bility of keeping an analytical check on a known amount of carbon-14 added in appropriate chemical form a t the beginning of, or during, an industrial organic chemical process affords many opportunities for controlling the process. The labeling of a particular constituent of the crude feed of an oil refinery for a fixed period would allow a detailed examination of the flow rates and patterns throughout the plant to be made-e.g., the contribution of this constituent to coke in the catalytic crackers, the completeness of the burnoff of the coke from this constituent in the burn-off cycle. LITERATURE CITED
(1) Aldrich, L. T., Wetherill, G. W., Tilton, 0. R., Davis, G. L.9 Phys. Rev. 103 1045-7 (1956). 12) Bothe, W., Ann. k’hysik. [a] 44(1949). (3) Curran, S. C., Dixon, D., Wilson, H. W., Phys. Rev. 84,151 (1951). (4) Curtiss, M. L., Heyd, J. W., ANAL. CHEM.27, 1073 (1955). (5) Danziger, H., 2. Phys. 128,79 (1950).
(6) Forro, M., Z. Physik. 138, 441 (1954); Phys. Rev. 92, 931 (1953). (7) Geese-Bahnisch,I., Huster, E., Naturwissenschaft& 39, 379 (1954). (8) Gleason, G. I., Tabern, D. L., Taylor, T. D., Nucleonics 8, No. 5, 112 (1951). (91 L.. Solomon. A. K.. . , Glendenin. Science i127623 (1950). (10) Graf, W. L., Comar, C. L., Whitnev, I. B., Nucleonics 9. No. 4. 22 (1951j, Grinberg, B., Le Gallic, Y., J . Phys. radium 17,35 (1956). Harley, J., Hallden, N., Nucleonics 13, No. 1 , 3 2 (1955).
Haxel, O., Houtermans, F. G., Kemmerich, M., Phys. Rev. 74, 1886 (1948).
Huster, E., Rausch, W., Univ. of Marburg, Germany, private communication to L. T .Aldrich, G. W. Wetherill, G. R. Tilton, G. L. Davis. (15) Kemmerich, M., Z. Phys. 126, 399 (1949). (16) Lerch, P., Helv. Phys. Acta 26, 663 (1953). (17) Lewis, G. M., Phil. Mag. 43, 1970 (1952). (18) Libby, W. F., ANAL. CHEM.19, 2 (1947).
(19) Libby, W. F., Phys. Rev. 46, 196 (1934). (20) Zbd., 103, 1900 (1956). (21) Libby, W. F., Lee, D. D., Ibid. 5 5 , 245 (1939). (22) MacGregor, M. H., Wiedenbeck, 98,420 (1952). M. L., IM., (23) Muller, R. H., Lo8 Alamos Scientific
Laboratory, Los Alamos, N. M,, private communication. (24) Seliger, H. H., Phys. Rev. 88, 408 (1952). (25) Sommermeyer, K., Walchter, K. H., 2.angew. Phys. 8, 53 (1955). (26) Steinberg, E. P., “Interpretation of
Counting Data,” Argonne National Laboratory Report ANL5622, September, 1956. (27) Stevenson, P. c., Univ. of California Radiation Laboratory, Livermore, Calif., private communication. (28) Strassman, F., Walling, E., Ber. deut. chem. Ges. 71, IB, 1 (1938). (29) Sugihara, T. T., Wolfgang, R. L., Libby, W. F., Rev. Sci. Instr. 24,511 (1953). (30) Suttle, A. D., Jr., Libby, W. F., ANAL. CHEM.27, 921 (1955).
RECEIVEDfor review June 23, 1956. Accepted June 13,1957.
