Tm (0.32 pg), and lo4 Bq of 141Cetracer were added to the sample. In all cases, 100ng of Y and Sm carrier were added to the 500-mL urine samples. The urine samples were not pooled, so that the effect of variability in the urine matrix on the analysisprocedure could be assessed. Initial experiments with urine spiked immediately before processing indicated that acidifying the urine to 0.5 M HN03 and heating to 80 OC resulted in complete recovery of the tracer lanthanides on a 10-mLcation-exchangecolumn. However,when this same procedure was applied to urine samples which had been spiked and then allowed to equilibrate for 4 days, only 4050 5% of the tracer inventory was retained on the cation resin. The poor recovery was attributed to incomplete destruction of organic lanthanide complexes that had formed in the urine and which were not retained on the ion-exchange column. Additional experiments using urine which had been spiked and allowed to equilibrate for 4 days indicated that acidifyingthe sample to 0.5 M HN03and boiling for 1h successfully destroyed any complexes which had formed and resulted in complete adsorption of the lanthanides on the cation column. In all subsequent experiments the urine specimens were acidified to 0.5 M HN08 and boiled for
a period of 1h before processing. The samples were then cooled to room temperature, fdtered by gravity using Whatman No. 41 filter paper and passed through a 10-mL bed of AG 50W-XS cation-exchange resin (100-200 mesh). The resin was washed in turn with 25 mL of water and 250 mL of 1.0 M NH4Cl (pH 5.0) and again with 25 mL of water, and the lanthanides were eluted from the resin with 35 mL of 0.03 M citric acid (pH 5.0). The citrate solutionwasevaporated to dryness and then ignited at 520-550 "C for 30 min. The lanthanides were converted to the nitrate form by dissolving the residue in 3-4 mL of concentrated HN03/2% HF and evaporating to dryness. Finally, the lanthanide nitrates were dissolved in 1.0 mL of HIBA solution (0.05 M at pH 3.5) containing 1-octanesulfonic acid (0.01 M). Sample Analysis by HPLC. The 1.0-mL sample loop was flushed with distilled-deionized water, the sample was loaded into the loop through a 0.45pm syringe cartridge filter, and then valved "in-line" with the eluent. The solvent for the gradient elution was varied from 0.05 M HIBA to 0.25 M HIBA over 15 min at a flow rate of 1.5 mL/min; for both eluents the pH was 4.6. The concentration of 1-octanesulfonate was constant to 1X throughout a given gradient, and ranged from 7.5 X 1V2M in the various studies. The eluted fractions of 147Pm and were collected using a manually operated switching valve, for subsequent determination by liquid scintillation counting and y spectrometry, respectively. The elution times required for collectingthe fractions were determined prior to the analysis using lanthanide standards and were verified during the sample runs using the Sm peak as a marker. Radiochemical Analysis. Two milliliters of the lr7Pm fractionwere mixed with 18.OmLof Scintiverseliquid scintillation cocktail (Fisher Chemical Co.) and counted on a Beckman LS 2800 counter (Beckman Instruments Inc., Nuclear Systems Operation, Irvine, CA) for 30 min. Calibration of the liquid scintillation counter using a 14'Pm standard gave a counting efficiency of 91 3% for Ir7Pm under these conditions. The background for the liquid scintillation counter was 100 cpm. The lrlCe fraction (2.0 mL) was counted on a EGLG Ortec PopTop high-purity germanium y spectrometer (30% relative efficiency). The recovery of the I41Ce was determined relative to 2.0 mL of eluent containing the same quantity of lrlCe as spiked into the urine. ICP Analyses. Samples from intial cation-exchange studies were analyzed with an Applied Research Laboratories 3520 B inductively coupled plasma-atomic emission spectrometer using a 1.0-mPawhen-Runge vacuummonochromatorwith a 1080linea/ mm grating. The specificconditions were rf power 1.5 kW, torch viewing height 14 mm above the load coil, nebulizer uptake rate -3.0 mL/min, integration time 5 s/step, and argon flow rates (nebulizer)1.5L/min, (plasma) l.OL/min,and (coolant)10L/min. The lines used for each element were Sm 359.260 nm, Nd 430.357 nm, Sr 407.771 nm, Ca 317.933 nm, and Y 371.030 nm.
