Liquid Scintillation Counting of Radioiodine BUCK A. RHODES Radiological Science Department, The Johns Hopkins School of Hygiene and Public Health, Baltimore, Md.
b Eight liquid scintillation systems were evaluated for counting radioiodide. The balanced quenching technique was adapted to the measurement of 1125. Moderately quenched ( 1 4 to 42y0 quenched) Ilz5 solutions may b e directly counted with a uniform efficiency of 28.6y0. The Ilz5 was counting efficiency of greatly increased compared to previous reports. A maximum efficiency of 56y0 was found. Two peaks of the I l 2 5 spectra can b e distinguished in the liquid scintillation system used.
L
SCIKTILLATION measurement of radioiodine has not been given sufficient attention, probably because of the fear of quenching by iodine and widespread use of other methods. However, the method can be as direct, simple, and efficient as N a I crystal counting. It is especially useful for counting the x-ray emitting isotopes 1 1 2 5 and which require for NaI crystal counting a special thin window and consideration of sample self-absorption (1). is included for comparative reasons. This investigation %-as made to find an efficient system for counting aqueous iodide solutions and to determine the applicability and extent of usefulness of the direct balanced quenching technique (8). IQUID
EXPERIMENTAL
Packard Tri-Carb Model 3003 automatic liquid scintillation spectrometer was used for this study. This unit has three independent differential analyzers. T h e pulses of both photomultipliers are summed to double t h e signal-to-noise ratio and increase efficiency and resolution. Counting was done in 20ml. polyethylene vials. Reagents. T h e hTaIIz5 used for efficiency measurements was calibrated by the National Bureau of Standards. Sa1129 and ?iaI131 were calibrated by t h e vendor. 2,5Diphenylosazole was employed as t h e primary phosphor and 1,4-bis(4methyl-5-phenyl-2-oxazolyl)benzene as the wavelength shifter. The receipts of the eight solutions tested are listed in Table I. Apparatus.
Procedures. To 15 inl. of each of eight different scintillation solutions 151 pg. of 1’29 in 1 ml. of absolute alcohol were added. Efficiencies at optimum gain were measured. Opti-
The balanced quenching technique mum gain-i.e. t h a t gain which gives makes use of a window in which the the maximum count rate in a wide relative counting efficiency is approxiwindow (30 to 1000 discriminator mately independent of quenching ( 8 ) . units)-was determined from a linear This was found by counting a series of plot of count rate us. gain for each 1 1 2 5 samples in Bray’s solution consolution. The solutions were also titaining increasing amounts of water trated with water t o determine their as the quencher. The upper and lower aqueous sample capacity. discriminators were varied until the At the optimum gain for each isotope minimum variation of the count rate in Bray’s solution, solution 4 of Table was obtained. The first approximation I (Z), the best operating conditions were t o the appropriate window was found determined by varying the upper and by starting with the spectrometer at lower discriminators until the maxithe optimum setting then lowering the mum value of the figure of merit was upper discriminator until the count obtained. The figure of merit is defined as [(loo ~.p.m./d.p.m.)~/background].rate was reduced by one half. Various substances which might be Apparent efficiencies defined as 100 encountered in measuring aqueous iadioc.p.m./d.p.m. were made a t these iodide were added to 15-ml. aliquots optimal conditions. The corresponding of Bray’s solution containing a known crystal counting efficiency of 1 1 ~ 5 nonquenched count of 11*5. The was measured in a 3- X 3-inch NaI(T1) count rate was measured in three difwell crystal with a thin aluminum ferent windows: (1) a window for window. optimum counting of nonquenched The spectrum of each isotope was solutions; (2) a window for balanced determined in Bray’s solution using a quenched counting; and (3) a monitor gain of 30Oj, which is optimum fo1 window in which the variation of Count rates were recorded a t count rate with quenching is high. each 2% interval of the 1000 unit scale The decrease in the count rate of of the lower discriminator. Table I.