Simple Liquid Scintillation Counter for Chemical Analysis with Radioactive Tracers WILLIAM SEAMAN Research Division, American Cyanamid Co., Bound Brook,
b A liquid scintillation counter with one photomultiplier tube and no refrigeration is simple, inexpensive, and compact. Because it requires a higher level of activity than would b e available for some applications, it is recommended for analytical uses where the necessary levels of activity would not b e disadvantageous. Examples of its use are given for counting tritium activity in tritiated triolein and for determining naphthalene in tar oil fractions by the isotope-dilution method using naphthalene tagged with carbon- 1 4. scintillation counting (1, 4, 7) is recognized as being advantageous for counting low-energy beta rays of radioisotopes such as carbon-14, tritium, and sulfur-35. It is also advantageous for relatively volatile substances such as naphthalene. McDonald and Turner (6) used carbon-14-tagged naphthalene to determine naphthalene in coal tar samples and in distillation fractions by means of the isotope dilution method. They employed the liquid scintillation counting apparatus described by Audric and
L
IQUID
1570
ANALYTiCAL CHEMISTRY
N. J.
Long (1). This apparatus had one unrefrigerated photomultiplier tube. However, because of the few details published, many difficulties had to be resolved by the author in using the method. As this technique is of great potential value to analytical chemists, this report is made to facilitate its more widespread use. The apparatus was also tested for counting tritium, with good results. One-tube refrigerated counters have also been reported (8, 9). The apparatus described handles the problem of tube noise merely by using a low-level discrimination circuit, which is part of a commercial scaler. This makes it possible to cut out all pulses below a definite predetermined threshold amplitude. The discrimination, however, also cuts out some of the desired pulses from the sample and does so increasingly as the threshold level is raised. Consequently, the apparatus is not recommended for counting low activities for which the statistical errors in counting the high noise background would be too great to permit adequate precision in the net count. However, for many analytical purposes, particularly with the isotope dilution
method, there is no compelling need to use low activities and the simple instrument described would often be adequate. APPARATUS
Figure 1 is a diagrammatic sketch of the apparatus used. An RCA 5819 photomultiplier tube, contained in a light-tight housing, has a retaining ring of pressed fiber cemented to the top of the photomultiplier tube with collodion cement. This is to hold some silicone fluid for coupling the sample cell optically to the photomultiplier tube. The latter is surrounded by a mu-metal shield for protection from stray magnetic fields, and is held in a tube socket containing dividers. The socket in turn rests in a lead base for loading the shock mounts. The whole is contained in a brass housing with a brass cover provided with a light baffle. The housing rests on two horizontal lead bricks, which in turn rest on a foam rubber pad underneath which is placed an ordinary rubber pad. This arrangement reduces vibrations to which the photomultiplier tube is sensitive. Some lead bricks also surround the apparatus for shielding. Figure 2 indicates the circuitry of the apparatus. There are 10 dynode stages.
The resistance between each stage is given in 1000-ohm units. A Model 182X Nuclear-Chicago scalins unit wa,s used which has . R ~ 5000-volt ~.~~~~~~ ...~ power supply, a high-gain linear amplifier, a preset timer, and gain controls to permit threshold discrimination settings. The sample cells were fahricated from tau-form, flat-topped weighing bottles, 40 mm. in internal diameter and 100 mm. in height. To these were fused optically clear, flat glass ends, after the weighing bottles had been cut down so that the over-all height, including the top cover, was 90 mm. Bright aluminnm foil was wrapped around the cell and a circle of bright aluminum foil was also attached to the inside of the cover of the cell in order to increase the righegathering efficiency. Figure 3 is a photograph of the assembled apparatus with the sample cell shown, hut without the shielding. ~
~~~
the discriminator setting is adjusted to the desired level, and counting is begun. The ability of the cells to reflect light may not be equal, so that factors must he established by which to convert the count of each of the cells to a common basis. This is done by countimg the activity of the same volume of a common scintiating solution in each of the cells and calculating conversion factors. The number of counts obtained with the RCA 5819 photomultiplier tube may be increased by raising the voltage applied to the dynode stages (up to a limit of about 1250 volts). However, to cut out some of the simultaneously increasing counts due to thermal noise, it is necessary not only to adjust the voltage to a suitable value, but also to set the gain control on the discriminator circuit a t a level which will allow only pulses above the desired threshold value to be counted. Figure 4 shows the change in counting
rate for a given preparation with a change in voltage a t several discrimination levels. Voltage and discrimination levels required to get the most advantageous differences between the total count and the background count must he chosen on the basis of the characteristics of a particular determination. If a voltage is chosen on the steep part of the curve, s n y imperfection in the voltage regulation of the power supply will cause fluctuations in the COUQt.