*
RESULTS AND DISCUSSION
Cation-Exchange Pretreatment. The recovery of the rare-earth elements from the cation exchange pretreatment was determined by adding 1aLa, 147Nd,149J51Pm,and 1 W e tracers to urine specimens obtained from seven individuals, along with 100 ng of Y and Sm carrier. The lanthanideamino acid complexes were destroyed by acidification and boiling, and then the metals were preconcentrated on a 10mL bed of AG6OW-X8 cation-exchange resin. Monitoring of the eluting urine matrix by y spectrometry indicated that less than 1% of the lanthanide tracers were lost during the preconcentration step. The bulk of the inorganic urine matrix retained on the cation exchanger was calcium. It was determined that citric acid quantitatively elutes the lanthanides while leaving calcium on the column. Best removal was achieved at pH < (12)Cassidy,R.M.;MiUer,F.C.;Knight,C.H.;Roddick,J.C.;Sullivan,4, but even at pH 9 only 1.6 pg/mL of Ca was present in the R.W. Anal. Chem. 1986,58, 1389-1394. citric acid. The pH of the citric acid eluent also affects the (13)Elchuk,S.;Burns,K.I.;Cassidy,R.M.;Lucy,C.A.J.Chromatogr.elution of the lanthanides (Figure l),with only a minor portion 1991,558,197-207. of the lanthanides being eluted below a pH of 6. However, (14)Cassidy,R.M.; Elchuk, S.; Dasgupta, P. K.Anal. Chem. 1987,59, 85-90. as the pH is increased above pH 6, the lanthanide recovery
ANALYTICAL CHEMISTRY, VOL. 64, NO. 20, OCTOBER 15, 1992
*
2341
m
U
n IW
a -1 W
U W
l
=I
I-
w
u
I
z W LL
n W
-_
, 0
2
I
I
L
I
I
6
8
aJ u
10
PH
Flgure 1. Effect of pH on elution of lanthanldes and yttrlum in urine from AQ50W-X8 catlonaxchange resin by 0.03 M citric ackl. Experlmental conditions are as described in the Sample Preparatbn. The sample was splked with 1 mg each of Sm, Nd, and Y, and their concentrations In the eluate were determined by ICP-AES.
gradually decreases. Based on Figure 1, a pH of 5 was used to elute the lanthanides. Initial studies were conducted using a citrate concentration of 0.3 M, however a reduced concentration of 0.03 M was found to elute the lanthanides equally well and reduced the subsequent combustion time to 30 min. Tracer studies and ICP analyses indicate that 70-90% of the lanthanides and 65% of the yttrium are recovered from this procedure. However, as can be seen in Figure 1, there are significant differencesin the recoveries of the lanthanides. This will impact on the internal standard, as discussed below. Dynamic Ion-Exchange Separation of Lanthanides. The chromatographic conditions used in this work were based on those previously developed for the separation of lanthanides in spent thorium-uranium dioxide fuele.4 Figure 2 shows the separation of an aqueous standard containing 100 ng/mL of the lanthanides, uranium, and yttrium and 8 pg/mL of Ca. The peak at 1.4 min is composed of transition-metal impurities from the HF. If calcium is not removed from the urine sample prior to the dynamic ion-exchange chromatography, the large concentration of calcium will overload the column and alter the elution times of the lanthanides,resulting in incorrect fraction collection and erroneous determination of the radioisotopes present. Determination of 14Tm and 144Ce in Urine. Thirteen urine samples were spiked with 11.0 Bq of 147Pm, 104 Bq of 141Ce tracer, and 100 ng of inactive Sm carrier and processed through the full separation and detection protocol. These samples were not pooled so that the effect of variations in the urine matrix on the analysis could be assessed. The final seven samples were spiked with a further 100 ng each of Y and U. The results for Y and U will be discussed below. A typical sample chromatogram is shown in Figure 3, where the peak at 1.4 min is attributed to the presence of transition metals in the urine. The baseline shift at the retention times corresponding to Pm and Ce results from the diversion of the eluent from the column to a collection vial at these points. The Pm and Ce fractions were analyzed by liquid scintillation counting and 7 spectrometry, respectively. The recoveries of 147Pm, "lee, and the inactive Sm carrier are reported in Tables I and 11. Sample 5 of Table I represents a "catastrophic" sample preparation incident, wherein the sample was spilled after the cation-exchange pretreatment resulting in the loss of approximately one-third of the sample solution. Nevertheless, the recovery of 147Pm is equivalent at the 955% confidence interval to that of Sm for this sample
.-c'
E m
Flgure 2. Dynamic bn-exchange separatlon of a standard solutbn contalning 100 ng of each lanthanide, yttrlum, and uranium and 8 pg of Ca. The standard solution was processed as described In the Experimental Section. m
u
E
VI
H
.-i E
Ln
Flgwe 9. Dynamk bn-exchange separatlon of a urine sample splked with 100 ng of Sm, Y, and U. Conditions are as described in the
Experimental Sectlon.