Comparison of Different Scintillation Solutions for Counting
Scintillation solution (1) 3 Grams of PPO,c 0.06 gram of DM-POPOPd/liter of toluene (2) 1.5 grams of PPO, 0.05 gram of DM-POPOP/liter of toluene-absolute ethanol (1: l ) (3) 1.8Grams of PPO, 0.06gram of DM-POPOP/liter of tolueneabsolute ethanol (1.5:1) (4)4 Grams of PPO, 0.2 gram of DN-POPOP, 60 grams of naphthalene, 20 ml. of ethylene glycol, 100 ml. of methanol with p-dioxane to make 1 litere (5) 10 Grams of PPO,0.5 gram of DM-POPOP, 80 grams of naphthalene/liter of xylenedioxane-ethylene glycol monoethyl ether (1: 3 :3) (6) 7 Grams of PPO, 0.04 gram of DM-POPOP, 80 grams of naphthalene/liter of dimethox yethane (7) 10 Grams of PPO, 0.4 gram of DAZ-POPOP, 80 grams of naphthalene/liter dioxaneanisole-dimethoxyethane
Gain requifed for Apparent maximum counting count rate, efficiency,n
1129
of
Water required to cause phase separation,
%
%
merit b
%
12.5
119
674
0
29.0
96
576
10.0
27.5
104
470
6.9
12.5
114
542
46.5
16.5
115
575
24.0
17.5
111
513
20.8
Figure
11.5 121 732 13.9 (6:l:l) (8) Same as 7 except the solvent 20.0 114 650 6.3 contains 20% ethylene glycol a 100 c.p.m./d.p.rn. Figure of merit = (100 c.p.m./d.p.m.)2/background.c PPO
2.5-diphenyloxazole. Bray’s solution.
= e
DhI-POPOP-1,4-bis( 4methyl-5-phenyl-2-oxazolyl)benzene.
VOL. 37, NO. 8, JULY 1965
a
995
-6
a A G. 3 . 4 5 0 1-125
A 1-129
u)
0 1-131
//7
600 5001
Rotlo* 2 . 9 5
430
>*01
0
"
IO
"
20 30 4 0
"
50
"
60 7 0
8.
80 90
PERCENT G A I N
01 0
I60
A0
360
4b0
560
D l S C R I M INATOR
Figure 1.
Liquid scintillation spectra of 1125,
window 1 compared to the nonquenched standard is a direct measure of quenching. The count rate in window 2 does not change appreciably with quenching. The difference in the relative efficiencies in this window is a direct measure Of the error Of the balanced quenching method. The count rate in window 3 can be used to correct for quenching when the error of the balanced quenching technique becomes excessive.
Table II.
660
760
800
900
Id00
UNITS Ilz9,
and
in Bray's solution
RESULTS A N D DISCUSSION
Comparison of Scintillation Solutions. T h e results are tabulated in
Table 1. -411 eight solutions tested may be used for counting iodide. In general i t appears that the lower the gain required for optimum counting> the higher the efficiency and figure of merit. Solution 7 , dek'eloped by Davidson and Fergelson ( 5 ) , gave the
at Critical Regions of Energy Spectra Showing Count Rates of Existence of Minimum between Two Maxima
Ijumbers at heads of columns are discriminator window setting. Values reported are net counts per minute with one standard deviation. 1'29 included for comparison 13-K.e.v. peak Valley 40-K.e.v. peak GAIY of 30 90-110 170-190 270-290 0.6 f0.2 0.7 f0 3 Blank 1.3f0.4 I126 I126
Blank
I126 I126 GAIX of 20
Blank
I129 1129 I129 1-129 1-129 1-129 Table 111.
