Ion Exchange Beads as Calibrated Microscopic Radioactivity Sources Paul B. Hahnl and Bernard Shleien Northeastern Radiological Health Laboratory, U. S. Department of Health, Education, and Welfare U . S. Public Health Service, Winchester, Mass. 01890 Spherical microscopic radioactivity sources with diameters ranging from 10-500 pm and activities from 1-25,000 dpm have been prepared by equilibrating milligram quantities of ion exchange resin beads with microcurie quantities of radioactivity in aqueous solution. The correlation between activity taken up by an individual bead and its measured diameter was shown to be excellent, the relative standard deviation of source specific activities (dpm/unit volume) for beads from the same equilibration being on the order of 2%. The demonstrated ability to prepare these sources ranging in activity over four orders of magnitude from a variety of radionuclides and ion exchangers and to precisely calibrate them in terms of a microscopic diameter measurement makes available to the nuclear industry a new approach for preparing point source radioactivity standards.
IT HAS BEEN DEMONSTRATED that ion exchange beads have the ability to contain a precise quantity of exchangeable material based on the number of exchangeable sites per unit weight or volume of the ion exchange material ( I , 2). Owing to the fact that individual particles of ion exchange material prepared in the bead form are uniformly spherical, the number of exchangeable sites in an individual bead will be directly proportional to the cube of the diameter of the bead which can be conveniently measured to a high degree of precision and accuracy by microscopic techniques (3). As a consequence, it should then be possible to saturate an ion exchange material with a given counterion, precisely measure by conventional microtechniques the density ( p ) and the counterion weight fraction (f)of the ion exchange material, and predict the quantity (m*)of the counterion on an individual bead as a function of its diameter (6).
Freeman et al. (2) have developed techniques for the production of microstandard ion exchange beads loaded with calcium and sodium. The beads range from 5 to 25 pm in diameter and contain from to gram of the element with the bead diameter representative of the ion content to within a few per cent. In the present work, ion exchange beads are employed for the production of calibrated microscopic radioactivity sources. Sources which ranged in diameter from 10-500 pm and in activity from 1-25,000 disintegrations per minute were prepared from 9OSr-g0Y, 204T1, 63Ni,and l 3 I I activities and several commercially available ion exchange resins. Possible applications of these sources which can be calibrated quite precisely in terms of their measured diameter, include the calibration of nuclear counting instruments or the evaluation of the dose response of nuclear Present address, Department of Chemistry, Iowa State University, Ames, Iowa 50010 (1) D. H. Freeman, Nature, 218, 563 (1968). (2) D. H. Freeman, L. A. Currie, E. C. Kuehner, H. D. Dixon, and R. A. Paulson, ANAL.CHEM., 42, 203 (1970). (3) D. H. Freeman ion “In Exchange,” Volume I, J. A. Marinsky, Ed., Marcel Dekker, Inc., New York, N. Y., 1966, Chapter 5. 1608
emulsions where point sources of activity may be desired. The activity may also be desorbed from a bead with an electrolyte to introduce a known activity into a tracer study. EXPERIMENTAL
Apparatus. Diameters of individual beads were measured with an American Optical Company Spencer Microstar binocular microscope with integral illuminator base and variable transformer fitted with a Model 426 filar micrometer eyepiece. Beads were assayed for beta activity either by direct counting with a Transnuclear Model CLL low background anticoincidence beta counter or by liquid scintillation counting of the desorbed activity with a Packard Model 3375 Tri-Carb liquid scintillation spectrometer. Gamma counting of individual sources was accomplished with a Harshaw Chemical Company Model 16MBS/16B 4-in. X 4-in. integral line NaI(T1) detector coupled to a Victoreen Model ST400 multichannel analyzer. Special Materials. Radionuclides of B3Ni and 204Tl used for source preparation were obtained from the New England Nuclear Corporation. Standard solutions of goSr-90Y, 204Tl, and lalI used for source preparation and/or for the calibration of counting instruments were obtained from Analytical Quality Control Service, Bureau of Radiological Health, U. S. Public Health Service. The ion exchange resins used in this study were reagent grade Dowex 5OW-X12, 