(3) Biemann, K., Bommer, P., Desiderio, D. >I., Tetrahedron Letters, Yo. 26, 1725 (1964). (4) Bommer, P., Mchlurray, W.J., Biemann, K., Twelfth Annual Cons. Mass Spectrometry and Allied Topics, Montreal, June 1964. (5) Brunnee, C., Jenckel, L., Kronenberger, K., 2. Anal. Chem. 189, 50 (1962); 197, 42 (1963). (6) Desiderio, 11. M., Biemann, K., Twelfth Annual Conf. Mass Spectrometry and Allied Topics, Montreal, June
1964. (7) Dorsey, J. A., Hunt, R. H., O'Neal, M. J., ANAL.CHEM.35, 511 (1963).
(8) Ebert, A. A,, ANAL.CHEM.33, 1865 (1961). (9) Gohlke, R. S., Ibad., 31, 535 (1959). (10) Gohlke, R. S., Ibid., 34, 1332 (1962). (11) Henneberg, D., 2. Anal. Chem. 183, 12 (1961). (12) Holmes, J. C., Morrell, F. A,, A p p l . Spectry. 11, 86 (1957). (13) Lindeman, L. P., Annis, J. L., ANAL. CHEM.32, 1742 (1960). (14) McFadden, UT.H., Teranishi, R., Black,'D. R., Day, J. C., J . Food Science 28, 316 (1963). (15) Miller, D. O., ANAL.CHEM.35, 2033 (1963). (16) Owens, E. B., Zbid., 35, 1172 (1963).
(17) Ryhage, R., Zbid., 36, 759 (1964). (18) Ryhage, R., J . Lipid Res. 9, 245 (19641. (19) Watson, J. T., Biemann, K., ANAL. CHEM.36, 1135 (1964). (20) Zemanv, P. D., J. A m ..l i e d Phvsics 23, 924 (i952). RBCEIVEDfor review January 26, 1965. Accepted March 24, 1965. Eastern Analytical Symposium, New York, N. Y., Kovember 11, 1964. Work supported by research grants from the National Science Foundation (G-21037) and the Kational Institutes of Health, Public Health Service (RG-9352).
Radiochemical Determination of Lead-21 0 after Solvent Extraction as Iodide and Dithizonate N. A. TALVlTlE and WILLIAM J. GARCIA Colorado River Basin Water Quality Control Project, Public Health Service, U . S. Department o f Health, Education, and Welfare, 1750 South Redwood Road, Salt lake City, Utah 8 4 1 04 Lead-210 is determined radiochemically in environmental samples. Iron, zinc, tin, and similar heavy metals are removed by extraction of the thiocyanates with methyl isopropyl ketone. Lead-2 10 and stable lead are isolated by successive extractions as the iodide and dithizonate. The activity of the lead-2 1 0 is determined by direct counting of its beta radiation or by counting the beta and alpha radiation resulting from ingrowth of bismuth-2 10 and polonium-2 1 0. The recovery determined with standard lead-2 10 is 95% with a relative standard deviation of 6.470 when polonium-210 ingrowth is measured.
L
~ ~ ~ - 2 1as0 ,a member of the uranium-238 chain, appears in the waste products of uranium ore processing. Because of its assimilable nature and because of the length of its half-life relative to human longevity, its presence in culinary and irrigational waters is of concern. A method for the determination of lead-210 was needed which would be applicable to source samples such as uranium mill tailings and effluents as well as to river waters, sediments, and biological specimens. Separation techniques applicable to radiometric determination have been reviewed by Gibson (3) and Millard (6). R e s t and Carlton ( 7 ) have shown that lead in hydrochloric acid solution can be separated from a large number of metals by a preliminary extraction of metal thiocyanates with methyl isopropyl ketone followed by the extraction of lead as the iodide with the same solvent. hlcCord and Zemp ( 5 ) separate lead from partially ashed urine by extraction of
lead iodide with methyl isopropyl ketone and determine the lead spectrophotometrically as the dithizonate. Frank ( 2 ) ,in a modification of the hlcCord and Zemp procedure, extracts lead iodide with a 6: 3 mixture of methyl isobutyl ketone and methyl ethyl ketone. I n the following procedure, iron and other heavy metals are removed by extraction of their thiocyanates with methyl isopropyl ketone after which radiolead and stable lead are separated by extraction as iodides. Following wet-ashing of the extract and a waiting period to allow for decay of short-lived lead isotopes, final purification of the lead and its separation from bismuth210 and polonium-210 are accomplished by extractions as the dithizonate. The lead-210 is determined radiometrically by direct count of its beta radiation or by counts of beta and alpha radiation resulting from the subsequent ingrowth of bismuth-210 and polonium-210. EXPERIMENTAL
