all perchlorates decomposed during the heating. If the salts stand a t room temperatures for any appreciable time, they frequently do not completely dissolve in the glycerol. Preheating the dry salts for approximately 1 hour a t 200" to 280' C. often proved helpful for preparing the glycerol solution. This suggests the possibility that a hydrated form of one of the salts may be formed and it is not readily soluble in the solvent system. The final solution has been found to be stable indefinitely, and the sample may be counted a t any time thereafter. Occasionally, with certain urine and large tissue samples (more than 500 mg.), the oxidation salts did not completely dissolve in the glycerol and complete solvent system. Analysis of smaller quantities of these samples indicated that the presence of this precipitate did not cause any loss of activity in the larger samples. If the specific activity in terms of the sulfur-35 or sulfate-35 content is desired, a sulfate determination can be conveniently made after the sample is counted. For this, the counted sample is transferred to a small separatory
funnel with several portions of water. After separation of the aqueous phase, the insoluble barium or benzidine sulfate can then be precipitated by the addition of the appropriate reagent. Khen duplicate aliquots of radioactive liver homogenates were oxidized with varying amounts (0.5 t o 3.0 ml.) of modified Pirie's reagent, no significant difference was observed in the counting rate. Furthermore, these results indicated that small errors in the measurement of the volume of glycerol or the ethyl alcohol-dimethylformamide mixture did not change the counting rate. It is absolutely necessary, however, that the sample to be counted be a clear colorless solution. To obtain the highest counting efficiency, the water impurities in the various solvents should be removed. However, i t may be more convenient to use the commercial grades of absolute ethyl alcohol and N,N-dimethylformamide directly, without further purification. If so, the efficiency would be 4 to 5% lower. Probably the greatest advantage of this method is the direct linearity of the observed counting rate with the true
counting rate over a very large range.
As a result, it is not necessary to apply any self-absorption or coincidence correction factors. Because nonradioactive materials present on the samples do not interfere with the counting rate, it is not necessary t'o isolate the radioactive sulfur compound in pure form. Although this determination has been described for the determination of S35 in biological materials, it should be applicable to all oxidizable materials containing S35. LITERATURE CITED
(1) Packard Tri-Carb Liquid Scintillation Spectrometer Operation l l a n u a l , Pack-
ard Instrument Co., LaGrange, Ill. Also, Personal communication from above company officials. (2) Passmann, J. If., Radin, Y S., Cooper, J. A. D., .4xa~.CHEY.28. 484 (1956). (3) Pirie, N. R., Biochern J. 26, 2041 (1932): (4) Radin, N. S.,Fried, R., A N ~ LCHEM. . 30, 1926 (1958). (5) Vaughan, M., Steinberg, D., Logan, J., Science 126, 446 (1957). RECEIVED for review September 10, 1959. Accepted December 23, 1959. Work supported by U. s. Public Health Service Grant -4-1543.
The Absolute Assay of Sulfur-35 by internal Gas Counting W. F. MERRITT and R. C. HAWKINGS Atomic Energy o f Canada limited, Chalk River, Ontario, Canada
b A method is described for measuring the absolute disintegrotion rate of 87.2-day sulfur-35, (j3-€,,, = 0.1 67 m.e.v.). The sulfur to b e assayed is converted to sulfur dioxide by ignition of barium sulfate with red phosphorus in an atmosphere of oxygen. The sulfur dioxide is then mixed with methane and counted in an internal gas proportional counter.