200-400 mesh, H+ form, lot 30335, and reagent grade Dowex 1-X8, 50-100 mesh, C1- form, lot 34508 from the J. T. Baker Chemical Co., analytical grade AG 50W-X8, 50-100 mesh, Hf form, lot 6622, from Bio-Rad Laboratories and a test sample of sulfonated poly (0.92 styrene/0.08 m DVB) beads 4-30 pm in diameter, supplied by the Separation and Purification Section of the National Bureau of Standards. Source Preparation. Microscopic radioactivity sources were prepared by equilibrating a known quantity-i.e., 10-50 mg of ion exchange resin with approximately 5 ml of solution containing 2-10 pCi of the desired radionuclide. The mixtures were tumbled slowly in a 6-ml polyethylene vial for periods of time ranging from several hours to several days. Equilibrating conditions varied. In some cases, cation exchange resin in the H+ form was converted to the Na+ form before equilibration; in other cases, the resin was converted to the chemical form isotopic with the equilibrating radionuclide. The equilibrating solutions which contained both isotopic carriers and acid or base were adjusted to as low an ionic strength as practicable to ensure maximum uptake of the desired radionuclide by the ion exchanger. A typical equilibration involved the competition between 10-300 pequiv of acid or base, 0.2-3 pequiv of isotopic carrier, and the 50-200 pequiv of the counterion originally bound to the resin for the 50-200 pequiv of ion exchange sites. Table I presents the conditions under which the various equilibrations were performed. After equilibration, the resin was filtered by suction on a Millipore Swinnex-13 filter unit and washed successively with 10 ml of distilled water, 10 ml of anhydrous ethyl alcohol, and 10 ml of cyclohexane. The resin was stored under 10-20 ml of cyclohexane in a glass vial. The microsources were mounted in lots of several hundred by spreading a few drops of the cyclohexane slurry on a glass microscope slide. Sizing and Activity Determinations. Individual sources from each equilibration were sized microscopically with a
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
Table I. Equilibration Conditions Equilibration
Nuclide
Resina 44 mg 2 pCi AG 50W-X8, Sr2+ 1 Y3r (50-100 mesh) 52 mg 10 pCi AG 50W-X8, Ni2+ 2 3Ni (50-100 mesh) 12 mg 4 pCi Dowex l-X8, C13 1311 (50-100 mesh) 12 mg Dowex 1-X8, C1(50-100 mesh) 15 mg Dowex 5OW-X12, Na+ (200-400 mesh) 13 mg 10 pCi Dowex 50W-Xl2, Na+ 6 6 3Ni (200-400 mesh) 11 mg 10 pCi 7 Dowex 5OW-X12, Ni2+ E 3Ni (200-400 mesh) 10 mg 10 pCi 8 Dowex 5OW-X12, Na+ 20 4 ~ 1 (20@-400 mesh) 10 mg 5 pCi 9 Microfine-X8, Na+ gOSr (4-30 pm) a Resin dried at 125 "C and desiccated over anhydrous CaS04. * Total volume for all equilibrations 5.0 ml.
filar micrometer eyepiece calibrated with a ruled scale. Diameter measurements were made at 100X ,430 X , and 970 x magnifications for the 50-100 mesh, 200-400 mesh, and the 4-30 pm beads, respectively. Before sizing, the beads were allowed to equilibrate with the humidity in the air for several hours. The beads were manipulated and transferred with a 3-mil tungsten wire or a small forcepts. Activity measurements of the smaller goSr-90Y sources (equilibrations 5 and 9) were made by counting individual beads in a low background anticoincidence beta counter. It was assumed that counting losses from the self-absorption of beta particles in the bead were negligible due to the small bead diameter and the relatively high energy of beta decay. The bead counting efficiency was determined by individually desorbing several beads in a solution containing strontium carrier and counting the desorbed activity in a previously calibrated geometry. The goSr-90Yactivities of the larger sources (Equilibration l), "Ni activities (Equilibrations 2, 6, and 7) and 2O4Tl activities (Equilibration 8) were determined by liquid scintillation counting of the desorbed activity. Each source assayed by this technique was transferred to 1.00-ml dilute acid containing suitable isotopic carriers and allowed to stand for at least 24 hours. The goSr-90Ywas desorbed in 0.15N " 0 3 containing 10 mg of Sr2+ and 10 mg of Y3+ per ml, the 63Niin 0.12N H C L containing 25 mg of Ni*+ per ml and the 204Tlin 0.1N "03 containing 10 mg of TI+ per ml. For counting, 15.0 ml of Bray's scintillation solution (4) was added to the desorbing solution. Calibration standards for efficiency and background for liquid scintillation counting were prepared similarly. The l 3 '1 sources were assayed by placing individual beads between two layers of Scotch Tape and counting the 0.36-MeV gamma ray on top of a 4 X 4 inch NaI(T1) detector.
