Preparation of Suspended Solids Samples for Radioactivity Counting R. L. Weatherford and T.
E. Larson, Illinois State
Water Survey, Urbang, 111.
the recommended alcohol I burn-off method (6) to determine the in
As the backscatter factor for thallium204 has been shown by Nader, Hagee, and Setter (1) t o be 1.375 on stainless steel, the origin of the connecting curve is indicated at this point. Previous measurements have indicated that the thickness of the cellulose acetate, immediately below the preparation, may vary by f 0.7 mg.per sq. cm., a variation of minor significance. The backscatter factor, which includw absorption by the cellulose acetate, for this metaod of preparation of suspended solids, may be taken as 1.27 with a vtiriation of f 0.02, equivalent to an error of about 1.6% in the estimation of counting efficiency. The data indicated no charge problem when the cellulose acetate was dissolved and dricd, but srparate tests have shown
N USINQ
radioactivity of suspended matter surface water samples, difliculty has been experienced, particularly for low suspended solids. Objections to the method have been largely due to indefinite counting area; occasional loss of suspended matter during burning or during gas purging in the proportional counting chamber, particularly for samples of low suspended matter; and loss of sample in handling or storage. T o overcome these problems, the Millipore filter can be used as a binder. The wet filter with the deposit of suspended solids is placed flat on an adsorbent blotter and set aside to airdry completely overnight. A small beaker is inverted over the filter to avoid curling. The filter with the suspended matter is then placed in the center of the steel planchet which has been cleaned and wetted with about 1 ml of acetone, and another 0.5 ml. of acetone is spread evenly over the surface of the dissolved Millipore filter. The steel planchet is then placed on a blotter and under a watch glass for at' least 2 hours. Or the sample is allowed to stand for l hour and then placed in an oven for 1 niinute a t 90" to 95" c. A stable, adherent deposit of the suspended matter develops on the surface of the cellulose acetate, which is tightly adherent to the planchet. The counting area is unaltered from the specific nrea dei.rloped during filtration. To obtain a satisfactory modification of the backscatter factor that would
itirlude :tbsorption by the cellulase ac*c4ato, thallium-204 was used as a rcyftwncv standard. I t was deposited on thr filter surface from a preparation of 200 ml. of water containing approxiin:it'c,l>-450 p.p.m. of fuller's earth and .io nil. of concentrated or a 1 to 10 dilution of thc thallium-204 standard. The prcpsration was neutralized with 1.65 nil. of Clorox and stirred for 30 niinutvs lx'fore filtration. Its rnergy is similar ( 2 ) to that for mixrd fission products (MFP), which has a recommended backsrnttw factor of 1.36 for stainlt5ss steel. From the National Bureau of Standards and ccrtifiration of activity subsequent meaauremrnts in this laboratory, a thallium-204 standard was deterniined t o have a half lifp of 2.46 0.18 years, in fair agreement Kith the 2.7 years reported by Seaborg (3). The latter figure was used to calculate
(.two)
*
THICKNESS, m g / c m 2
Figure 1. Relation of effective backscatter factor to thickness of cellulose acetate between suspended solids deposit and stainless steel planchet
Table I.
Influence of Cellulose Acetate on Effective Backscatter Factor
Cellulose Acetate Prepara- Thickness, tion Mg./Sq. Cm. F-6
F-9 F-10 F-11
a
4 5 9.5 14.5 19.4 4 1 8.3 12 6 4 8 9.3 14.0 4.3 9.7 14.8
D.P.M.
C.P.M.
48,650
18,718 17,059 16,047 15,222 1,983 1,806 1,705 1,960 1,782 1,645 1,917 1,721 1.605
4,670 5,120 4,944
Sample Thickness, Cm.
lIg./Sq.
12.0
A0 0.61
8.5
0.66
12.7
0.60
10.7
0.62
1 1 1 1 1 1 1 1 1 1 1
B 2G 15 98 02 29 17 11 28
16 07 25 1.12 1 05
Estimated from ( 4 ) .
the activity of measurcd aliquots used as reference standards. Four preparations nerc made and after determination of the activity, the cellulose acetate with the preparation was removed from the planchet and placed on a second Millipore filter which had been dissolved, but not dried on the planchet. After drying, the activity was again determined. This was repeated with the third AIillipore filter and the activity r a s again determined. Using the equation B =
cpm ~
dpm
X
1
-
GA
3
where G is the geomrtry factor, 0.5, A is the self-absorption factor (Q), and B is the effective backscatter factor, the data in Table I were obtained, as shown graphically in Figure 1 .
the counting rittc to be reduced by 11 to 15% \\ hen thr dried but not dissolved filter pad was placed directly on the planchet for counting. The data presentrd i n Tnbk I and Figure 1 nerr the result of trirlicntc %minute counting on dup1ir:ctc. prqxxations and were within the limit of crror at the 95% confidence levcl. LITERATURE CITED
( 1 ) Nader, J. S., Hagee, G . R., Setter, L. R., Nucleonics 12, No. 6,29 (1954).
( 2 ) Reynolds, S. A., Brooksbank, W. A., Jr., Zbid., 11, No. 11, 46 (1953). ( 3 ) Seaborg, G. T., Perlman, I., Reus. Modern Phys. 20, 631 (1948). ( 4 ) Setter, L. R., Goldin, A. S., Nader, J. S.,ANAL.CHEM.26, 1304 (1954). ( 5 ) Setter, L R., Hagee, G. R., Straub, C. P., .4STM Bull. 227, 35 (19%).
VOL. 31, NO. 11, NOVEMBER 1959
1931