S
ULFUR-35is a pure negatron emitter with a half life of 87.2 days and a maximum beta energy of 0.167 m.e.v. ( 7 ) . ilbsorption losses in solid sources of such a low energy beta emitter are extremely variable and difficult to estimate (6). Internai gas counting (1) is a n-ell established technique for the assay of low energy beta emitters which can be prepared in a gaseous form and do not give poor counting characteristics. Wilkinson has stated (9) that any gas will function in a proportional counter. For the purpose of absolute 308
ANALYTICAL CHEMISTRY
counting, however, one restriction is imposed: The gas used must be such that one, and only one, pulse be recorded for every disintegration which takes place within the sensitive volume of the counter-that is, the gas must neither capture electrons nor release extra electrons in such a way as t o affect the observed number of pulses significantly. Of the two common gaseous compounds of sulfur, hydrogen sulfide, and sulfur dioxide, sulfur dioxide was selected as most desirable. An investigation v a s therefore undertaken of the performance of sulfur dioxide in methane-filled proportional counters, and of a suitabk procedure for the preparation of sulfur dioxide from sulfates without isotopic fractionation. EFFECT O F SULFUR DIOXIDE ON COUNTER EFFICIENCY
Simple tests n ith external sources showed that a proportional gas counter filled n-ith methane would tolera:i> small
quantities of sulfur dioxide nithout apparent, ill effects. A more precise evaluation of possible losses or spurious pulses was made as follows. A gas counter was filled with a mixt,ure of 2 em. of mercury pressure of hydrogen containing tritium (H3) and 74 cm. of mercury pressure of methane. Such a mixture has been shown to haye n high counting efficiency (2). The voltage characteristic for this mixture v a s determined and the plateau counting rate recorded. Successive amounts of inactive sulfur dioxide were then added to the filling and the counting characteristics redetermined. The results of these measurements are shown in Figure 1. The addition. of up to 4.5-mm. Hg pressure of sulfiiv dioxide to 760-mm. Hg pressure n i methane causes no change in t h plateau counting rate to within f 0.1L7;,. In general with increasing concentratior of sulfur dioxide, the plateau region is moved to higher and higher voltages while the voltage region for multiple counting remains essentially const;int.
As a result the plateau becomes shorter with increasing concentration of sulfur dioxide. No significant error is introduced until the pressure of sulfur dioxide exceeds 7.5 mm. Hg (about 1% SOZ in CHI). Beyond this a semblance of a plateau may be obtained with a n appreciably lower counting rate. For example, in Figure 1 using 10.5-mm. Hg pressure of sulfur dioxide, the apparent plateau counting rate is 1% lower than the true rate. At a pressure of 20 mm. Hg, the plateau has disappeared completely. The effect of pressure of sulfur dioxide containing sulfur-35 on an SOZ-CH, counter filling was then measured. The results are shown in Table I. The rates observed for 0.54-mm. and 1.1-mm. pressures of sulfur dioxide in Table I are believed to be in error due to the difficulties in measuring the small pressures of sulfur dioxide used. The value for 16.2-mm. Hg pressure of sulfur dixoide filling is lower by about 2% than the average of the three previous measurements. This is in accord with the observations made with the tritium and inactive sulfur dioxide filling in Figure 1. The foregoing experiments indicate that up to 1% sulfur dioxide may be used in methane-filled beta proportional counters with an efficiency of 99.9% relative to that of tritium as H H S in mixture with methane. PREPARATION
OF SO2
Having established that S"OZ could be used successfully as a counter gas with methane, the next problem was the development of a suitable procedure for the preparation of radioactive sulfur dioxide. As sulfur-35 is most frequently used as the sulfate and most other compounds of sulfur can be converted quantitatively to sulfates by standard chemical procedures, sulfate was selected as the starting point for the preparation of W02. Methods investigated include (A) thermal decomposition of CuS04 to SOa with reduction to SO2a t l l O O o C. over a platinum catalyst, (B) reduction of CaS04to CaS with carbon and oxidation to SOZ,(C) direct reduction with hydrogen a t 1100O C., and (D) ignition of BaS04 n 3 h red phosphorus in an oxygen atmosphere (3). Method D proved to be the most suitable for our purposes. hlethod A showed promise but gave erratic results in actual use. This inconsistency was probably due to insufficient carrier sulfate in the sulfur35 solution, which was not discovered until method D n a s developed. PROCEDURE
A solution of sulfur-35 as sulfate was diluted to about lo5 counts per second per ml. usirig 25 p.p.m. of lithium
9 5 0 d '
B
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o
105mm5G2 20mmSG1
-0-
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90 0
1
1
2400
2560
2800
2720
3040
,
3200
V O L T S
Figure 1. Effect of sulfur dioxide concentration on counting characteristics Figure 3.
Gas-mixing line
J, K , M , P . Stopcocks I. Methane storage bulb N. Manometer 0. Mixing bulb Q. Differential counters
"I;
4RMSTRONG CEMENT
c
c OUARTZ
Figure 2. Apparatus for preparing sulfur dioxide
. GUARD RING 8 . '
A.