(4) E. A. Bray, Anal Biochem., 1,279 (1960).
Duration 3 days 5 days
6 hours 3 days 2 days 5 days
Equilibration solutionb 50 pg Sr*+ 50 pg Y3+ 0.01N "03 25 pg Ni2+ 0.02N HCI 125 pg I250 pg Na28Oa 0.001N NaOH 125 pg I250 pg Na&03 0.001N NaOH 125 pg Sr2+ 125 pg Y3+ 0.03N "08 25 pg Ni2+ 0.02N HCl 25 pg Ni2+ 0.02N HCI
5 days
40 pg T1+ O.06N "03 125 pg Sr2+ 125 pg Y3+ 0.03N "03
3 days 2 days
Table 11. Evaluation of Individual Beads for Equilibration 7 Diameter Activity Specific activity Bead (pm) (dpm) (dpm/pm3 X IO4) 7-1 37.3 64.1 23.7 i 2.30 7-2 46.8 132 24.7 =t1 . 3 7-3 55.3 211 23.9 i 0 . 9 7-4 62.3 310 24.5 f 0 . 7 7-5 72.2 48 1 24.4 f 0 . 5 7-6 81.1 688 24.6 f 0 . 4 7-7 82.5 715 24.4 f 0 . 4 7-8 97.4 1200 24.8 f 0 . 3 7-9 108.4 1620 24.3 f 0 . 3 7-10 110.7 1780 25.1 i 0.3 7-1 1 124.1 2530 25.3 f 0 . 2 7-12 132.1 3000 24.8 f 0.2 7-1 3 183.4 8040 24.9 f 0 . 1 Average 24.6 i 0 . 5 b (1.973 One sigma counting error associated with the activity measurement of an individual bead. Standard deviation of specific activities for beads from the given equilibration. ~~
RESULTS AND DISCUSSION
For all equilibrations, the activity retained in the resin phase was greater than 90% and approached 100% when the ion of interest was exchanged with one having a much lower ion exchange selectivity; e . g . , I- for C1- and NiZffor Na+. In general, the equilibrating conditions in Table I produced and 4 X sources with specific activities between 2 X dpm/pm3per pCi of activity/mg of resin in the equilibration mixture. Activity, ( A ) , and diameter, (4,measurements for 13 sources from equilibration 7 are presented in Table I1 along with the corresponding specific activity values (activity/ unit volume). Table I11 summarizes the results obtained from the evaluation of individual beads from the other successful equilibrations. Also presented in Table I11 are the linear
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
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Table 111. Evaluation of Individual Beads (Summary of Results) Specific Number Diameter Activity activity average Equilibration assayed range (,urn) range (dprn) (dpm/prn3) X lo4 Regression equation 1 QOSr 10 127.8-467.0 154-7910 1.45 f 0.02* In A = 3.026 o,013c) In d - 9.638 (2.1-0,0%). (1.723 2 B3Ni 7 189.3-459.2 1850-25900 5.21 i 0.07 2b,018) 979 In d - 8.088 In A = ( 1 . 0 4 . 4%) (1.473 7 270.0-452.3 3710-17130 3.56 i 0.05 4 1311 2b,029) 993. I n d - 8.548 In A = (1,1-0,6z) (1.473 5 gOSr 11 22.5-188.6 7.2-4220 1 2 . 4 2 ~0 . 6 3b,028) 014 In d - 7.404 In A = (5.0-0.8Z) (4.973 7 GaNi 13 37.3-183.4 64.1-8040 24.6 f 0 . 5 In A = In d - 6.788 (9.7-0.473 (1.973 8 zo4Tl 11 39.9-121.1 154-4520 4 8 . 4 k 1.0 In A = In d - 6.048 (1.5-0.273 (2.1%) 9 @OSr 10 12.3-26.2 1.61-16.9 17.2 f 0 . 6 l n d - 7.284 1nA = (5.4-3.3 Z) (4.1 Counting statistics range (relative standard deviation). Standard deviation of specific activities for beads from given equilibration. Standard deviation in linear regression coefficient.