Another limitation is reproducibility of setting the discrimination level. I n the naphthalene analysis, counting was done a t 1200 volts with a discrimination setting of either 15 mv. or 20 mv. The over-all precision was poorer than that which would be expected from the statistics of the random radiation emission. It is likely that some of the loss in precision was caused by working on the steep portions of a curve. However, working a t too low a voltage or too high a discrimination setting may reduce the count so much that inordinately long counting periods for background and sample may be necessary to get satisfactory precision. I n addition, lower voltages and lower threshold values may make it difficult to distinguish the dark-noise counting rate from the net counting rate. A more favorable spread may he obtained a t the higher voltages and a t higher threshold values. If the high voltage is turned on and counting is begun soon thereafter, an exceedingly high dark-noise value will be obtained. This decreases as the scaler warms up and ultimately settles down to a reasonably low and fairly constant value, the magnitude of which depends upon the voltage and the discriminator setting. It normally takes a t least 2 or 3 hours (with the particular apparatus used) for stahilisation t o occur, so that it is advisable to leave the high voltage supply turned on without interrnption day and night while the scaler is being used for connb ing.
COUNTING TECHNIQUE
Many liquid scintillators have been described (t-4). I n this work 2.5 grams of p-terphenyl and 5 mg. of diphenylhexatriene were dissolved in toluene to ength ,egion I tube
scincell mina,lume le cell ahout t the Nrning 1000 I fiher tuhe. and iy air ttight high talue,
ANODE
DYNODE IC DYNOOES
H?
J
3
D"NoDf-,'
150K
DYNODE 2 DYNODE I CATHODE
*
150 K
r
'y
~
15p photomultiplier
.
so-239 RLCLPTAICLE
ibe housing and Figure 3.
Assembled apparatus
VOL. 29, NO. 1 1 , NOVEMBER 1957
1571
Another precaution, when replacing one sample cell with another, is to turn down the high voltage. It should, however, be turned on again with no delay when the sample has been inserted and the lighttight cover has been replaced on the cell holder. [A report published subsequent t o the work reported here (9) describes a device which permits insertion of a sample without turning down the high voltage.] Furthermore, a n additional minute should elapse after the high voltage has reached its proper level before any counting is begun, otherwise there will be an initially high dark noise which may falsify the count.
Table I.
Instrument
Counting Efficiencies for Tritiated Triolein" in Toluene
A
Scintillator 0 . 3 % diphenyloxazole
B C
phenyloxazolylbenzene 0.37@diphenyloxazole 0 .3Ye diphenyloxazole
+
This paper a
O.5ye
=a
B
I 0,000
Naphthalene per
to Sample
Solution
1:51 1:51 1:87 1:87
Xaphthalene Found, 70
44.5 49.4 71.8 73.2
74.4 143.0 50.8 25.0 49.7 24.3
Mean Std. dev. of single value = i ~ 1 . 0 5 7(abs.) ~ or 325.2% (rei.) Std. dev. of mean = =k0.43% (abs.) or 2Z2.193 (rel.)
I1
5,000
APPLIED VOLTAGE Figure 4. Count-voltage curves at various discrimination levels
.
The efficiency of counting a particular isotope depends on the experimental conditions. With naphthalene-l-C14, the efficiency decreases as the concentration of naphthalene in solution increases because of the quenching effect of naphthalene. I n a typical experiment the counting efficiency was about 29%. With tritium in the form of tritiated triolein, a lower counting efficiency is obtained than with carbon-14. This is because of the lower energy of the tritium beta rays. Therefore, a discrimination level which is high enough to reduce the tube noise sufficiently will also cut out a greater portion of the 1572
Weight Ratio,
1:90 1:90 1:90 1:90 1:56 1:56
20,000
3
13.4 6.4 13.6 11.1 9.4 3.7 9.8
17.6 16.3 16.6 16.0 16.6
Std. dev. of single value = 320.707e (abs.) or 324.2% (rel.) Std. dev. of mean = &0.35y0 (abs.) or 2 ~ 2 . 1 %(rel.)