and for all other samples. Whereas the 141Ce and 147Pm recoveries differed at the 95 % confidence interval for 3 of the
2342
ANALYTICAL CHEMISTRY, VOL. 64, NO. 20, OCTOBER 15, 1992
Table I. Recovery of Lanthanide Spikes from Urine Samples. Sm yield 147Pm yield 141Ce Yield (HPLOb sample (LSC)b (7 spec)b 85f 5 82 f 2 80 f 3 1 73 f 3 9Of3 79 f 5 2 91 f 5 82 f 3 93*3 3 90 f 5 82 f 3 91 f 3 4 67 f 3 50 f 4 60 f 5 5c 71 f 3 85 9
6
avd
68f 5
64 1 4
83 f 9
77f8
A 500-mLaliquot of each urine sample was spiked with 11.0 Bq of 147Pm, 104 Bq of 141Ce,and 100 ng of Sm and handled as described in the Experimental Section. b Errors are the standard deviations (at 1s level) associatedwith the peak area measurement (HPLC) or the counting statistics (liquid scintillation counting and y spectroscopy). c Samplewas accidentallyspilled,resultingin the loss of about one-thirdof the samplevolume. Samplenumber 5 is excludedfrom the average. Table 11. Recovery of Lanthanide, Yttrium, and Uranium Spikes from Urine Samples. Sm yield 147Pmyield 14lCe yield Y yield U yield sample (HPLC)b (LSC)b (y spec)* (HPLOb (HPLC)* 7
8 9 10 11 12 13
av
70f3 70f3 8113 67f3 83f3 92f3 82f3 78f9
67f5 67f5 87*4 70f5 85f4 89i4 83*4
84f3 79f3 9Of3 76f3 90f3 91f3 93*3
50f4 63f4 80f3 50f4 72f4 86f4 78*4
77f6 73f6 60f7 81f6 81f6 85f5 85f5
78*10
86&7
68f14
77f9
A 500-mL aliquot of each urine sample was spiked with 11.0 Bq of 147Pm, 104 Bq of 141Ce, and 100 ng of Sm, U,and Y and handled as described in the Experimental Section. Errors are the standard deviations (1s) associatedwith the peak area measurement (HPLC) or counting statistica (liquid scintillation counting and y spectroscopy). (1
~~~~
~
~
~
~
13 samples. Therefore the internal standard should be as chemically similar to promethium as possible-Le. samarium or neodymium. An additional urine sample was spiked with 147Nd and 141Ce, and passed through the cation-exchange and HPLC separation stages. The resultant Pm fraction was analyzed by y spectrometry, and contained undetectable quantities of these radionuclides. Thus, on the basis of the minimum detectable activity for 147Nd, the decontamination factor for adjacent lanthanides is 2790. On the basis of an average recovery of 80 % and a counting efficiency of 91% ,the limit of detection for 147Pmin 500 mL of urine is 0.1 Bq for a 30-min count on a liquid scintillation counter with a background of 100 cpm. Use of a lowbackground liquid scintillation counter or a longer count time would further improve the sensitivity of the method; however the current sensitivity was sufficient for our application. Determination of Other Radionuclides. Preliminary studies were also performed on two other radionuclides, U and Y. Uranium and yttrium were separated from the bulk cations of the urine matrix using a cation-exchangecolumn in the same fashion as for the lanthanides. Tracer studies determined that U recoveries from this initial step were comparable to those of the lanthanides. However ICP-AES analyses showed that Y had significantly lower recoveries (Figure 1). To improve the dynamic ion-exchange separation between U and Y, the octanesulfonate concentration was reduced to 7.5 x 10-3 M. Yttrium and the lanthanides are retained solely by ion exchange with the octanesulfonate, whereas the uranium forms a much more hydrophobic complex with HIBA
.-c'
E
L n
Flgurr 4. Dynamic ion-exchange separation of a standard solution of urine containing 100 ng of each lanthanlde, uranium, and yttrium and 8 lis of Ca. Condltkns are as deecribed in the Experimentalseotkn, except that the loctanesulfonicacid concentratbn Is 7.5 X lo4 M.