(* (* (* (+ (2:g9)
(:z44)
m
t 0.21
c;
I
I
I
I
+O.81
A
I
I - \
5 t0.1t \
-O.L)[
I
I
1
I
W
W
a
a -0.2 I
250
I
300
I I 400 450 DIAMETER ( p m )
I
350
-0.8 0
500
Figure 1. Relative deviations of individual 1311-Dowex 1-X8 (50-100 mesh, C1- form) source specific activities, ( S A , - sA)/SX, as a function of bead diameter, comparing equilibration times of 6 hours (A) and 3 days ( 0 )
50
1 4
100 150 DIAMETER ( p m )
200
Figure 2. Relative deviations of individual 6 3 N i - D o ~ e ~ 5OW-X12 (200-400 mesh) source specific activities as a function of bead diameter, comparing 5 day equilibrations with resin in Na+ (A) and Ni2+( 0 )forms
regression equations (5) of In A us. In d and the standard deviation in the calculated linear regression coefficient for the sources from each equilibration. The proportionality between the activity held by a n individual bead and its volume or the cube of its diameter is clearly apparent from the excellent agreement between the specific activities of beads from the same equilibration and from the fact that the calculated linear regression coefficient of In A c’s. In d for each series of sources is extremely close to and within experimental error of the expected value of 3.00. The ability to predict the activity of a source from a diameter measurement is reflected by the estimated relative standard deviation in specific activities of sources from a given equilibration. For several equilibrations this value was found to be on the order of 2 or less, illustrating the potential for predicting source activities within 3 4 % a t the 95% confidence level. Proportionality between source activity and volume is dependent both upon particle to particle homogeneity of the
ion exchange resin (in terms of capacity, density, and swelling ratio) and allowing the resin to equilibrate with the radionuclide solution for a period of time long enough for the achievement of complete equilibrium between activity and resin. Although previous studies (6, 7) concluded that the majority of commercially available ion exchange resins did not exhibit a high degree of interparticle homogeneity, the results of this investigation indicated that the four ion exchange resins used for source preparation were homogeneous from bead to bead. In two of the nine equilibrations performed, 3 and 6, complete exchange between solution and resin was not achieved as specific activities of individual sources were found to decrease with increasing bead diameter. Both situations were corrected however, by increasing the equilibration time in the case of I 3 l I on Dowex 1-X8, C1- form (Figure l), and by charging the resin with the same cation as the radionuclide in the case of 63Ni on Dowex 5OW-X12, Ni2+ form, (Figure 2). Effect of Relative Humidity on Bead Volume. Previous
(5) R. G. D. Steel and J. H. Torrie, “Principles and Procedures of
( 6 ) E. Hogfeldt, Science, 128, 1435 (1958). (7) D. H. Freeman, V. C . Patel, and Mary Hood, .I. Polym. Sci., Part A , 3, 2893 (1965).