a
0
2.1 1.0 2.2 1.8 1.5 0.6 1.6
Mean
w
0
4760 2269 4846 3942 3347 1328 3472
Tagged Saphthalene 10 M1. of Scintillator
30,000
2
103 240 275 275 10, 36gb 7,100~ 22,539
Determination of Naphthalene by Isotope Dilution blg. Recovered
I
z
(bis)-2,5-
+
Fraction Sample A
w
Calcd. Countinn .. Activity, Effi--= Mpc. per ciency, per bfg. Mg. %
Net C.P.M.
triene Specific activity by gas counting = 16 mpc. per mg. Voltage 1000, discriminator a t 1 mv. Voltage 1100, discriminator a t 4 mv. Voltage 1200, discriminator a t 6 mv.
Tar Oil
t
Background, C.P.M.
0.577@ terphenyl 0.01% diphenylhexa-
Table II.
1I
-
~
ANALYTICAL CHEMISTRY
desired pulses. Table I gives figures for background, net counting rate, and counting efficiency versus activity by gas counting for tritiated triolein for three commercially available liquid scintillation counters in comparison with that just described (6). Different scintillators were used and other conditions were not controlled so that the comparisons should not be pressed too far. However, it is evident that under some conditions of voltage and discrimination, the apparatus described here gives an efficiency of counting which compares favorably with the others. Of course, the higher background would necessitate longer counting periods to attain a desired precision. Furthermore, the apparatus would not do for counting low activity. DETERMINATION
OF NAPHTHALENE
To illustrate the utility of the liquid scintillation counter in analysis, it was used to determine naphthalene in tar oil fractions by the isotope dilution method. Naphthalene-l-CI4 of a specific activity of 20 pc. per mg. was diluted with pure inactive naphthalene to a specific
21.3 18 3 20.6 20 3 20.7 19.6 20.1
activity of 0.002 pc. per mg., which was not altered by recrystallizing the naphthalene. Keighed portions of this standard naphthalene were dissolved in about 10-gram portions of the tar oil sample in the ratios given in the second column of Table 11. By a process of distillation and recrystallization, naphthalene of high purity was isolated (as indicated by melting point values). Portions of this recovered naphthalene were dissolved in the scintillator solution. These were counted in comparison with solutions of known weights of the standard naphthalene sufficient to give a counting rate not far removed from that of the recovered naphthalene. To the standard naphthalene there were added additional amounts of pure inactive naphthalene sufficient to equalize the total weight of naphthalene in the sample cell and the comparison cell (in order to balance self-quenching effects). T o avoid errors due to any possible gradual change in the count caused by instrumental variations, the counting was carried out in a definite sequence so that, for example, a 20minute count on the comparison cell was interspersed between two 10-minute
counts on the sample cell. The average of these two 10-minute counts was then compared with the interspersed coniparison count. Table I1 gives the results of some typical determinations. ACKNOWLEDGMENT
Grateful acknowledgment is made to G. E. Gerhardt who designed and constructed the housing and associated circuity for the photomultiplier tube and furnished valuable assistance in overcoming many instrumental difficulties
which arose during the course of this work. LITERATURE CITED
(1) Audric, B. N., Long, J. V. P., Research
5,46 (1952). (2) Hayes, F. N., Nucleonics 12, S o . 3, 27 (1954). (3) Kallman, H., Furst, M., Sucleonics 8. KO. 3. 32 (1951). (4) Kafiman, H., 'Furst, AT., Phys. Rev. 79,857 (1950); 81, 853 (1951). (5) Kritchevsky, D., American Cyanamid Co., Pearl River, N. Y., personal communication. (6) McDonald, W. S., Turner, H. S., Chem. & Ind. 1952, 1001.
(7) Raben, XI. S., Bloembergen, K., Science 114,363 (1951). (8) Rosenthal, D. J., Anger, H. O., U. S. Atomic Energy Comm. UCRL-2320 (Aug. 21, 1953). (9) Weinberger, -4.J., Davidson, J. B., Ropp, G. A., -&SAL. CHEW28, 110 (1956). RECEIVED for review December 26, 1956. Accepted May 27, 1957. Presented in part, Meeting-in-Miniature of the Analytical Group, North Jersey Section, ACS, Xewark, K.J.,January 24,1955, and Section on Radiochemical Methods, XVth Congress of Pure and Applied Chemistry, Lisbon, Portugal, September 14,1056.