Apparatus. Counts were obtained with Suclear Measurements Corp., Model PC-3A, internal proportional counters and with a n Isotopes, Inc., Model CLL-4-C, low-level beta counter. Reagents. Methyl isopropyl ketone (3-methyl 2-butanone), redistilled. Immediately before use, shake with 10% of its volume of 5y0 (0.6A7) hydrochloric acid. Weak and strong dithizone solutions. 25 and 500 mg. of diphenylthiocarbazone per liter of chloroform. Buffer solution. Adjust 0.05.11 sodium formate to pH 3.4 with 90% formic acid. Ammonia-cyanide solution. Mix 2 liters of 1 : 1 ammonium hydroxide with 400 ml. of a 1% solution of anhydrous
sodium sulfite and 400 ml. of a 2% sodium cyanide solution. Procedure. THIOCYAKATE EXTRACTION. Transfer a 50-ml. aliquot of the prepared sample solution in 5oj, (0.6S) hydrochloric acid to a 250-ml. separatory funnel. Add 10 ml of 20.11 ammonium thiocyanate and 25 ml. of methyl isopropyl ketone and shake for one minute. Drain the aqueous phase into a second separatory funnel and discard the organic phase. Repeat the extraction with additional 25-1111. portions of the ketone until the aqueous phase returns to a nearly colorless state. IODIDE EXTRACTION. Add 10 ml. of 9 X sodium iodide and 70 ml. of methyl isopropyl ketone to the aqueous phase from the thiocyanate extraction and shake for 2 minutes. Drain the aqueous phase into a separatory funnel and the organic phase into a 25O-ml. beaker. Repeat the extraction of the aqueous phase with an additional 70-ml. portion of the ketone and evaporate the combined extracts to dryness. ASHING OF RESIDUE. Cover the beaker containing the dried extract and cautiously add a few drops of concentrated nitric acid. When the foaming subsides, continue a d d q nitric acid in small increments until 20 ml. have been added. Digest just below the boiling point on a hot plate until a clear solution results. Inasmuch as the decomposition of the organic matter proceeds slowly and time should be alloued for the decay of short-lived lead isotopes, it is advantageous to continue the digestion overnight or longer. Raise the temperature of the hot plate to cause moderate boiling of the acid. When the volume has been reduced to about 5 ml., add 2 ml.of 70y0 perchloric acid. Continue to heat, while covered, to fumes of perchloric acid and then evaporate to dryness. Ilissolve the residue with 4 ml. of concentrated nitric acid and dilute with 10 ml. of water. VOL. 37, NO. 7 , JUNE 1965
851
FIRSTDITHIZONE EXTRACTION. To the nitric acid solution from the iodide extraction add in order with mixing after each addition: 10 ml. of 5% ammonium citrate, 4 ml. of concentrated ammonium hydroxide, and 1 ml. of 2070 hydroxylamine hydrochloride. Adjust to p H 9.4 with concentrated ammonium hydroxide and then add 10 ml. of 270 sodium cyanide. Transfer the sample to a separatory funnel and shake for one minute with 20 ml. of the weak dithizone solution (25 mg. per liter). If the aqueous phase does not have the amber color of excess dithizone, add 2-ml. increments of the strong dithizone solution (500 mg. per liter) and shake after each addition. When the dithizone is in excess, drain the chloroform layer into a second separatory funnel containing 25 ml. of the pH 3.4 formate buffer. Continue extracting with 5-ml. portions of the weaker dithizone solution until the last portion remains green. Shake the combined extracts with the buffer for two minutes. Drain the chloroform layer into a third separatory funnel containing an additional 25 ml. of formate buffer and shake for two minutes. Discard the chloroform layer and combine the two buffer solutions. Add 10 ml. of the 25 mg. per liter dithizone solution to the combined buffer solutions and shake for one minute. Record the time of completion of this shaking as the zero time for ingrowth of bismuth-210 and polonium210. Discard the chloroform layer and wash the buffer solution by shaking briefly with 5 ml. of chloroform. SECOND DITHIZONE EXTRACTION. Add 25 ml. of the ammonia-cyanide solution to the buffer solution and mix. Then add 20 ml. of the 25 mg. per liter dithizone solution and shake for one minute. If necessary, add a few drops at a time of the stronger dithizone solution until the aqueous