Statistics,” McGraw-Hill Book Co., Inc., New York, N. Y., 1960, pp 161-164. 1610
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
studies have shown that a dried ion exchange resin can absorb up to 10 millimoles of H 2 0 per milliequivalent of resin as a function of the relative humidity of the air in which the resin is stored (8,9). The resulting volume changes in a given type of ion exchange bead would consequently have to be determined as a function of relative humidity if one desired to calculate the activity of a bead on the basis of a diameter measurement made at a future date. The magnitude of these volume change corrections was determined for three resins by repeatedly drying individual beads at 125 “C, allowing them to equilibrate for several hours over sulfuric acid-water mixtures having a known water activity (IO),and sizing the individual beads. Relative bead volumes us. relative humidity are presented in Figure 3. All relative bead volume measurements were normalized to the volume obtained for the same bead after drying and cooling over anhydrous calcium sulfate (0 relative humidity). An interesting observation of hysteresis was made when relative volume measurements on AG 50W-X8 (Ni2+ form) beads were repeated, but in this case, by first allowing the bead to equilibrate at 100% relative humidity before allowing it to equilibrate at a lower relative humidity. Relative bead volumes appeared independent of the two drying techniques when the final equilibrating relative humidity was greater than approximately 30 %. Below 30 relative humidity, however, relative bead volume also depended on whether the bead was gaining or losing water to reach its final state of hydration. Since bead volume or calculated specific activity is a function of the resin drying technique as well as final relative humidity conditions, stringent and reproducible drying and storing procedure is needed if day to day comparisons of specific activities are to be referred to a calibration curved based on air humidity. Prediction of Source Specific Activity from Original Equilibrating Conditions. By carefully evaluating the radionuclide uptake per unit weight of resin in an equilibration and the density of the resin, the final specific activity of a series of sources may be predicted by
1.2
I
I
I
I
I
I
I
I
[/
I .I
5 g
1.0
x
x
(AIV) =
I
0
I
20 40 60 80 100 RELATIVE HUMIDITY ( % I
Figure 3. Relative bead volume as a function of relative humidity for three ion exchange resins 0 Bead dried at 125 “C and then equilibrated at specified relative humidity for 5 hours before sizing A Bead equilibrated at 100% relative humidity and then equilibrated at specified relative humidity for 16 hours before sizing
10-12A,qRp m
where ( A / V )is the specific activity (dpm/pm3), A,, is the quantity of activity (dpm) and m is the weight of resin (gram) introduced into the equilibration mixture, R is the fractional uptake of the radionuclide by the resin, and p is the resin density (gram/cm3). For equilibrations 1 and 7, the parameters in Equation 2 were evaluated and are presented in Table IV. Density measurements of 0.5-1 gram of the resin dried at 125 “C (8) G. E. Boyd and B. A. Soldano, 2.Electrochem., 51, 162 (1953). (9) H. P. Gregor, K. M. Held, and J. Bellin, ANAL.CHEM.,23, 620 (1961). (10) R. A. Robinson and R. H. Stokes, “Electrolyte Solutions,” 2nd ed., Butterworth and Co., Ltd., London, England, 1959, p 510.
were made in octane, a nonswelling medium (3), using standard pycnometric techniques. The fractional uptake of the radionuclide was determined by difference by radioassaying the combined filtrate and washes from the equilibration mixture. The measured specific activity values presented in Table IV were obtained from the corresponding activity measurements in Table I11 and volume measurements determined under the same drying conditions used for the gross density measurements. Although precautions were taken to minimize moisture uptake by the dried resin when weighing and during the gross density measurements, it is possible that they were not perfect. A low value for the density, or a high value for the quantity of resin transferred to the equilibrating solution (m) may explain at least part of the approximately 4 4 % devia-
Table IV. Comparison of Predicted and Measured Specific Activities AIV
Equilibration 1 7
m (grams)
0.0440 0.0107
A,, (dpm) 4.74 x 106 2.22 x 107
R
0.928 0.941
p
(gramsIcm3) 1.637 1.542
Predicted 1.63 x 10-4 2.99 x
(S) Measured 1.69 x 10-4 3.24 x 10-3
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