Modified Determination of Radium in Water F. B. BARKER
U. S. Geological Survey, Denver, Colo. L. L. THATCHER U. S. Geological Survey, Washington, D. C. The proposed method embodies a barium sulfate carrier precipitation, filtration through molecular filter membranes, and collection of activity after prescribed aging period. The method is sufficiently accurate and precise to indicate the potability of water and for use in general studies of radium in chemical hydrology. Amounts of radium as low as 0.1 ppc. can be detected by using 1-hour counting times. Radium-226 is used as the standard and the results indicate about 100 to 1 1 0 of the activity of the alpha-emitting radium isotopes as radium-223, radium-224, and radium226.
yo
C
interest in radioactivity in water has developed within the past sereral years. T o the public health official and the n-aterworks engineer, radioactivity in the water supply is important as a possible health hazard. To the industrial engineer, it is a variable n-hich may affect the quality of his industrial products. To the geologist and geochemist, it is a clue to subsurface conditions and processes. T o the hydrologist, radioactivity in water is a potential tool for the study of \T ater movement. Radium is a source of radioactivity in water. The isotope radium-226 is a member of the uranium series, radium228 and radium-224 are members of the thorium series, and radium-223 is a member of the actinouranium series. From the standpoint of health, radium-226 is the most important because of its very OXSIDERABLE
low permissible tolerance level in safe drinking water (IO), 40 ppc. per liter. However, from the geochemical and hydrological standpoint, the other isotopes are also of interest. Several methods suitable for determining radium in mater have been reported. The emanation method (2, 3, 8 ) has been used extensively and is capable of yielding very good results for the isotope radium-226; however, the equipment is expensive t o install and operate. Stehney (12) has determined both radium-226 and radium-224 in water by a modification of a method developed by Ames and coworkers ( I ) and revised by Russell, Lesko, and 8chubert ( I I ) , which involves doublecarrier precipitation followed by alpha counting. A simple barium sulfatecarrier precipitation method for determining radium in urine was reported by Harley and Foti ( 7 ) ) and a similar method vas described by Gubeli and Jucker ( 5 ) . The U.S. Geological Survey uses a modification of the method of Harley and Foti ( 7 ) for the routine determination of radium in natural waters. The method embodies a barium sulfatecarrier precipitation, filtration through a molecular filter membrane, and counting of activity after a prescribed aging period. Although this method is less accurate for some waters than more elaborate procedures, it requires less time of the technicians and less equipment. It is sufficiently accurate and precise to indicate the potability of water and for general studies of radium in chemical hydrology.
REAGENTS
Standard Radium Solution. ,4 Kational Bureau of Standards radium226 gamma-ray standard, containing 1.0 x 10-7 gram of radium, is broken under approximately a liter of distilled water, and the solution is transferred t o a 2-liter volumetric flask. The broken ampoule is leached with 50 ml. of concentrated hydrochloric acid and washed with distilled water; the leach and washings are added to the original solution. Dilution t o 2 liters provides a stock solution containing 5 X 10-l' gram per ml. of radium. A working standard is prepared by diluting 10 ml. of the stock solution plus 10 ml. of concentrated hydrochloric acid to 500 nil. The usual precautions required when handling radioactive alpha emitters niust be observed to prevent the contamination of personnel and equipment. Barium Carrier Solution. TWOand a half grams of reagent grade barium chloride dihydrate are dissolved in 1 liter of distilled water. Ammonium Sulfate Solution. Four hundred grams of reagent grade ammonium sulfate are dissolved in 1 liter of hot distilled water; t h e solution is cooled and filtered. Sulfuric Acid Wash Solution. Concentrated sulfuric acid is diluted 1 t o 200; 0.15 gram of Aerosol OT, 10070 (American Cyanamid Co.) is added for each liter of solution. PROCEDURE
Water samples containing less than 350 mg. of calcium are diluted t o 1 liter in 2-liter beakers and the acidity of each is adjusted t o approximately p H 3 with hydrochloric acid. A 1-liter disVOL. 29, NO. 11, NOVEMBER 1957
0
1573