phase shows a slight excess of dithizone after shaking Drain the chloroform layer into a second separatory funnel containing 15 ml. of 1: 99 ammonium hydroxide. Shake for one minute and drain the chloroform into a 50-ml. beaker. Repeat the extraction with 5-ml. portions of the weaker dithizone solution until the last portion, after having been washed with the 1:99 ammonium hydroxide, is colorless or nearly so. Evaporate the combined chloroform extracts to dryness. Dissolve the residue in nitric acid and transfer to a stainless steel planchet. Counting and Computations. Any of the several radiations from the principal modes of decay in the scheme below can be used for qurtntification of the separated lead-210:
Depending upon whether alpha or beta radiations are counted, the following experimental data are obtained : to = Start of ingrowth of bismuth-
210 and polonium-210
R1 = Counts per minute of lead-210 beta plus ingrowth of bismuth210 beta immediately following planchet preparation tl = Ingrowth time in hours between tu and midpoint of counting interval of R1 Rz = Counts per minute of lead-210 beta plus ingrowth of bismuth210 beta after planchet has aged for 7 to 9 days tz = Ingrowth time in hours between to and midpoint of counting interval of RP a3= Counts per minute of polonium210 alpha ingrowth after planchet has aged for 30 to 60 days. t3 = Ingrowth time in days between &, and midpoint of counting interval of Ra I n addition, the following predetermined calibration data are required :
E
=
S
=
Fractional efficiency of the counter-planchet combination for the measured radiation Self-absorption due to stable lead expressed as a fractional transmittance factor
Compute the activity of the separated lead-210 by the following formulae in which the subscripts 1, 2, and 3, if not previously identified, refer to the radiations of lead-210, bismuth-210, and polonium-210, respectively. The value of t h e disintegration constant for bismuth-210, X2, is 0.005761 per hour and 2.22 is the factor for converting disintegrations per minute to micromicrocuries. The factor, K3, is the ratio of polonium210 activity a t fa to the activity of lead210 a t &, as found in the ingrowth table computed by Kirby ( 4 ) . Immediate count of lead-210 beta:
RI
2.22 El& (2
- e-"'')
=
~ , . L cPbZ1' . (1)
Count of bismuth-210 beta after an ingrowth period of 7 to 9 days:
= ~ , . L cPb210 . (2)
Count of polonium-210 alpha after an ingrowth period of 30 to 60 days:
Equation 2 requires an additional correction for the contribution of the alpha activity resulting from a 1 to 27, ingrowth of polonium-210. Because only a small error is involved, the decay of lead-210 has been ignored in Equation 2. The most straightforward of the counting methods is the measurement of polonium ingrowth with an internal proportional counter, inasmuch as the efficiency and trammittance factors can be obtained directly from counts of an aged calibrated lead-210 solution. Sample Preparation. T h e pretreatment of samples should minimize t h e introduction of anions other than chloride in the final solution which should be 5 7 , with respect to concentrated hydrochloric acid. Concentrate water samples b y evaporating to dryness in the presence of an excess of hydrochloric acid. Acidify uranium mill effluents with 570 by volume of hydrochloric acid, add a small excess of hydrogen peroxide, and heat to destroy the peroxide. If organic matter is present, evaporate to dryness with an excehs of nitric acid, convert the residue to chlorides, and redissolve in 5% hydrochloric acid. Decompose 1-gram amounts of 100mesh sediment samples by repeated evaporations with hydrofluoric and hydrochloric acids. Fuse any remaining acid-insoluble residue with a minimum amount of potassium pyrosulfate in a small platinum dish. If necessary, add a drop of hydrofluoric acid and 10 drops of sulfuric acid and fuse again. When uranium mill tailings are treated as above, a precipitate of barium sulfate may remain after the pyrosulfate fusion. Transpose to barium phosphate by heating below redness in a platinum dish with 1 ml. or less of 857, phosphoric acid. Avoid excessive dehydration of the acid by cooling and repeating the heating cycle after the addition of a few drops of perchloric acid. Add dilute hydrochloric acid and evaporate a t a low temperature to hydrolyze polyphosphates. Redissolve in 5y0 hydrochloric acid and carry through the iodide extraction separately but recombine, after ashing, with the other portion of the sample for the dithizone extraction. Dry ash biological materials a t 550' C. or digest, with nitric acid as described for the-dried residue from the iodide extraction. Convert the ash to chlorides and treat any insoluble residue of silics, with hydrofluoric acid. RESULTS A N D DISCUSSION
