---
In certain applications the stirring mechanism can be eliminated by introducing nitrogen a t low pressure into the vacuum flask, M , to achieve a stirring act,ion by ebulation through the sintered disk, N . SUGGESTED MODIFICATIONS
Several possible modifications have become apparent with continued use of this apparatus. Where very precise temperature control is necessary, a suitable control, preferably a thermistor, could be inserted in the flask, 0,and used to control the circulating pump. An electric heating tape might be preferable to the steam jacket, D,for some applications where reactions or crystallizations are to be carried out a t controlled temperatures above room temperature. Most organic solvents, except ethyl alcohol, attack the graphited asbestos packing in the pump. If it were necessary to circulate a higher boiling liquid, a rotary mechanical seal instead of the standard stuffing box would serve the purpose. Valve G could be moved to a position between the pinch clamp, B, and the bypass line. In this position it would serve the additional purpose of allowing one to use a n external jacketed filtering vessel through A and K . Tubing, K , is connected to a tee in the inlet side of the pump. LITERATURE CITED
Craig, L. C., IND.ENG.CHEM.,ANAL. ED. 12, 773 (1940).
(1)
Figure 1.
Diagram of low temperature crystallization apparatus
(2) Piper, J. D., Kerstein, N. A,, Fleiger, A. G., Ibid., 12, 738 (1942). (3) Quakenbush, F. W., Steenbock, H., Ibid., 14, 736 (1942). (4) Scheraga, H. A., Manes, M., ANAL. CHEM.21, 1581 (1949).
(5) Wendland, R., Zbid., 28, 282 (1956). The mention of firm names or trade-products does not imply that they are endorsed or recommended by the Department of Agriculture over other firms or similar
products not mentioned.
Use of Submicron Silica to Prevent Count Loss by Wall Adsorption in Liquid Scintillation Counting
F. A.
Blanchard and 1. T. Takahashi, Radiochemistry Laboratory, The Dow Chemical Co., Midland, Mich.
ERIOUS ERRORS
in liquid scintillation
S radioassays of part per million level
solutions can be caused by adsorption of the radioactive compound to the wall of the counting vial. Such adsorption was reported by Hayes (2) with dilute solutions of benzoic acid-C14 in toluene scintillator. At 0.1 p.p.m., the counting efficiency was only half that a t concentrations of 2 to 1000 p.p.m. He attributed this to benzoic acid molecules being adsorbed on the walls of the glass container and thereby removed from 4 to 2 T geometry. A number of tracer compounds have shown this effect in our laboratory. A counting sample containing polya~rylamide-c'~,after 1, 10, 100, and 1000 hours in the counter, gave 291,
246, 225, and 175 c.p.m. The scintillation solution, when transferred into a clean vial, gave only a background count rate. However, the sample vial itself, filled with fresh scintillator, gave the full equilibrium count rate of 175 c.p.m. Thus, the compound seemed to be completely adsorbed on the wall of the counting vial. An internal standard used with such a sample would give an entirely erroneous counting efficiency. Of course many compounds do not show such adsorption-for example, the monomer from which the polyacrylamide was prepared. The adsorption might be mitigated by varying the solvent or by adding a carrier. Another approach is to make use of the adsorption by controlling its
effects. Thus, if the adsorption occurs primarily on material suspended in the midst of the solvent, the 4 T geometry is preserved, provided the suspension particles are small compared to the beta-particle range. Such adsorbing material should have an active surface area, for the compound, much greater than that of the glass wall of the counting vial. It should remain dispersed and be relatively transparent. Cab-0-Si1 M5 silica (99.0 to 99.770 SiOt, Cabot Corp., Boston, Mass.) is such a material. The particles are reported to range in size from 0.015 to 0.020 micron, giving rise to a surface area of 175 to 200 square meters of surface per gram. The absorption properties of several polymers on such VOL. 33, NO. 7, JUNE 1961
975
a material have been reported (4). Its use as a suspending agent for liquid scintillation counting has been described
Table I.
Effect of Cab-0-Si1 on Counting Rates of Several Compounds
Counts Per Minute Without Cab-0-Si1 With Cab-0-Si1
(1, 3).