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tions between predicted and observed specific activity values. Also, we have assumed total conversion to the strontium or nickel form. Fractional activity uptakes, however, indicate partial return to the hydrogen form during equilibration. Investigations by Gregor et al. ( I I ) , have shown that dry ion exchange resin volume varies with the bound counterion. Further, the dry volume of a strongly acidic cation exchanger in the hydrogen form is less than that in any other form. This would produce a smaller final volume than calculated on the basic of total conversion to the strontium or nickel form. In spite of these uncertainties, it was possible to predict final source specific activities within 4-8 % on the basis of evaluated terms in Equation 2. In retrospect, two modifications in technique should reduce the errors in predicting specific activities from equilibrating conditions. The first would be to perform weighing, density, and sizing measurements on the resin in the partially hydrated state (e.g., equilibrated at 40% relative humidity). This would reduce the rapid volume and weight changes in the ion exchange beads when exposed to the atmosphere during these operations. The second would be to reduce the acid in the radionuclide solution by evaporation or dilution before mixing the beads with the radionuclide solution. This should reduce the partial conversion to the hydrogen form and permit a more accurate relation between the calculated and measured specific activities of individual sources. Stability of Prepared Activity Sources. Over a period of several days, loss of activity from partially hydrated ion exchange beads appeared to be negligible when stored on glass microscope slides and exposed to atmospheric conditions of 30-60z relative humidity. Such source integrity should not be assumed however, for extended exposure to normal laboratory conditions where acid and ammonium chloride vapors or dusts could provide foreign ions for exchange. For long storage times, it was demonstrated both here and elsewhere (2) that ion exchange resin integrity can be maintained if stored in an inert solvent such as cyclohexane or hexane. CONCLUSION The results of this investigation have demonstrated the feasibility of preparing microscopic radioactivity sources with activities in the pCi and nCi ranges. The unique properties of these sources including their extremely small size, uniformity of shape and composition, and their ability to be calibrated in terms of their diameter offer many advantages in evaluating and calibrating nuclear counting systems. As point sources (11) H.P. Gregor, F. Gutoff, and J. I. Bregman, J. Colloid Sci., 6, 245 (1951).
1612
for P-y or y-y coincidence determination of absolute disintegration rates and the measurement of intrinsic efficiency and resolution of y detectors, errors associated with finite sample size may be avoided. Such sources may also have utility in the measurement of absolute disintegration rates by 4i$ counting where corrections for self absorption can be made by assaying a series of sources of decreasing size from the same equilibration and extrapolating specific activity to zero source diameter. Once the absolute disintegration rate of an individual bead has been ascertained by coincidence or 4 a p counting, the activity may be quantitatively desorbed and used as a primary radiation standard. Other uses, based on the availability of a wide range of activities from a single equilibration, might include the calibration of the dose response of photographic emulsions for the determination of particulate radioactivity by autoradiography and the evaluation of counter dead times as a function of increasing activity. The techniques used in this study for radioactive source preparation and evaluation may also be employed in the area of chemical microstandards. They have demonstrated the availability of homogeneous ion exchange resins having beads in the 25-200 pm and 200-500 pm ranges for the preparation of standards containing ng and pg quantities of a given element. In addition, these same techniques may be used for the direct determination of the counterion content of an ion exchange bead by isotope dilution offering a method independent of that implied by Equation 1 for microstandard evaluation. The degree of success achieved in this study with four radionuclides and four widely differing ion exchangers coupled with the work of Freeman et al. (2) strongly emphasizes the credibility of a single ion exchange bead acting as a microstandard for practically any radionuclide or stable element. ACKNOWLEDGMENT The authors acknowledge David H. Freeman and Herbert D. Dixon of the Institute for Materials Research, National Bureau of Standards, for their helpful suggestions and for providing one of the ion exchangers used in this study. RECEIVEDfor review May 28, 1970. Accepted August 26, 1970. Paper presented at the Thirteenth Conference on Analytical Chemistry in Nuclear Technology, September 30October 2,1969, Gatlinburg, Tenn., and at the Eastern Analytical Symposium, November 19-21, 1969, New York, N. Y. Representative products and manufacturers are named for identification only, and listing does not imply endorsement by the Public Health Service and the U. S. Department of Health, Education, and Welfare.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