Nuclide
Half-Life 19.4Y 5.013 1) 138.4 D
Stable 852
ANALYTICAL CHEMISTRY
-
Mode of Decay 85% 0.017 M.e.v. p 15% 0 061 M.e.v. p 857, 0.0465 h1.e.v. y l O O ~ , 1 155 Pvl.e.v. p l O O ~ ,5 3
1M.e.v. a
The recovery of stable lead extracted as the iodide from 570 nitric acid ranged from 52 to 56Y0 by the McCord and Zemp (5) procedure and increased to 67% when the Frank ( 2 ) modification was used. Extraction from 570 hydrochloric acid, as specified by West and Crtrlton (?'), resulted in a recovery of
97y0 when the ratio of aqueous solution to methyl isopropyl ketone was 5 : 1 and 99% when the ratio was 1 : l . The concentration of hydrochloric acid was not critical in the range of 5 to 8%. Analyses of a uranium mill effluent at differing acidities indicated trends toward lower recovery of lead-210 below 5% acidity and lower efficiency of extraction of metal thiocyanates above 8%. The use of ammonium and sodium iodides in place of potassium iodide resulted in a slight but not significant increase in recovery of lead-210. Introduction of potassium salts would be more detrimental in work involving only the iodide separation, such as gamma counting of lead-210, than in the procedure as decribed which provides for additional separations with dithizone. The procedure presumes the presence of large amounts of extraneous salts in the prepared sample. I n the absence of significant amounts of heavy metals, the thiocyanate extraction can be omitted. When the sample is low in alkaline earth salts as well, the dithizone separation can be applied directly. I n the latter case, the interference from shortlived lead isotopes is circumvented by alpha counting of the polonium-210 ingrowth. Alpha and beta counts of the spent solutions remaining from the se,3aration of lead-210 from a standard solution at transient equilibrium showed, in addition to the anticipated complete removal of bismuth and polonium in the dithizone steps, that a fair amount of bismuth and 97% of the polonium are removed by the preliminary thiocyanate extraction. Comparative analyses of a uranium mill effluent with and without the thiocyanate extraction step indicated that lead is not extracted to a significant extent as the thiocyanate. A loss of about 3y0 was observed but could be attributed largely to mechanical loss of aqueous phase in the four successive extractions with methyl isopropyl ketone. While no indication was observed that heavy metals which are extractable as thiocyanates interfere with the extraction of lead as the iodide, omission of the thiocyanate extraction with some uranium mill effluent samples resulted in acid-insoluble precipitates on ashing of the iodide extract and in excessive consumption of dithizone solution. When lead is extracted as the dithizonate, bismuth is often separated by the Bambach and Burkey ( I ) method in which lead is selectively stripped from a chloroform solution of the mixed dithizonates with a potassium biphthalate buffer solution of p H 3.4. T o eliminate the possibility of potassium40 carrying through and contributing to the beta counts of lead-210 and bismuth210, the buffer was changed to a sodium
Table I.
Calibration and Comparisons of Counting Methods Obtained by Quadruplicate Analyses of Lead-2 10 Standard
Rel. std. dev. of Total counting quadruplicate efficiency” analyses, yo 0.642 64% immediately 5.1 0.660 457, at 8 days ingrowth 2.9 0.439 30Y0 at 8 days ingrowth 1.8 0.420 77, at 45 days ingrowth 6.4
Fractional efficiency, E
Counting method Pb2l0p in internal proportional counter Bizlop in internal proportional counter Bizlop in low-level beta counter Poz10a in internal proportional counter a With respect to activity of lead-210
Table II.