The effectiveness of Cab-0-Si1 silica in preferentially adsorbing polyacrylamide was measured by centrifuging a silica-containing counting sample and then radioassaying the supernate and the emptied vial. This showed that 99% of the activity was taken up by the silica. A control consisting of the nonadsorbing acrylamide showed no adsorption with a recovery of 101% in the solution. EXPERIMENTAL
Scintillator solutions were prepared with and without Cab-0-Si1 silica. The solution itself was made up of 70 parts of toluene, containing 0.4% PPO (2,5diphenyloxazole) and 0.01% POPOP [1,4-bis-2-(5-phenyloxazolyl)benzene], mixed with 30 parts of absolute ethyl alcohol. A portion of this solution was stirred into the silica to give a n approximately 3% wt./ v. suspension. Aqueous solutions of one sulfur-35 and five carbon-14 compounds were prepared at 1, 10, and 100 p.p.m. Adsorbing compounds: NalSOa polyacrylamide A polyacrylamide B poly-N-vinyl-5methyl-2-oxazolidinone (PVOM) Nonadsorbing compounds: acrylamide N-vinyl-5-methyl2-oxazolidinone WOM)
ionic inorganic ionic polymer, 4’% hydrolyzed ionic polymer, 30y0 hydrolyzed nonionic polymer
nonionic monomer nonionic monomer
The polyacrylamide A and the acrylamide had the same specific activities; likewise, the PVOM and VOM.
976
ANALYTICAL CHEMISTRY
Adsorbing compounds NanSOs Polyacrylamide A Polyacrylamide B PVOM Nonadsorbing compounds Acrylamide VOM
Time in Counter 1 day 1 week 1 day 1 week 1 day 1 week 1 day 1 week
1 day 1 week 1 day
1 week
P.P.M. of Compounds in Test Solution 10
L
32 27 28 19 118
77 9
8
35 35 20
17
346 22 1
306 205 1,150 718 104 89
337 335 176 173
100
1
10
3,725 2,500 2,974 1,965 10,950 7,543
39
422
1,414
44
35 37 128 128 16
1,245
15
3 ,432 3,435 1,655 1,657
37 38
164
4,211 4,180 3,509 3,529 13,084 13,160 1,651 1,655
329 325 178 176
3,559 3 ,508 1,677 1,674
427 348 355 1,278 1,293 161
21 21
100
Counting samples were prepared in duplicate from 0.1-ml. aliquots of these test solutions in 20 ml. of each scintillator solution. The samples were counted in a Tri-Carb liquid scintillation spectrometer Model 314X (Packard Instrument Co.: LaGrange, Ill.) during the first day after preparation and again after 1 week in the counter freezer. The average of at least two 10-minute counts of each sample, corrected for background, was averaged with its duplicate. Sulfur-35 data were corrected for decay to the time of the first counts.
1-week period with count rates as high as homogeneous systems (polyacrylamide A with acrylamide and PVOlM with VOM). Obviously the silica particles were too small to introduce appreciable self-absorption of the beta radiation from the adsorbed compounds. The nonadsorbing compounds gave equally good efficiencies (48 to 50%) with or without silica. Compounds with questionable adsorption properties can thus be counted in silica as a precautionary measure at no cost in efficiency.
RESULTS A N D DISCUSSION
LITERATURE CITED
The count rates observed are given in Table I. The adsorbing compounds, when counted without silica, gave low initial counts which after 1 week approached values indicating extensive adsorption with a close approach to 2 ?r geometry. However, with silica, these samples gave stable values over the
(1) Gordon, C. F., Wolfe, A. L., ANAL.
CHEM.32,574 (1960).
(2) Hayes, F. N., U. S. At. Energy
Comm. Document, LA-1639 (1953). (3) Ott, D. G., Richmond, C. R., Trujillo, T. T., Foreman, H., Nucleonics 17, 106 (1959). (4) Stromberg, R. R., Quasius, A. R., Toner, S. D., Parker, M. S., J. Research Natl. Bur. Standards 62, 71 (1959).