B. C. D.
E. F. G.
H.
1 -liter reaction flask Glass wool plug Alundum boat Sand Ignition wire Tungsten seals Trap Sample flask
03
S S T E E L CATHODE
Table I. Effect of Sulfur Dioxide Concentration on Observed Counting Rate for Sulfur-35
Pressure of SOZ, Mm. Hg
Counts/Sec./Cm. SO1
0.54 1.13 2.13 4.32 8.28 16.2
3220 3662 3582 3585 3589 3506
Table II. Precision of Analysis for Sulfur-35 b y Internal Gas Counting
1 2
3 4
INVAR ARMSTRONG CEMENT
~
t-
- ,
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7
OUARTZ
Preparation
"ODE
Counts/Sec./Ml. S3602at N.T.P. 4168 4232 4200 4162 4212 4174 4230 4172 .4v. 4194 i- 29 (k0.770S.D.)
sulfate as carrier and 2 p.p.m. of potassium dichromate as a bacterial inhibitor. Sufficient standardized c o p per sulfate solution was added to give, upon completion, a convenient volume of sulfur dioxide. The sulfate was then precipitated as barium sulfate, filtered, and ignited, according to standard analytical procedures (4). The pulverized barium sulfate was mixed with an excess of red phosphorus in an Alundum boat, and placed on a bed of sand in a combustion flask (Figure 2). The flask
SPRING COPPER GLASS SEAL
-m', -w'm y
-(a-
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_ t
Internal gas counter
was evacuated and filled to a pressure of 50 cm. Hg with oxygen and the contents of the boat were ignited electrically. Combustion was completed in about 20 seconds. A flask of liquid oxygen was placed around the trap and the system was evacuated. Suspended phosphorus pentoxide was filtered out on the glass-wool plug and the sulfur dioxide was condensed in the liquid oxygen-cooled trap. It is essential to use liquid oxygen a t this stage to prevent condensation of oxygen in the trap. The sulfur dioxide was then distilled into a glass sample tube for storage. Before counting, the sample was purified on a high vacuum line by condensing the sulfur dioxide in liquid nitrogen, warming to dry ice temperatures, and "flashing" to vacuum (8). The sulfur dioxide was then diluted quantitatively with methane and suitable aliquots were used for counting. The sample was introduced into the counter and manometer system from the sample flask, H (Figure 3). The pressure was measured on manometer N by means of a cathetometer sensitive to 0.002 cm. The rest of the system was evacuated and the gas in the counters VOL. 32, NO. 3, MARCH 1960
309
a n d associated manifold pumped into t h e mixing flask, 0, by means of the Toepler pump. The tapered leads to stopcock K assured that the gas was fransferred quantitatively. A suitable amount of methane was pumped into the mixing flask from the reservoir, L, and the gases were mixed by repeated compression and expansion cycles. The mixture was then displaced into the counter system and the counters were sealed off and removed from the line for counting.
and corrected for decay to an arbitrary zero time. The pressure measurements were corrected for temperature and for change in density of mercury with temperature. The exact amount of gas in the differential volume of the counters could then be determined. This was expressed as a fraction of the total gas evolved from the known weight of sulfate used in the dilution. From these data the disintegration rate per milliliter of sulfur-35 stock solution was calculated.
COUNTING
DISCUSSION A N D RESULTS
LITERATURE CITED
Two counters, differing only as to length, were used (Figure 4). The volumes of the two counter cathodes were measured by filling them with water from a calibrated buret before assembly. The sensitive volume of the counter was accurately defined by the field tubes. All dimensions were measured to A0.002 em. The long counter had a volume approximately twice that of the short counter. With 3 inches of lead for shielding, these counters had background counting rates of approsimately 2 and 1 counts per second, respectively. Counting apparatus consisted of a scaler, H.T. set, and pulse amplifier. A voltage plateau was determined for each counter. The data were corrected for dead time losses and for background. The differential plateau was derived by subtracting the results of the short counter from those of the long. The differential plateau was usually 100 volts in length with a slope of 1% per 100 volts.