Determination of Lead-2 10 in Uranium Mill Effluent Samples
(Quadruplicate analyses of 50-ml. aliquots) CRBP 3653 CRBP 3662 llean Mean Rel. Rel. std. Pb2l0 std. Pb2l0 dev., found, dev., found, Counting method ppc. % WC. % Pbz100 in internal proportional counter 2390 1.8 365 5.6 Bizlo p in internal proportional counter 3270 1.1 382 5.5 Bizlop in low-level beta counter 3320 1.1 364 1.4 Po210 a in internal proportional counter 3280 5.0 370 4.6
formate-formic acid system at equivalent concentration. Two successive portions of either buffer were required for reliable stripping of lead. I n order to assure quantitative recovery of lead, it was necessary to maintain favorable ratios of organic to aqueous phases, not only in the dithizone extractions, but also in the iodide extraction. This is reflected in the volumes specified. I n order to minimize losses of lead-210 by adsorption on the glassware, 20 pg. of stable lead were routinely added to samples suspected to be low in stable lead. A final wash of the lead dithizonate extract with 1: 99 ammonium hydroxide was introduced for removal of sodium salts which would otherwise carry through to the planchet and contribute to self absorption. Because the ammonia removes the greater part of the excess dithizone, the washed extract is in a form suitable for the spectrophotometric determination of total lead for use in self-absorption corrections. The accuracy of the method, as described, was determined by quadruplicate analyses of lead-210 standard. Aliquots containing 1012 ppc. of lead210 and 20 pg. of stable lead were evaporated to dryness and redissolved in 5% hydrochloric acid. After separation of the lead, count rates of the four planchets were obtained a t appropriate intervals by each of the counting methods given in Table I. The mean recovery of lead-210 as computed from the polonium-210 ingrowth, was 95.1%.
CRBP 3663 Mean Rel. Pbz10 std. found, dev., PPC.
96
17
30
19
13
18
12
19
3.6
The efficiencies of the beta counting methods were determined by substitution of the beta count rates and t h e computed lead-210 activities of the same planchets in Equations 1 and 2. The efficiencies found for each of the counting methods are given in Table I which also shows the total counting efficiencyas well as the relative standard deviations found for each of the methods. The precision and accuracy obtainable with samples was determined by replicate analysis of uranium mill effluents which were selected on the basis of previous analyses to represent a wide range of lead-210 content as well as high levels of extraneous salts and activities. The fractional efficiencies reported in Table I were used for quantification but no self-absorption corrections were made. The results of the precision study are presented in Table 11. The greater deviations in the beta count methods at the lower activity levels can be attributed largely to unnecessarily short immediate counts of the lead-210 radiation in an attempt to avoid significant bismuth-210 ingrowth. Sample No. 3653 contained a fair amount of stable lead which is reflected in the lower lead-210 value found by the direct counting of lead-210 beta radiation. Although necessary for the weak lead-210 beta, corrections for selfabsorption of bismuth-210 and polonium-210 radiations can be neglected for the amounts of stable lead which can be extracted conveniently with dithizone. A mean recovery of 93.2% was found for 1012 ppc. of lead-210 VOL. 37, NO.
7,JUNE 1965
853
added to 50-ml. aliquots of Sample KO. 3663. Beta and alpha counts obtained immediately after separation of lead from samples which were high in radioactivity but low in lead-210 content have given no indication of measurable contamination from other radionuclides. Although smaller amounts can be detected in low-background counters, about 1 ppc. is required to establish by means of ingrowth curves that the measured activity is due to lead-210.
ACKNOWLEDGMENT
The authors are indebted to D. E. Rushing for suggestions and advice and to Boyd C. Sorenson for assistance in performing the analyses. LITERATURE CITED
(1) Bambach, K., Burkey, R. K., IND.
EKG.CHEM.,ANAL.ED. 14, 904 (1942). ( 2 ) Frank, Adrian, $m. Ind. H y g . Assoc. J . 23, 424 (1962). (3) Gibson, W. XI., "The Radiochemistry of Lead," Kational Academy of Sci-
ences, National Research Council, Nuclear Science Series, KAS-KS 3040, Office of Technical Services. IT. 8. DP~ a r t m e n t of Commerce, kashington, D.
c.,lg6'.
(4) Kirby, H. W., AKAL.CHEM.26, 1063 114.54) ,-"--,.
(5) McCord, W. M . , Zemp, J. W., Ibid., 27, 1171 (1955). (6) Millard, H. T., Jr., Ibid., 35, 1017 (1963) ( 7 ) West, P. W., Carlton, J. K., Anal. Chim. Acta 6, 406 (1952). RECEIVEDfor review February 8, 1965. Accepted March 30, 1965.