609 f 8 6451 f 80 1.6 f 0.4 662 f 8 7063 f 84 70-90 4.1 f0.6 187.5 f 4 . 4 1521.2 f 1 2 . 4 14150 f 42 497.4 f 7 . 1 539.3 f 7 . 3 511.2 f 7 . 2
534 f 7 6112 f 25 0.5 f0.2 556 f 7 6324 f 25 150-170 1.0 f0.3 160.6 f 4 . 0 1298.3 f 1 1 . 4 12212 f 62 635.1 f 8 . 0 674.8 f 8 . 2 625.1 f 7 . 9
567 f 8 6728 f 82 0.5 f0.2 508 f 7 6676 f 82 230-250 0.3 f0.1 174.9 f 4 . 2 1431.2 f 12.0 13256 i 36 643.9 f 8 . 0 649.4 f 8 . 1 606.0 f 7 . 5
Comparative Integral Count Rates of Duplicate Aliquots of Calibrated and ILZ9 Solutions 10 cc. of 3- x 3-inch NaI(Th) thin % Bray's soln., % A1 window, Efficiency, gain of 20, Efficiency, 15-80 k.e.v. 100 window 25100
Sample net c.p.m. c.p.m./d.p.m. net c.p.m. c.p.m./d.p.m. Doubly calibrated (30,800 d.p.m.) 15,845 51.4 1126 (28,963 d.p.m.)= 14,900 14,862 51.3 1129 (22,385 d.p.m.)* 12,879 57.5 25,350 113.2 Blank 125 62 a This solution counted in glass container identical to that of standard I1=; volume, is in Bray's solution. 1 cc., is the same. 1lz6of standard is in water, second b Calcd. d.p.m. based on 60.5 pg. of with reported [Studier et. al., ANL-6577, (1962)] specific activity of 370 d.p.m. per pg. Liquid scintillation count rates of 11% and 1129 can be increased over reported values by optimizing for each particular isotope. Settings tised here allow for highly efficient counting of both isotopes at identical instrument settings.
996
ANALYTICAL CHEMISTRY
Figure 2 When peak heights vs. gain are plotted for the two peaks of 1125, ratios of their relative energies is 3, CIS is the case of the two most probable absorption peaks, 13 and 40 k.e.v.
highest values. However solutions containing 1'2-dimethoxyethane (solutions 6 and 7) become yellow in the presence of iodide. Bray's solution was chosen for further studies because it combined high efficiency and high water holding capacity. A similar comparison for tritium and carbon counting is contained in Rapkin's review (7'). Liquid Scintillation Spectra. Figure 1 shows the pulse height distributions of t h e three isotopes a t a gain of 30y0 which is optimum of I I z 5 counting. = 1.56 X lo7 years) decays with simultaneous beta and gamma emission (9). The average beta energy is 30-33 k.e.v.; the 39-k.e.v. gamma ray is internally converted yielding a 29-k.e.v. x-ray. The similarity of this spectra to that of 1 1 2 5 would make counting of mixtures of these two isotopes difficult but either should be easily counted in the presence of 1131. The 1 1 2 5 liquid scintillation spectra appeared bimodal. This observation was verified by additional tests made a t various levels of activity and instrument gains. Table I1 shows that a statistically discernible minimum was always present with I l Z 5 (and not with the control, 1129), verifying the bimodal character of this spectra. Further analysis of the data shows that the energy ratio of the two peaks is approximately 3 (Figure 2). By considering the probability of the various events occurring during the decay of II26 by electron capture, Cavanaugh (4) calculated that the deposition of 13 k.e.v. and 40 k.e.v. of energy in the scintillation solution occurs with the respective probabilities of 0.45 and 0.41. Considering the excellent energy resolutions being obtained with modern liquid scintillation systems ( 6 ) , it is not surprising that these two peaks are resolvable. Efficiency Measurement. Using a newly calibrated National Bureau of
Table IV. Comparison of Optimum Spectrometer Setting and Corresponding Efficiencies for Three Isotopes of Iodine
APparent effi-
Isotope
Gain Window 25 25-900 12.5 30-600 4.5 30-900
1'26
1'"
I131
ciency,
76
56 106 102
Figure of
merit 134 666 297
Standards IlZ5solution a maximum efficiency of 55.7yo was obtained. This is a significant improvement over the previously reported value of 13.8% (10). Table I11 compares liquid and NaI scintillation counting of and 1129.