and so is less affected by changes in the ion-exchangecapacity of the column.6JsJ6 Thus by decreasing the concentration of 1-octanesulfonate, the retention of uranium on the column can be adjusted so that it does not coelute with any of the lanthanides or yttrium (Figure 4). Table I1 shows the recoveries of each of the radionuclides in the urine samples. The results for the lanthanides were discussed above. The standard deviation for the U recovery is greater than that of Sm, which was also determined using postcolumn reaction with Arsenazo 111,because the U peak tends to tail, making peak area determinations less precise. For all cases except sample3, the U recoveriea were equivalent to those of Sm, 1r7Pm,and 141Ce within the 95 9% confidence interval. Sample 3 was within the 99 ?4 confidence interval. Therefore, it is concluded that a lanthanide carrier can be used as a recovery marker for U as well as 147Pm. The limit of detection for U in urine by HPLC Separation and postcolumn reaction detection with Arsenazo-111 is 10 ng in a 500-mL urine sample. This is much more sensitive than the 1-rg detection limit achieved by using anion-exchange purification followed by spectrophotometric determination of the uranium-Arsenazo I11 complex,17 or the 1 pg/L U detection limit of the fluorometric method.18 The recoveries observedfor yttrium for the cation-exchange pretreatment and overall process were consistently lower than those obtained for the rarsearth elements(Figure1and Table 11). Therefore inactive Y would be the only accurate internal standard which could be used to assess the recovery of (15) Caeeidy, R. M.;Elchuk, S.; Green, L. W.; Knight, C. H.; Miller, F. C.; Recoskie, B. M. J. Radioanal. Nucl. Chem., Art. ISSO, 199,5&64. (16) Kerr, A.; Kupferachmidt,W.; Attas, M. Anal. Chem. 1988,60,
2729-2733. (17) Kreasin, I. K. Anal. Chem. 1984,56, 2269-2271. (18) Kramer, G. H.; Johnson, J. R.; Green, W. Report AECL-8251; Atomic Energy of Canada Ltd.: Chalk River, Ontario, 1984.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 20, OCTOBER 15, 1992
This would preclude collection of the Y fraction prior to the addition of the Arsenazo I11 postcolumn reagent, and so it would be necessary to decolorize the solution by wet ashing with HN03prior to analysis by liquid scintillation counting. Thorium could not be determined using this procedure, since it was lost during filtration (most likely as an insoluble phosphate) prior to the initial cation exchange.
2343
gratefully acknowledged. This work was presented in part a t the International Ion ChromatographyForum, September 17-19, 1989, Boston, MS.
RECEIVED for review March 20, 1992. Accepted July 15, 1992.
ACKNOWLEDGMENT Robert Martin performed the calibration of the 147Pm standard. Helpful discussions with Stan Linauskas are
Registry No. 147Pm,14380-75-7; l4La, 13981-28-7; 147Nd, 14269-74-0; 141Ce,13967-74-3; U, 7440-61-1; Y, 7440-65-5.