A typical set of results is shown in Table 11. Four generations of S3502 were made from samples containing 5 ml. of standardized copper sulfats solution, and 3 ml. of sulfur-35 stock solution. Duplicate counts mere made on each gas sample. There is excellent agreement between both duplicate counts and between the different generations of V502. The errors involved in the measurement of low pressures of sulfur dioxide may be avoided by a suitable dilution of the gas with methane. The end effects of the counters are avoided by the differential counter technique. Wall effects were investigated by using counters of different diameters, but were not detected. This is in agreement with the results reported by Mann (6). There is little chance of isotopic fractionation in the high temperature solid state reaction used for preparing the sulfur dioxide. By suitable modification of the gashandling system, it should be possible to generate the sulfur dioxide, pump it directly into a small known volume,
(1) Bernstein, W., Ballantine, R., Rev. Sci. Znstr. 21,158-63 (1950).
CALCULATIONS
The observed counting rate was obtained from the differential plateau
measure the relatively high pressure on a mirror scale, and then mix with methane and transfer to a calibrated counter. Such a system would enable accurate, routine measurements of sulfur-35 to be performed. A study of the accuracy of the method and a comparison with other methods for the absolute assay of S35 have been carried out a t Chalk River and have been described by Merritt and others (6).
(2) Hawkings, R. C., Merritt, W. F., Atomic Energy of Canada Ltd., Chalk River, Ontario, AECL 94 (1954). (3) Johnson, R. E., Huston, J. L., J . Am. Chem. SOC.72, 1841-2 (1950). (4) Kolthoff, I. M., Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” pp. 329-43, Macmillan, New York. 1943. ~ ~ (5) Merritt, J. S., Taylor, J. G. V , Merritt, W. F., Campion, P. J., ANAL. CHEM.32,310 (1960).(6) National Academy of Sciences, National Research Council, Washington, D. C., “Measurements and Standards of Radioactivity,” Publ. 573, 102-3 (1958). ( 7 ) Slack, L., Way, X., “Radiations from Radioactive btoms,” U. S. Atomic Energy Comm., 1959. (8) Thode, H., Macnamara, J., Collins, C. B., Can. J. Research 27B, 361-73 (1949). (9) Wilkinson, D. H., “Ionization Chambers and Counters,” p. 152, Cambridge University Press, Cambridge, 1950. I
RECEIVED for review September 17, 1959. Accepted December 1, 1959. Division of Analytical Chemistry, Symposium on Radiochemical Analysis, 136th Meeting, ACS, Atlantic City, N. J., September 1959.
The Absolute Counting of Sulfur-35 JANET S. MERRITT, J. G. V. TAYLOR, W.
F. MERRITT,
and P. J. CAMPION
Atomic Energy of Canada limited, Chalk River, Ontario, Canada
b Several methods for determining the absolute disintegration rate of a stock solution of sulfur-35 are presented and compared. These include differential gas counting as sulfur dioxide, 4irb counting with suitable corrections, and a new tracer method. A technique is described for preparing sources for 47rO counting in which a minute quantity of colloidal silica is added to improve the uniformity of the deposit, The corrections associated with 474 counting are described, the largest being that due to self-absorption. In the tracer method, a P-y-emitting nuclide suitable for coincidence counting is combined with the sulfur-35 and used as a tracer to 310
ANALYTICAL CHEMISTRY
follow the efficiency for counting sulfur-35. Experimental justification for this method i s presented. The mean of these independent determinations has a standard deviation of about 1
yo.
T
published proceedings of the Easton Conference on Measurements and Standards of Radioactivity (10) suggest a general lack of agreement on the absolute standardization of sulfur-35, a radionuclide which decays by beta emission only. Because of the low beta end point energy (167 k.e.v.) self-absorption effects are large in most counting techniques (4, 6, ‘7, 9,13, 16), HE
and this may have been responsible for the discrepancies. The wide use of this nuclide makes better agreement desirable; therefore we have examined and compared several methods of standardizing a stock solution of sulfur-3.5. EXPERIMENTAL
The specific activity and radioactive concentration of the sulfur-35 stock solution were approximately 20 me. per mg. of lithium sulfate and 1 me. per ml., respectively. I n addition, the solution contained 2 p.p.m. of potassium dichromate to prevent bacterial growth. Because the results of this analysis were obtained over a period of several