Table IV compares the three isotopes a t their optimum counting conditions. 1 1 2 9 and 1131 should approach maximum counting efficiencies of 100%. The small positive error in these measurements is probably the dilution error. Some of the error in the IlZ9measurement may also be attributed to the uncertainty of its specific activity. Balanced Quenching Technique. T h e relative efficiency remains nearly constant (Figure 3) when t h e balanced quenching technique is used as quenching increases from 14 t o 42%. Thus, t o count quenched samples, all samples including t h e standard are made to contain a small amount of some mild quenching agent, for example 1 ml. of water. T h e results are obtained by direct comparison of the balanced quenched count rate of the sample to that of the standard. The
Table V. Quenching, %, of Ilz6 Count Rate Caused by Various Substances and Resultant Relative Balance Point Counting Efficiency with Associated Error
Rel. balanced quenched' efficiency, %
Quenching
Sample
54 (1) 1 ml. of 1N "03 39 (2) 1 ml. of 0.5N "08 18 (3) 1 ml. of 0.1N HNO, 9 (4) 1 ml. of 0.002N HKOs 18 (5) 1 ml. of 30% NHI 10 (6) 1 ml. of 3% NHa 9 (7) 1 ml. of 0.37, S H , 9 (8) 1 ml. of 0.037,S H s 11 (9) 1 ml. of 0.00370KHB 11 (10) 1 ml. of HzO i2 (11) 1 ml. of 0.05M (SH4)2SzOa 26 (12) 1 ml. of SH41(200fig. of I-) 82 (13) 1 ml. of 6.37,H2S03 10 (14) 1 ml. of 0.002M (NH,)&Or 10 (15) 0.1gram of 3,5-diiodo-l-tyrosine 0 (16)Unquenched standard (17) 1 ml. of I - carrier (2mg. of I-, 2 mg. of Br-) 0.5ml. of 6.3% 28 H P S O P ;0.5 ml. of 30Y0 NH, Relative to 10 which contains 1 ml. of HzO.
count rates from the other two channels allow for quenching correction by the channels ratio method (3) if quenching exceeds the range of this direct technique. The efficiency of IlZ5in the channel for balanced quenched counting was 28.6%. The wide range of application of this technique is demonstrated in Table V. Sample 17 illustrates how quenching is easily handled. Excess iodine carrier was added to this solution causing i t to become an intense yellow. HzS03 and NH3 solutions were added to reverse the iodine-iodide reaction. The resultant clear solution was counted by the balanced quenching technique with the same efficiency as the water quenched standard. The measured error is 2.3%.
-0
m
30 m
5
50 m
60 I
70
3 0
00 90
00 0
I 2 3 P E R C E N T W A T E R A D D E D T O B R A Y ' S SOLUTION
102.3
+2.3
ACKNOWLEDGMENT
Thanks to Henry N. Wagner, Jr., Arthur Karmen, and Robert Cavanaugh for suggestions. LITERATURE CITED
10
40
Error, -24.3 -5.9 +5.6 +2.8 +2.4 f1.6 +2.2 +1.3 +0.9 0 +1.9 +2.2 -68.8 fO.9 -1.3 -9.2
Sample 15 containing 3,5-diiodo-1tyrosine illustrates that iodo-organic quenchers may also be counted directly b y this technique. High acid concentrations quench so much that these samples are outside the range of the balanced quenching technique. However, if these samples are neutralized with "3, then they may also be counted. Note again sample 17. The average error of the method is 1.8%, excluding the unneutralized acidic samples.
0
2o
75.7 94.1 105.6 102.8 102.4 101.6 102.2 101.3 100.9 100.0 101.9 102.2 31.2 100.9 98.7 90.8
4
Figure 3. Effect of quenching may b e minimized b y balanced quenching technique (curve 2) Efficiency Is reduced to approximately one half the maxlmum, but itlll It Is over 25%, whlch compares favorably with other methods of counting
(1) Bakhle, Y.S., Prusoff, W. H., McCrea, J. F., Sci. 143, 799 (1964). (2) Bray, E. A., Anal. Biochem. 1, 279 (1960). ( 3 ) Bush, E. T., ANAL.CHEM.35, 1024 (19631. (4).Cavanaugh, R., private communication, Packard Instrument Co., 2200 \ - - - - ,
Warrenville Road, Downers Grove, Ill.,
1965. (5) Davidson, J. D.,Fergelson, P., Intern. J . Appl. Radiation Isotopes 2, 1 (1957). (6) Horrachs, D. L., Studier, H. H., ANAL.CHEM. 36, 2077 (1964). (7)Rapkin, E., Intern. J . Appl. Radiation Isotopes 15, 69 (1964). ( 8 ) Ross, H.H., Ibzd., p. 273. (9)Russell, H.T.,ORNL-2293 (1957). (10)Yerick, R. E., Ross, H. H., Oak Ridge Radioisotopes Conference, p. 20, (1963).
RECEIVEDfor review August 4, 1964. Resubmitted April 12, 1965. Accepted April 23, 1965. Supported by U. S. Public Health Service Grant No. GM 10548.
VOL. 37, NO. 8, JULY 1965
997