Measurement of Low Energy Beta-Emitters in Aqueous Solution by Liquid Scintillation Counting of Emulsions MICHAEL S. PATTERSON and RONALD C. GREENE Radioisotope Service, Veterans Administration Rospital and the Department of Biochemistry, Duke University School of Medicine, Durham, N. C. Triton X-100 has been found to be an acceptable agent for the formation of stable emulsions of water and toluene for liquid scintillation counting. The properties of three counting mixes a r e described. These are: toluene: Triton, 2 : 1 , which can b e mixed with 23% water and give counting efficiencies of 10% for H3 and 68% for C I 4 ; toluene: Triton : ethanol, 8 : 4 : 3, which can hold 4 3 % water with counting efficiencies as high as 58% for C I 4 ; toluene:triton, 7:6, which forms a fluid emulsion with 43% water that sets to a rigid gel on cooling in which CI4 can be counted with 5770 efficiency.
I
liquid scintillation counting has become the method of choice for measurement of low energy @-emitters such as C14, P5, or H3. Unfortunately, solvents like the alkylated benzenes which give the highest yield of photons per p particle do not dissolve significant quantities of many biological materials, while conversely, some of these biological compounds and water are pot'ent quenching agents. Thus an increase in sample size generally results in a decrease in efficiency, and specimens with low concentrations of radioactivit,y are difficult' to measure with accuracy. Many techniques have been developed, some simple and some complex, some general and some specific, to overcome the problems of counting polar, water soluble compounds. The specific procedures, which have been developed by many workers, are too numerous to be individually cited here but they are well covered in the recently published book by Schram ( 3 ) and in a comprehensive review by Rapkin ( 2 ) . N R E e m T YEARS
854
ANALYTICAL CHEMISTRY
I n spite of the wide variety of techniques which have been developed, the counting properties of emulsions containing high percentages of water have not been evaluated; although on theoretical grounds their properties would be expected to be highly superior to most of the previously described mixtures. We have tested a number of emulsifiers for this purpose. Of those tested, which satisfactorily emulsify toluene and water, only Triton X-100 (Rohm & Haas, Inc.) has acceptable counting properties, in that it does not quench light emission from the scintillators, and it has low concentration of phosphorescent contaminants which can easily be removed by treatment with silica gel. Emulsification with toluene solutions of PPO (2,5-diphenyloxazole) and POPOP [1,4-bis-2-(5 phenyloxazoly1)-benzene] has been found to allow counting of large volumes of aqueous solutions with high efficiency. After completion of the Rork reported here, a paper by Meade and Stiglitz ( 1 ) was called to our attention. These workers used a mixture of Triton X-100 and a toluene solution of PPO and POPOP to suspend small quantities (10 to 100 mg.) of tissue for counting, but did not investigate emulsification of aqueous solutions. EXPERIMENTAL
Scintillators, radioactive standards (benzoic acid C1*, tritiated water and toluene, calibrated in d.p.m.), and counting vials were obtained from the Packard Instrument Company, La Grange, Ill. All other chemicals, except Triton X-100 were reagent grade and were used nithout further purification. Triton X-100 contained small quantities of phosphorescent materials and
had to be purified by the following procedure before it was suitable for use in scintillation counting: one-tenth part of silica gel (6-16 mesh) is added to the Triton X-100 and the mixture is vigorously mechanically stirred (Eberbach Con-Torque stirrer) for 15 to 20 minutes. The mixture is allowed to sit for 15 minutes, whereupon most of the silica gel settles out. The supernatant Triton X-100 is then decanted through a thin layer of glass wool spread across a Buchner funnel. A second similar filtration is often required to remove the last traces of silica gel. Material prepared in this manner can be stored in the laboratory without special precautions for several months. The counting and emulsification properties of a given batch of Triton X-100 remain constant on storage but considerable vbriation has been observed between different batches of Triton. Therefore when a new lot of Triton is used, it is advisable to redetermine the optimal proportions of the counting mixes. All s'amples were counted in 20-ml. screw-cap glass vials in a Model 314 EX-2 Packard TriCarb liquid scintillation spectrometer. Throughout this work, constant discriminator settings were used (red channel 80-1000, green channel 400-1000) and optimal counting rates for each mixture were obtained by adjustment of the phototube dynode voltage. The properties of the various emulsions are dependent on temperature. As the emulsions used here are cooled, each clears at a temperature (between 25" C. and 0' C.) characteristic of the particular emulsion. T o assure formation of adequate emulsions, the mixtures must be agitated after they have been cooled below their respective clearing points. Evaluation of Counting Mixtures. I n order to allow ready comparison of the counting properties of Triton X-100 emulsions with other mixes,
